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Hard braking and the Physics of Coming-to-Rest
Hard braking and the Physics of Coming-to-Rest
Colonel John Stapp rode a rocket sled 29 times during research at
Edwards Air Force Base in the 1950's. He was already 43 year old when
he made run # 29, 0-to-630mph in 5 secs (not too shabby), but his
"braking" was literally eye-watering: 630-to-0 in 1.4 seconds ---
oooof! The record books say it registered about 40G's of
deceleration.
Nonetheless Stapp lived to the ripe old age of 92.
Seriously: What's the physics of coming-to-rest? We've all seen that
oddity of tires slipping slightly when a vehicle comes to rest, when
the dying speed is minuscule --- it can't just be "marbles" and grit
under the tire. Race cars often skid the last inches into the pits,
when the tires had evidently gripped effectively in high-speed braking
and equally at maybe 60kph on approach when aero had no effect. (I
should declare my innocence of these matters; my school in England
almost 50 years ago forbade me to re-take "O Level" physics yet again,
to protect its reputation.)
Colonel John Stapp rode a rocket sled 29 times during research at
Edwards Air Force Base in the 1950's. He was already 43 year old when
he made run # 29, 0-to-630mph in 5 secs (not too shabby), but his
"braking" was literally eye-watering: 630-to-0 in 1.4 seconds ---
oooof! The record books say it registered about 40G's of
deceleration.
Nonetheless Stapp lived to the ripe old age of 92.
Seriously: What's the physics of coming-to-rest? We've all seen that
oddity of tires slipping slightly when a vehicle comes to rest, when
the dying speed is minuscule --- it can't just be "marbles" and grit
under the tire. Race cars often skid the last inches into the pits,
when the tires had evidently gripped effectively in high-speed braking
and equally at maybe 60kph on approach when aero had no effect. (I
should declare my innocence of these matters; my school in England
almost 50 years ago forbade me to re-take "O Level" physics yet again,
to protect its reputation.)
Re: Hard braking and the Physics of Coming-to-Rest
On Apr 17, 4:46 pm, "brafi...@hotmail,com " <brafi...@hotmail,com >
wrote:
> Colonel John Stapp rode a rocket sled 29 times during research at
> Edwards Air Force Base in the 1950's. He was already 43 year old when
> he made run # 29, 0-to-630mph in 5 secs (not too shabby), but his
> "braking" was literally eye-watering: 630-to-0 in 1.4 seconds ---
> oooof! The record books say it registered about 40G's of
> deceleration.
>
> Nonetheless Stapp lived to the ripe old age of 92.
>
> Seriously: What's the physics of coming-to-rest? We've all seen that
> oddity of tires slipping slightly when a vehicle comes to rest, when
> the dying speed is minuscule --- it can't just be "marbles" and grit
> under the tire. Race cars often skid the last inches into the pits,
> when the tires had evidently gripped effectively in high-speed braking
> and equally at maybe 60kph on approach when aero had no effect. (I
> should declare my innocence of these matters; my school in England
> almost 50 years ago forbade me to re-take "O Level" physics yet again,
> to protect its reputation.)
For the first part of the deceleration, the grip between the tyre and
the surface exceeds the grip of the pads over the disc. At high
speeds the downforce on the car adds "weight" to the vehicle too so
the wheels grip more effectively.
As speed decreases the downforce reduces and the driver makes
adjustments (presumably) to reduce the braking force to compensate for
the lower levels of grip between the road surface and the tyre. Also
the effectiveness of the pads / discs combination varies throughout
the braking cycle as the various components heat to optimal efficiency
(and then exceed it). As lower and lower speeds are reached,
downforce reduces and the amount of brake "power" increases relative
to momentum so unless the driver reduces input absolutely in line with
the theoretical maximum its likely that he will apply more brake
pressure than is required and the wheels will lock.
I know nothing about these things, but that would be my guess!! It
was interesting (for me) thinking it through though.
Matt
On Apr 17, 4:46 pm, "brafi...@hotmail,com " <brafi...@hotmail,com >
wrote:
> Colonel John Stapp rode a rocket sled 29 times during research at
> Edwards Air Force Base in the 1950's. He was already 43 year old when
> he made run # 29, 0-to-630mph in 5 secs (not too shabby), but his
> "braking" was literally eye-watering: 630-to-0 in 1.4 seconds ---
> oooof! The record books say it registered about 40G's of
> deceleration.
>
> Nonetheless Stapp lived to the ripe old age of 92.
>
> Seriously: What's the physics of coming-to-rest? We've all seen that
> oddity of tires slipping slightly when a vehicle comes to rest, when
> the dying speed is minuscule --- it can't just be "marbles" and grit
> under the tire. Race cars often skid the last inches into the pits,
> when the tires had evidently gripped effectively in high-speed braking
> and equally at maybe 60kph on approach when aero had no effect. (I
> should declare my innocence of these matters; my school in England
> almost 50 years ago forbade me to re-take "O Level" physics yet again,
> to protect its reputation.)
For the first part of the deceleration, the grip between the tyre and
the surface exceeds the grip of the pads over the disc. At high
speeds the downforce on the car adds "weight" to the vehicle too so
the wheels grip more effectively.
