axis of rotation (very simple question) [Physics]

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From: Tom on 12 Jun 2010 12:25

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?
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?

t.

From: Androcles on 12 Jun 2010 12:56

"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 14:04

"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.

The wheel continues to rotate about the axle, but the axle starts to move
forward as soon as the tyre hits the ground. The reason the axle starts
moving is because the frictional force that 'prevents' the tyre from
slipping on the road surface is transmitted up the sidewall of the tyre to
the hub, where it causes the axle to accelerate forward.

If there is absolutely no slipping, there will be a torsion in the sidewall
of the tyre such that radii are twisted, which transmits the force to the
axle.

> 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?

Newton's laws, Forces and acceleration

From: Matthew Lybanon on 12 Jun 2010 14:47

In article <MxOQn.76222$HG1.20002(a)newsfe21.iad>, "Tom" <junk(a)junk.com>
wrote:

> This is a simple (ignorance-based) question.... please forgive me.
>
> If I hold a car above a road, the wheels rotate about the axle.

Car does not move forward.

>
> 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?

Wheels on road, no slipping. Car must be moving forward now.

> 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?

Do you see a difference in the two situations?

>
> t.

From: Cwatters on 12 Jun 2010 15:13

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