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From: Tom on 12 Jun 2010 15:49
Is it fair to say that just by letting the wheels hit the ground that it is
now a physically
different problem?

"Androcles" <Headmaster(a)Hogwarts.physics_z> wrote in message
news:Y_OQn.27082$g76.18234(a)hurricane...
>
> "Tom" <junk(a)junk.com> wrote in message
> news:MxOQn.76222$HG1.20002(a)newsfe21.iad...
> | This is a simple (ignorance-based) question.... please forgive me.
> |
> | If I hold a car above a road, the wheels rotate about the axle.
> |
> | But then I drop the car onto the road and (assuming no slipping), the
> axis
> | of rotation
> | is no longer the axle. It has become an instantaneous axis of rotation
> | about the contact
> | point.
> |
> | Could someone explain what is happening here?
> =======================================
> Of course I can. You assumed no slipping and you assumed
> instantaneous. That's what happening here.
> ======================================
>
> | How does simply letting the wheel touch the ground, "transform" this
> problem
> | and
> | convert a stationary axis of rotation into one that is no longer
> stationary?
> |
> It's your assumption, you deal with it. (very simple answer)
>

From: Tom on 12 Jun 2010 15:54
nevermind...

clear as day... fog is gone.


"Tom" <junk(a)junk.com> wrote in message
news:zxRQn.4025$1Q5.1203(a)newsfe08.iad...
> Is it fair to say that just by letting the wheels hit the ground that it
> is now a physically
> different problem?
>
> "Androcles" <Headmaster(a)Hogwarts.physics_z> wrote in message
> news:Y_OQn.27082$g76.18234(a)hurricane...
>>
>> "Tom" <junk(a)junk.com> wrote in message
>> news:MxOQn.76222$HG1.20002(a)newsfe21.iad...
>> | This is a simple (ignorance-based) question.... please forgive me.
>> |
>> | If I hold a car above a road, the wheels rotate about the axle.
>> |
>> | But then I drop the car onto the road and (assuming no slipping), the
>> axis
>> | of rotation
>> | is no longer the axle. It has become an instantaneous axis of rotation
>> | about the contact
>> | point.
>> |
>> | Could someone explain what is happening here?
>> =======================================
>> Of course I can. You assumed no slipping and you assumed
>> instantaneous. That's what happening here.
>> ======================================
>>
>> | How does simply letting the wheel touch the ground, "transform" this
>> problem
>> | and
>> | convert a stationary axis of rotation into one that is no longer
>> stationary?
>> |
>> It's your assumption, you deal with it. (very simple answer)
>>
>

From: Androcles on 12 Jun 2010 16:41

"Tom" <junk(a)junk.com> wrote in message
news:MBRQn.4026$1Q5.1417(a)newsfe08.iad...
| nevermind...
|
| clear as day... fog is gone.
|
Good. Does a golf ball change its shape and the golf club's shaft bend
when they meet at the tee? You'll need a high speed camera to see it,
but they do.
Watch: http://www.youtube.com/watch?v=iUzr-4W3imw
There is no room for assumption in any science. When you drop
the car the wheel will slip because it takes finite time to accelerate;
even if that time is very short it is still non-zero and hence not
instantaneous.



|
| "Tom" <junk(a)junk.com> wrote in message
| news:zxRQn.4025$1Q5.1203(a)newsfe08.iad...
| > Is it fair to say that just by letting the wheels hit the ground that it
| > is now a physically
| > different problem?
| >
| > "Androcles" <Headmaster(a)Hogwarts.physics_z> wrote in message
| > news:Y_OQn.27082$g76.18234(a)hurricane...
| >>
| >> "Tom" <junk(a)junk.com> wrote in message
| >> news:MxOQn.76222$HG1.20002(a)newsfe21.iad...
| >> | This is a simple (ignorance-based) question.... please forgive me.
| >> |
| >> | If I hold a car above a road, the wheels rotate about the axle.
| >> |
| >> | But then I drop the car onto the road and (assuming no slipping), the
| >> axis
| >> | of rotation
| >> | is no longer the axle. It has become an instantaneous axis of
rotation
| >> | about the contact
| >> | point.
| >> |
| >> | Could someone explain what is happening here?
| >> =======================================
| >> Of course I can. You assumed no slipping and you assumed
| >> instantaneous. That's what happening here.
| >> ======================================
| >>
| >> | How does simply letting the wheel touch the ground, "transform" this
| >> problem
| >> | and
| >> | convert a stationary axis of rotation into one that is no longer
| >> stationary?
| >> |
| >> It's your assumption, you deal with it. (very simple answer)
| >>
| >
|
|

From: OG on 12 Jun 2010 17:44

"Androcles" <Headmaster(a)Hogwarts.physics_z> wrote in message
news:MhSQn.15844$EK1.15271(a)newsfe15.ams2...
>
> "Tom" <junk(a)junk.com> wrote in message
> news:MBRQn.4026$1Q5.1417(a)newsfe08.iad...
> | nevermind...
> |
> | clear as day... fog is gone.
> |
> Good. Does a golf ball change its shape and the golf club's shaft bend
> when they meet at the tee? You'll need a high speed camera to see it,
> but they do.
> Watch: http://www.youtube.com/watch?v=iUzr-4W3imw
> There is no room for assumption in any science. When you drop
> the car the wheel will slip because it takes finite time to accelerate;
> even if that time is very short it is still non-zero and hence not
> instantaneous.

It may slip, but it is not *necessary* for it to slip against the road
surface.

The initial contact causes acceleration of the tread surface of the tyre,
and torsion in the sidewall transfers the force from the tread to the hub.

The torsion increases until the elastic properties of the tyre prevents any
more twist in the sidewall, at which point the tread stops rotating relative
to the axle. However, even before that point is reached, the sidewall will
be giving a forward impulse to the axle, which accelerates the car.

If the force can be transferred sufficiently quickly to the hub that the
tyre's rotation slows down somewhat while the car itself accelerates, then
it is not absolutely necessary for the tyre to lose grip and 'skid'.

Granted, if the force is insufficient to slow the rotation of the tyre and
accelerate the axle enough, then the tyre /will/ skid on the surface, but
it's not absolutely inevitable.

Hence, contrary to what you have claimed, the 'instantaneous' reaction in
the situation described is the twisting of the tyre sidewall, adding torsion
to the system and providing torque to the axle; any skidding would have come
later - or not at all.

From: rabid_fan on 12 Jun 2010 20:40
On Sat, 12 Jun 2010 09:25:25 -0700, Tom wrote:

>
> Could someone explain what is happening here? How does simply letting
> the wheel touch the ground, "transform" this problem and
> convert a stationary axis of rotation into one that is no longer
> stationary?
>

The only thing that is happening is that you are choosing
to shift your focus to the wheel-road interface by defining
an axis at that point. But why?

Angular momentum can be recognized anywhere:

L = r x p

Here, L is angular momentum, r is the position vector relative
to the point of interest, p is the momentum vector, and "x"
is the cross product operator.

Using this equation, we can easily define an "axis" at the
wheel-road interface, but we can also define an axis anywhere
else. If we wanted, we could define your head, as you stood
there to watch the car race past, as an axis.

But we usually select our point of interest (i.e axis) to be
meaningful vis-a-vis some problem or utility that we confront.
In this case, the road-wheel interface serves us no real purpose
as an axis.

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