As speed decreases the downforce reduces and the driver makes
adjustments (presumably) to reduce the braking force to compensate for
the lower levels of grip between the road surface and the tyre. Also
the effectiveness of the pads / discs combination varies throughout
the braking cycle as the various components heat to optimal efficiency
(and then exceed it). As lower and lower speeds are reached,
downforce reduces and the amount of brake "power" increases relative
to momentum so unless the driver reduces input absolutely in line with
the theoretical maximum its likely that he will apply more brake
pressure than is required and the wheels will lock.
I know nothing about these things, but that would be my guess!! It
was interesting (for me) thinking it through though.
Matt
Re: Hard braking and the Physics of Coming-to-Rest
larkim wrote:
> On Apr 17, 4:46 pm, "brafi...@hotmail,com " <brafi...@hotmail,com >
> wrote:
> > Colonel John Stapp rode a rocket sled 29 times during research at
> > Edwards Air Force Base in the 1950's. He was already 43 year old
> > when he made run # 29, 0-to-630mph in 5 secs (not too shabby), but
> > his "braking" was literally eye-watering: 630-to-0 in 1.4 seconds
> > --- oooof! The record books say it registered about 40G's of
> > deceleration.
> >
> > Nonetheless Stapp lived to the ripe old age of 92.
> >
> > Seriously: What's the physics of coming-to-rest? We've all seen
> > that oddity of tires slipping slightly when a vehicle comes to
> > rest, when the dying speed is minuscule --- it can't just be
> > "marbles" and grit under the tire. Race cars often skid the last
> > inches into the pits, when the tires had evidently gripped
> > effectively in high-speed braking and equally at maybe 60kph on
> > approach when aero had no effect. (I should declare my innocence
> > of these matters; my school in England almost 50 years ago forbade
> > me to re-take "O Level" physics yet again, to protect its
> > reputation.)
>
> For the first part of the deceleration, the grip between the tyre and
> the surface exceeds the grip of the pads over the disc. At high
> speeds the downforce on the car adds "weight" to the vehicle too so
> the wheels grip more effectively.
>
> As speed decreases the downforce reduces and the driver makes
> adjustments (presumably) to reduce the braking force to compensate for
> the lower levels of grip between the road surface and the tyre. Also
> the effectiveness of the pads / discs combination varies throughout
> the braking cycle as the various components heat to optimal efficiency
> (and then exceed it). As lower and lower speeds are reached,
> downforce reduces and the amount of brake "power" increases relative
> to momentum so unless the driver reduces input absolutely in line with
> the theoretical maximum its likely that he will apply more brake
> pressure than is required and the wheels will lock.
>
> I know nothing about these things, but that would be my guess!! It
> was interesting (for me) thinking it through though.
>
What I was thinking, that is relevant at the very lowest speeds, is
that the reverse of your first sentence becomes true. That is at some
point the grip of the friction material of the brakes exceeds that of
the tyre on the surface.
You feel the same thing happen in road cars (I don't mean the skidding)
when at the end of braking, if you don't ease of the brake, retardation
increases significantly at which point the passenger is thrown toward
the dashboard.
Same disclaimer applies.
--
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larkim wrote:
> On Apr 17, 4:46 pm, "brafi...@hotmail,com " <brafi...@hotmail,com >
> wrote:
> > Colonel John Stapp rode a rocket sled 29 times during research at
> > Edwards Air Force Base in the 1950's. He was already 43 year old
> > when he made run # 29, 0-to-630mph in 5 secs (not too shabby), but
> > his "braking" was literally eye-watering: 630-to-0 in 1.4 seconds
> > --- oooof! The record books say it registered about 40G's of
> > deceleration.
> >
> > Nonetheless Stapp lived to the ripe old age of 92.
> >
> > Seriously: What's the physics of coming-to-rest? We've all seen
> > that oddity of tires slipping slightly when a vehicle comes to
> > rest, when the dying speed is minuscule --- it can't just be
> > "marbles" and grit under the tire. Race cars often skid the last
> > inches into the pits, when the tires had evidently gripped
> > effectively in high-speed braking and equally at maybe 60kph on
> > approach when aero had no effect. (I should declare my innocence
> > of these matters; my school in England almost 50 years ago forbade
> > me to re-take "O Level" physics yet again, to protect its
> > reputation.)
>
> For the first part of the deceleration, the grip between the tyre and
> the surface exceeds the grip of the pads over the disc. At high
> speeds the downforce on the car adds "weight" to the vehicle too so
> the wheels grip more effectively.
>
> As speed decreases the downforce reduces and the driver makes
> adjustments (presumably) to reduce the braking force to compensate for
> the lower levels of grip between the road surface and the tyre. Also
> the effectiveness of the pads / discs combination varies throughout
> the braking cycle as the various components heat to optimal efficiency
> (and then exceed it). As lower and lower speeds are reached,
> downforce reduces and the amount of brake "power" increases relative
> to momentum so unless the driver reduces input absolutely in line with
> the theoretical maximum its likely that he will apply more brake
> pressure than is required and the wheels will lock.
>
> I know nothing about these things, but that would be my guess!! It
> was interesting (for me) thinking it through though.
>
What I was thinking, that is relevant at the very lowest speeds, is
that the reverse of your first sentence becomes true. That is at some
point the grip of the friction material of the brakes exceeds that of
the tyre on the surface.
You feel the same thing happen in road cars (I don't mean the skidding)
when at the end of braking, if you don't ease of the brake, retardation
increases significantly at which point the passenger is thrown toward
the dashboard.
Same disclaimer applies.
--
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Re: Hard braking and the Physics of Coming-to-Rest
Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>What I was thinking, that is relevant at the very lowest speeds, is
>that the reverse of your first sentence becomes true. That is at some
>point the grip of the friction material of the brakes exceeds that of
>the tyre on the surface.
Braking is simply an energy conversion process...kinetic energy is
converted into heat.
The rate that the discs dissipate the heat produced by the pad/disc
dictates the rate of deceleration.
--
Peter
Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>What I was thinking, that is relevant at the very lowest speeds, is
>that the reverse of your first sentence becomes true. That is at some
>point the grip of the friction material of the brakes exceeds that of
>the tyre on the surface.
Braking is simply an energy conversion process...kinetic energy is
converted into heat.
The rate that the discs dissipate the heat produced by the pad/disc
dictates the rate of deceleration.
--
Peter
Re: Hard braking and the Physics of Coming-to-Rest
peter wrote:
> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > What I was thinking, that is relevant at the very lowest speeds, is
> > that the reverse of your first sentence becomes true. That is at
> > some point the grip of the friction material of the brakes exceeds
> > that of the tyre on the surface.
>
> Braking is simply an energy conversion process...kinetic energy is
> converted into heat.
Given.
>The rate that the discs dissipate the heat
> produced by the pad/disc dictates the rate of deceleration.
Is this meant to be helpful in any way?
--
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peter wrote:
> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > What I was thinking, that is relevant at the very lowest speeds, is
> > that the reverse of your first sentence becomes true. That is at
> > some point the grip of the friction material of the brakes exceeds
> > that of the tyre on the surface.
>
> Braking is simply an energy conversion process...kinetic energy is
> converted into heat.
Given.
>The rate that the discs dissipate the heat
> produced by the pad/disc dictates the rate of deceleration.
Is this meant to be helpful in any way?
--
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Re: Hard braking and the Physics of Coming-to-Rest
Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>peter wrote:
>> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>> > What I was thinking, that is relevant at the very lowest speeds, is
>> > that the reverse of your first sentence becomes true. That is at
>> > some point the grip of the friction material of the brakes exceeds
>> > that of the tyre on the surface.
>>
>> Braking is simply an energy conversion process...kinetic energy is
>> converted into heat.
>
>Given.
>
>>The rate that the discs dissipate the heat
>> produced by the pad/disc dictates the rate of deceleration.
>
>Is this meant to be helpful in any way?
>
Adding an explanation to an effect...what was I thinking!
--
Peter
Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>peter wrote:
>> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>> > What I was thinking, that is relevant at the very lowest speeds, is
>> > that the reverse of your first sentence becomes true. That is at
>> > some point the grip of the friction material of the brakes exceeds
>> > that of the tyre on the surface.
>>
>> Braking is simply an energy conversion process...kinetic energy is
>> converted into heat.
>
>Given.
>
>>The rate that the discs dissipate the heat
>> produced by the pad/disc dictates the rate of deceleration.
>
>Is this meant to be helpful in any way?
>
Adding an explanation to an effect...what was I thinking!
--
Peter
Re: Hard braking and the Physics of Coming-to-Rest
peter wrote:
> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > peter wrote:
> >>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> >>> What I was thinking, that is relevant at the very lowest speeds,
> is >>> that the reverse of your first sentence becomes true. That is
> at >>> some point the grip of the friction material of the brakes
> exceeds >>> that of the tyre on the surface.
> > >
> > > Braking is simply an energy conversion process...kinetic energy is
> > > converted into heat.
> >
> > Given.
> >
> > > The rate that the discs dissipate the heat
> > > produced by the pad/disc dictates the rate of deceleration.
> >
> > Is this meant to be helpful in any way?
> >
> Adding an explanation to an effect...what was I thinking!
Is it an explanation of the effect?
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peter wrote:
> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > peter wrote:
> >>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> >>> What I was thinking, that is relevant at the very lowest speeds,
> is >>> that the reverse of your first sentence becomes true. That is
> at >>> some point the grip of the friction material of the brakes
> exceeds >>> that of the tyre on the surface.
> > >
> > > Braking is simply an energy conversion process...kinetic energy is
> > > converted into heat.
> >
> > Given.
> >
> > > The rate that the discs dissipate the heat
> > > produced by the pad/disc dictates the rate of deceleration.
> >
> > Is this meant to be helpful in any way?
> >
> Adding an explanation to an effect...what was I thinking!
Is it an explanation of the effect?
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Re: Hard braking and the Physics of Coming-to-Rest
Bigbird wrote, On 18/04/2008 09:13:
> peter wrote:
>
>> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>>> peter wrote:
>>>> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>>>>> What I was thinking, that is relevant at the very lowest speeds,
>> is >>> that the reverse of your first sentence becomes true. That is
>> at >>> some point the grip of the friction material of the brakes
>> exceeds >>> that of the tyre on the surface.
>>>> Braking is simply an energy conversion process...kinetic energy is
>>>> converted into heat.
>>> Given.
>>>
>>>> The rate that the discs dissipate the heat
>>>> produced by the pad/disc dictates the rate of deceleration.
>>> Is this meant to be helpful in any way?
>>>
>> Adding an explanation to an effect...what was I thinking!
>
> Is it an explanation of the effect?
>
The car coming to an abrupt stop under gentle braking is not due to the
tyres slipping, so is not really the reverse of what larkim said.
If the brakes are applied with a low but constant force they will
gradually slow the car. During which time it's momentum exceeds the
friction between pads and discs required to make them lock. Once the
momentum of the car becomes sufficiently low, and if the braking is not
adjusted for speed, the pads will grab on the discs and probably cause
an immediate stop. The momentum of the car is now too low to make the
tyres skid. The driver will over react and lift off the brake causing
passengers to jolt forward and back in their seats.
I've noticed this as well, usually with drivers that ride the brake
pedal instead of choosing the correct gear for their speed.
--
DavidM newsNO@ SPAMdjmorgan.org.uk
www .djmorgan.org.uk
Bigbird wrote, On 18/04/2008 09:13:
> peter wrote:
>
>> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>>> peter wrote:
>>>> Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
>>>>> What I was thinking, that is relevant at the very lowest speeds,
>> is >>> that the reverse of your first sentence becomes true. That is
>> at >>> some point the grip of the friction material of the brakes
>> exceeds >>> that of the tyre on the surface.
>>>> Braking is simply an energy conversion process...kinetic energy is
>>>> converted into heat.
>>> Given.
>>>
>>>> The rate that the discs dissipate the heat
>>>> produced by the pad/disc dictates the rate of deceleration.
>>> Is this meant to be helpful in any way?
>>>
>> Adding an explanation to an effect...what was I thinking!
>
> Is it an explanation of the effect?
>
The car coming to an abrupt stop under gentle braking is not due to the
tyres slipping, so is not really the reverse of what larkim said.
If the brakes are applied with a low but constant force they will
gradually slow the car. During which time it's momentum exceeds the
friction between pads and discs required to make them lock. Once the
momentum of the car becomes sufficiently low, and if the braking is not
adjusted for speed, the pads will grab on the discs and probably cause
an immediate stop. The momentum of the car is now too low to make the
tyres skid. The driver will over react and lift off the brake causing
passengers to jolt forward and back in their seats.
I've noticed this as well, usually with drivers that ride the brake
pedal instead of choosing the correct gear for their speed.
--
DavidM newsNO@ SPAMdjmorgan.org.uk
www .djmorgan.org.uk
Re: Hard braking and the Physics of Coming-to-Rest
DavidM wrote:
> Bigbird wrote, On 18/04/2008 09:13:
> > peter wrote:
> >
> >>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > > > peter wrote:
> >>>>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > > > > > What I was thinking, that is relevant at the very lowest
> > > > > > speeds,
> >>is >>> that the reverse of your first sentence becomes true. That is
> >>at >>> some point the grip of the friction material of the brakes
> >>exceeds >>> that of the tyre on the surface.
> > > > > Braking is simply an energy conversion process...kinetic
> > > > > energy is converted into heat.
> > > > Given.
> > > >
> > > > > The rate that the discs dissipate the heat
> > > > > produced by the pad/disc dictates the rate of deceleration.
> > > > Is this meant to be helpful in any way?
> > > >
> > > Adding an explanation to an effect...what was I thinking!
> >
> > Is it an explanation of the effect?
> >
>
> The car coming to an abrupt stop under gentle braking is not due to
> the tyres slipping, so is not really the reverse of what larkim said.
"the grip between the tyre and the surface exceeds the grip of the pads
over the disc."
The reverse being
"the grip between the tyre and the pads exceeds the grip between the
tyre and the surface"
leading to "that oddity of tires slipping slightly when a vehicle comes
to rest"
HTH.
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DavidM wrote:
> Bigbird wrote, On 18/04/2008 09:13:
> > peter wrote:
> >
> >>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > > > peter wrote:
> >>>>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > > > > > What I was thinking, that is relevant at the very lowest
> > > > > > speeds,
> >>is >>> that the reverse of your first sentence becomes true. That is
> >>at >>> some point the grip of the friction material of the brakes
> >>exceeds >>> that of the tyre on the surface.
> > > > > Braking is simply an energy conversion process...kinetic
> > > > > energy is converted into heat.
> > > > Given.
> > > >
> > > > > The rate that the discs dissipate the heat
> > > > > produced by the pad/disc dictates the rate of deceleration.
> > > > Is this meant to be helpful in any way?
> > > >
> > > Adding an explanation to an effect...what was I thinking!
> >
> > Is it an explanation of the effect?
> >
>
> The car coming to an abrupt stop under gentle braking is not due to
> the tyres slipping, so is not really the reverse of what larkim said.
"the grip between the tyre and the surface exceeds the grip of the pads
over the disc."
The reverse being
"the grip between the tyre and the pads exceeds the grip between the
tyre and the surface"
leading to "that oddity of tires slipping slightly when a vehicle comes
to rest"
HTH.
--
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Re: Hard braking and the Physics of Coming-to-Rest
Bigbird wrote:
> DavidM wrote:
>
> > Bigbird wrote, On 18/04/2008 09:13:
> > > peter wrote:
> > >
> > >>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > > > > peter wrote:
> > >>>>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > > > > > > What I was thinking, that is relevant at the very lowest
> > > > > > > speeds,
> > >>is >>> that the reverse of your first sentence becomes true. That
> > is >>at >>> some point the grip of the friction material of the
> > brakes >>exceeds >>> that of the tyre on the surface.
> > > > > > Braking is simply an energy conversion process...kinetic
> > > > > > energy is converted into heat.
> > > > > Given.
> > > > >
> > > > > > The rate that the discs dissipate the heat
> > > > > > produced by the pad/disc dictates the rate of deceleration.
> > > > > Is this meant to be helpful in any way?
> > > > >
> > > > Adding an explanation to an effect...what was I thinking!
> > >
> > > Is it an explanation of the effect?
> > >
> >
> > The car coming to an abrupt stop under gentle braking is not due to
> > the tyres slipping, so is not really the reverse of what larkim
> > said.
>
> "the grip between the tyre and the surface exceeds the grip of the
> pads over the disc."
>
> The reverse being
>
> "the grip of the pads over the disc exceeds the grip between the
> tyre and the surface"
[corrected]
>
> leading to "that oddity of tires slipping slightly when a vehicle
> comes to rest"
>
> HTH.
--
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Bigbird wrote:
> DavidM wrote:
>
> > Bigbird wrote, On 18/04/2008 09:13:
> > > peter wrote:
> > >
> > >>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > > > > peter wrote:
> > >>>>Bigbird <Bigbird.UsenetREMOVTHS@Gmail,com > writes
> > > > > > > What I was thinking, that is relevant at the very lowest
> > > > > > > speeds,
> > >>is >>> that the reverse of your first sentence becomes true. That
> > is >>at >>> some point the grip of the friction material of the
> > brakes >>exceeds >>> that of the tyre on the surface.
> > > > > > Braking is simply an energy conversion process...kinetic
> > > > > > energy is converted into heat.
> > > > > Given.
> > > > >
> > > > > > The rate that the discs dissipate the heat
> > > > > > produced by the pad/disc dictates the rate of deceleration.
> > > > > Is this meant to be helpful in any way?
> > > > >
> > > > Adding an explanation to an effect...what was I thinking!
> > >
> > > Is it an explanation of the effect?
> > >
> >
> > The car coming to an abrupt stop under gentle braking is not due to
> > the tyres slipping, so is not really the reverse of what larkim
> > said.
>
> "the grip between the tyre and the surface exceeds the grip of the
> pads over the disc."
>
> The reverse being
>
> "the grip of the pads over the disc exceeds the grip between the
> tyre and the surface"
[corrected]
>
> leading to "that oddity of tires slipping slightly when a vehicle
> comes to rest"
>
> HTH.
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Re: Hard braking and the Physics of Coming-to-Rest
DavidM <newsNO@_SPAMdjmorgan.org.uk> writes
>The car coming to an abrupt stop under gentle braking is not due to the
>tyres slipping, so is not really the reverse of what larkim said.
>If the brakes are applied with a low but constant force they will
>gradually slow the car. During which time it's momentum exceeds the
>friction between pads and discs required to make them lock. Once the
>momentum of the car becomes sufficiently low, and if the braking is not
>adjusted for speed, the pads will grab on the discs and probably cause
>an immediate stop. The momentum of the car is now too low to make the
>tyres skid. The driver will over react and lift off the brake causing
>passengers to jolt forward and back in their seats.
>
You don't think that the way a car can appear to come to an abrupt stop
under gentle braking is actually not because of any change in the grip
of the tyres or change in pad friction coefficient but simply the
reaction to the sudden removal of a force?
--
Peter
DavidM <newsNO@_SPAMdjmorgan.org.uk> writes
>The car coming to an abrupt stop under gentle braking is not due to the
>tyres slipping, so is not really the reverse of what larkim said.
>If the brakes are applied with a low but constant force they will
>gradually slow the car. During which time it's momentum exceeds the
>friction between pads and discs required to make them lock. Once the
>momentum of the car becomes sufficiently low, and if the braking is not
>adjusted for speed, the pads will grab on the discs and probably cause
>an immediate stop. The momentum of the car is now too low to make the
>tyres skid. The driver will over react and lift off the brake causing
>passengers to jolt forward and back in their seats.
>
You don't think that the way a car can appear to come to an abrupt stop
under gentle braking is actually not because of any change in the grip
of the tyres or change in pad friction coefficient but simply the
reaction to the sudden removal of a force?
--
Peter
Re: Hard braking and the Physics of Coming-to-Rest
peter wrote:
> DavidM <newsNO@ SPAMdjmorgan.org.uk> writes
>> The car coming to an abrupt stop under gentle braking is not due to the
>> tyres slipping, so is not really the reverse of what larkim said.
>> If the brakes are applied with a low but constant force they will
>> gradually slow the car. During which time it's momentum exceeds the
>> friction between pads and discs required to make them lock. Once the
>> momentum of the car becomes sufficiently low, and if the braking is not
>> adjusted for speed, the pads will grab on the discs and probably cause
>> an immediate stop. The momentum of the car is now too low to make the
>> tyres skid. The driver will over react and lift off the brake causing
>> passengers to jolt forward and back in their seats.
>>
> You don't think that the way a car can appear to come to an abrupt stop
> under gentle braking is actually not because of any change in the grip
> of the tyres or change in pad friction coefficient but simply the
> reaction to the sudden removal of a force?
The jerking you feel if you keep your foot on the brake rather than
smoothly lifting it as you come to a halt is basically due to the
resulting discontinuity in your rate of deceleration - the car goes from
decelerating at, say, 0.3g, to decelerating at 0g, very suddenly - not
instantaneously but close. Your body however has momentum and is not
rigidly fixed to the car, so it keeps going until decelerated to a halt
and probably pulled backwards either by the reaction of your muscles to
the relative motion between body and car or by the seatbelt, depending
on which acts first. So it's true to say that the rate of change of
retardation suddenly increases, but the rate of retardation of the car
itself suddenly goes to zero, and that's why you get chucked at the
dashboard.
--
Phil
http :// www .flickr,com /photos/tmc1979/
peter wrote:
> DavidM <newsNO@ SPAMdjmorgan.org.uk> writes
>> The car coming to an abrupt stop under gentle braking is not due to the
>> tyres slipping, so is not really the reverse of what larkim said.
>> If the brakes are applied with a low but constant force they will
>> gradually slow the car. During which time it's momentum exceeds the
>> friction between pads and discs required to make them lock. Once the
>> momentum of the car becomes sufficiently low, and if the braking is not
>> adjusted for speed, the pads will grab on the discs and probably cause
>> an immediate stop. The momentum of the car is now too low to make the
>> tyres skid. The driver will over react and lift off the brake causing
>> passengers to jolt forward and back in their seats.
>>
> You don't think that the way a car can appear to come to an abrupt stop
> under gentle braking is actually not because of any change in the grip
> of the tyres or change in pad friction coefficient but simply the
> reaction to the sudden removal of a force?
The jerking you feel if you keep your foot on the brake rather than
smoothly lifting it as you come to a halt is basically due to the
resulting discontinuity in your rate of deceleration - the car goes from
decelerating at, say, 0.3g, to decelerating at 0g, very suddenly - not
instantaneously but close. Your body however has momentum and is not
rigidly fixed to the car, so it keeps going until decelerated to a halt
and probably pulled backwards either by the reaction of your muscles to
the relative motion between body and car or by the seatbelt, depending
on which acts first. So it's true to say that the rate of change of
retardation suddenly increases, but the rate of retardation of the car
itself suddenly goes to zero, and that's why you get chucked at the
dashboard.
--
Phil
http :// www .flickr,com /photos/tmc1979/
Re: Hard braking and the Physics of Coming-to-Rest
Phil Newnham <pnewnham@yahoo,com > writes
>peter wrote:
>> DavidM <newsNO@_SPAMdjmorgan.org.uk> writes
>>> The car coming to an abrupt stop under gentle braking is not due to the
>>> tyres slipping, so is not really the reverse of what larkim said.
>>> If the brakes are applied with a low but constant force they will
>>> gradually slow the car. During which time it's momentum exceeds the
>>> friction between pads and discs required to make them lock. Once the
>>> momentum of the car becomes sufficiently low, and if the braking is not
>>> adjusted for speed, the pads will grab on the discs and probably cause
>>> an immediate stop. The momentum of the car is now too low to make the
>>> tyres skid. The driver will over react and lift off the brake causing
>>> passengers to jolt forward and back in their seats.
>>>
>> You don't think that the way a car can appear to come to an abrupt stop
>> under gentle braking is actually not because of any change in the grip
>> of the tyres or change in pad friction coefficient but simply the
>> reaction to the sudden removal of a force?
>
>The jerking you feel if you keep your foot on the brake rather than
>smoothly lifting it as you come to a halt is basically due to the
>resulting discontinuity in your rate of deceleration - the car goes from
>decelerating at, say, 0.3g, to decelerating at 0g, very suddenly - not
>instantaneously but close. Your body however has momentum and is not
>rigidly fixed to the car, so it keeps going until decelerated to a halt
>and probably pulled backwards either by the reaction of your muscles to
>the relative motion between body and car or by the seatbelt, depending
>on which acts first.
>So it's true to say that the rate of change of
>retardation suddenly increases, but the rate of retardation of the car
>itself suddenly goes to zero, and that's why you get chucked at the
>dashboard.
>
Or rather chucked at your seat?
--
Peter
Phil Newnham <pnewnham@yahoo,com > writes
>peter wrote:
>> DavidM <newsNO@_SPAMdjmorgan.org.uk> writes
>>> The car coming to an abrupt stop under gentle braking is not due to the
>>> tyres slipping, so is not really the reverse of what larkim said.
>>> If the brakes are applied with a low but constant force they will
>>> gradually slow the car. During which time it's momentum exceeds the
>>> friction between pads and discs required to make them lock. Once the
>>> momentum of the car becomes sufficiently low, and if the braking is not
>>> adjusted for speed, the pads will grab on the discs and probably cause
>>> an immediate stop. The momentum of the car is now too low to make the
>>> tyres skid. The driver will over react and lift off the brake causing
>>> passengers to jolt forward and back in their seats.
>>>
>> You don't think that the way a car can appear to come to an abrupt stop
>> under gentle braking is actually not because of any change in the grip
>> of the tyres or change in pad friction coefficient but simply the
>> reaction to the sudden removal of a force?
>
>The jerking you feel if you keep your foot on the brake rather than
>smoothly lifting it as you come to a halt is basically due to the
>resulting discontinuity in your rate of deceleration - the car goes from
>decelerating at, say, 0.3g, to decelerating at 0g, very suddenly - not
>instantaneously but close. Your body however has momentum and is not
>rigidly fixed to the car, so it keeps going until decelerated to a halt
>and probably pulled backwards either by the reaction of your muscles to
>the relative motion between body and car or by the seatbelt, depending
>on which acts first.
>So it's true to say that the rate of change of
>retardation suddenly increases, but the rate of retardation of the car
>itself suddenly goes to zero, and that's why you get chucked at the
>dashboard.
>
Or rather chucked at your seat?
--
Peter
Re: Hard braking and the Physics of Coming-to-Rest
peter wrote:
> Phil Newnham <pnewnham@yahoo,com > writes
>> peter wrote:
>>> DavidM <newsNO@ SPAMdjmorgan.org.uk> writes
>>>> The car coming to an abrupt stop under gentle braking is not due to the
>>>> tyres slipping, so is not really the reverse of what larkim said.
>>>> If the brakes are applied with a low but constant force they will
>>>> gradually slow the car. During which time it's momentum exceeds the
>>>> friction between pads and discs required to make them lock. Once the
>>>> momentum of the car becomes sufficiently low, and if the braking is not
>>>> adjusted for speed, the pads will grab on the discs and probably cause
>>>> an immediate stop. The momentum of the car is now too low to make the
>>>> tyres skid. The driver will over react and lift off the brake causing
>>>> passengers to jolt forward and back in their seats.
>>>>
>>> You don't think that the way a car can appear to come to an abrupt stop
>>> under gentle braking is actually not because of any change in the grip
>>> of the tyres or change in pad friction coefficient but simply the
>>> reaction to the sudden removal of a force?
>>
>> The jerking you feel if you keep your foot on the brake rather than
>> smoothly lifting it as you come to a halt is basically due to the
>> resulting discontinuity in your rate of deceleration - the car goes from
>> decelerating at, say, 0.3g, to decelerating at 0g, very suddenly - not
>> instantaneously but close. Your body however has momentum and is not
>> rigidly fixed to the car, so it keeps going until decelerated to a halt
>> and probably pulled backwards either by the reaction of your muscles to
>> the relative motion between body and car or by the seatbelt, depending
>> on which acts first.
>
>
>> So it's true to say that the rate of change of
>> retardation suddenly increases, but the rate of retardation of the car
>> itself suddenly goes to zero, and that's why you get chucked at the
>> dashboard.
>>
> Or rather chucked at your seat?
You probably chuck yourself at your seat, at the end, or your seatbelt
does it for you in the case of harder braking. Yes, really the dashboard
is chucking itselt at you, rather than the other way round.
--
Phil
http :// www .flickr,com /photos/tmc1979/
peter wrote:
> Phil Newnham <pnewnham@yahoo,com > writes
>> peter wrote:
>>> DavidM <newsNO@ SPAMdjmorgan.org.uk> writes
>>>> The car coming to an abrupt stop under gentle braking is not due to the
>>>> tyres slipping, so is not really the reverse of what larkim said.
>>>> If the brakes are applied with a low but constant force they will
>>>> gradually slow the car. During which time it's momentum exceeds the
>>>> friction between pads and discs required to make them lock. Once the
>>>> momentum of the car becomes sufficiently low, and if the braking is not
>>>> adjusted for speed, the pads will grab on the discs and probably cause
>>>> an immediate stop. The momentum of the car is now too low to make the
>>>> tyres skid. The driver will over react and lift off the brake causing
>>>> passengers to jolt forward and back in their seats.
>>>>
>>> You don't think that the way a car can appear to come to an abrupt stop
>>> under gentle braking is actually not because of any change in the grip
>>> of the tyres or change in pad friction coefficient but simply the
>>> reaction to the sudden removal of a force?
>>
>> The jerking you feel if you keep your foot on the brake rather than
>> smoothly lifting it as you come to a halt is basically due to the
>> resulting discontinuity in your rate of deceleration - the car goes from
>> decelerating at, say, 0.3g, to decelerating at 0g, very suddenly - not
>> instantaneously but close. Your body however has momentum and is not
>> rigidly fixed to the car, so it keeps going until decelerated to a halt
>> and probably pulled backwards either by the reaction of your muscles to
>> the relative motion between body and car or by the seatbelt, depending
>> on which acts first.
>
>
>> So it's true to say that the rate of change of
>> retardation suddenly increases, but the rate of retardation of the car
>> itself suddenly goes to zero, and that's why you get chucked at the
>> dashboard.
>>
> Or rather chucked at your seat?
You probably chuck yourself at your seat, at the end, or your seatbelt
does it for you in the case of harder braking. Yes, really the dashboard
is chucking itselt at you, rather than the other way round.
--
Phil
http :// www .flickr,com /photos/tmc1979/
Re: Hard braking and the Physics of Coming-to-Rest
"peter" <scoular@blackhole.do-not-spam.me.uk> wrote in message
news:adCvZuAjSJCIFwYo@ntlworld,com ...
> DavidM <newsNO@_SPAMdjmorgan.org.uk> writes
>>The car coming to an abrupt stop under gentle braking is not due to the
>>tyres slipping, so is not really the reverse of what larkim said.
>>If the brakes are applied with a low but constant force they will
>>gradually slow the car. During which time it's momentum exceeds the
>>friction between pads and discs required to make them lock. Once the
>>momentum of the car becomes sufficiently low, and if the braking is not
>>adjusted for speed, the pads will grab on the discs and probably cause
>>an immediate stop. The momentum of the car is now too low to make the
>>tyres skid. The driver will over react and lift off the brake causing
>>passengers to jolt forward and back in their seats.
Interesting in a strange sort of way but complete gobbledygook I'm afraid.
>>
> You don't think that the way a car can appear to come to an abrupt stop
> under gentle braking is actually not because of any change in the grip of
> the tyres or change in pad friction coefficient but simply the reaction to
> the sudden removal of a force?
Indeed. Nothing at all unusual happens in the last few inches of braking to
a halt other than at some point the car actually achieves zero mph at which
point the acceleration changes abruptly from whatever negative value it had
been at previously to zero. The rate of change of distance with respect to
time is velocity, the rate of change of velocity wrtt is acceleration, the
rate of change of acceleration wrtt is called jerk and so on ad infinitum to
further derivatives wrtt which don't have universally accepted names but
have at some point been called snap, crackle and pop for the next three in
the series.
So that very last jolt you get if the brakes are not feathered as the car
comes to rest is not a more abrupt acceleration than the car had been
undergoing previously but an abrupt change in the rate of acceleration i.e.
jerk, something that rollercoaster designers and public transport system
designeers have to take into account if the passengers aren't to be reduced
to vomiting all over each other. To minimise the amount of jerk the rate of
acceleration itself has to be reduced just before velocity drops to zero so
as to smooth the transition to the zero point.
From a purely mathematical point of view I'm not absolutely sure why the
jerk just at the point at which velocity changes from zero to something non
zero (or vice versa) isn't actually infinite because another value
(acceleration) is changing in an infinitesimal amount of time. I guess in
the real world that transition is cushioned by the elasticity of matter to a
value that has to be both finite and generally not actually that large.
Presumably in a theoretical world if you applied a force (or removed a
force) to an infinitely rigid body it would encounter an instantaneous
change in acceleration and hence an infinite amount of jerk.
--
Dave Baker
Puma Race Engines
"peter" <scoular@blackhole.do-not-spam.me.uk> wrote in message
news:adCvZuAjSJCIFwYo@ntlworld,com ...
> DavidM <newsNO@_SPAMdjmorgan.org.uk> writes
>>The car coming to an abrupt stop under gentle braking is not due to the
>>tyres slipping, so is not really the reverse of what larkim said.
>>If the brakes are applied with a low but constant force they will
>>gradually slow the car. During which time it's momentum exceeds the
>>friction between pads and discs required to make them lock. Once the
>>momentum of the car becomes sufficiently low, and if the braking is not
>>adjusted for speed, the pads will grab on the discs and probably cause
>>an immediate stop. The momentum of the car is now too low to make the
>>tyres skid. The driver will over react and lift off the brake causing
>>passengers to jolt forward and back in their seats.
Interesting in a strange sort of way but complete gobbledygook I'm afraid.
>>
> You don't think that the way a car can appear to come to an abrupt stop
> under gentle braking is actually not because of any change in the grip of
> the tyres or change in pad friction coefficient but simply the reaction to
> the sudden removal of a force?
Indeed. Nothing at all unusual happens in the last few inches of braking to
a halt other than at some point the car actually achieves zero mph at which
point the acceleration changes abruptly from whatever negative value it had
been at previously to zero. The rate of change of distance with respect to
time is velocity, the rate of change of velocity wrtt is acceleration, the
rate of change of acceleration wrtt is called jerk and so on ad infinitum to
further derivatives wrtt which don't have universally accepted names but
have at some point been called snap, crackle and pop for the next three in
the series.
So that very last jolt you get if the brakes are not feathered as the car
comes to rest is not a more abrupt acceleration than the car had been
undergoing previously but an abrupt change in the rate of acceleration i.e.
jerk, something that rollercoaster designers and public transport system
designeers have to take into account if the passengers aren't to be reduced
to vomiting all over each other. To minimise the amount of jerk the rate of
acceleration itself has to be reduced just before velocity drops to zero so
as to smooth the transition to the zero point.
From a purely mathematical point of view I'm not absolutely sure why the
jerk just at the point at which velocity changes from zero to something non
zero (or vice versa) isn't actually infinite because another value
(acceleration) is changing in an infinitesimal amount of time. I guess in
the real world that transition is cushioned by the elasticity of matter to a
value that has to be both finite and generally not actually that large.
Presumably in a theoretical world if you applied a force (or removed a
force) to an infinitely rigid body it would encounter an instantaneous
change in acceleration and hence an infinite amount of jerk.
--
Dave Baker
Puma Race Engines
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