Quite easy mechanics problem

1. Jun 9, 2005

C0nfused

Hi everybody,
I need your help in one easy-looking problem that has confused me a bit.

Suppose we have a small cyclic object of mass m, just like in the picture, on a smooth surface. A steady force F is pulling it. The force is always on point A, like in the picture. Describe the object's movement. (for t=0, v=0)

Thanks

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2. Jun 9, 2005

arildno

1. Since the surface is smooth, only normal forces act upon the cylinder from the ground.

2. Use Newton's 2.law of motion to determin the acceleration of the center of mass
3. Use the moment-of momentum equation with respect to the center of mass to determine the angular acceleration.

3. Jun 9, 2005

C0nfused

I have found that if a is the acceleration of the center of mass then a=F/m. My problem is the angular acceleration. I am not sure if there's any at all. Of course there is N=RxF so a(ang)=(RxF)/I(cm) ? (supposing we know I(cm)) Is it that simple? The angular accelaration seems a bit weird for me for some reason because there is no resistance from the surface and because the force is always on A and not on a specific point of the mass.

4. Jun 9, 2005

arildno

It is no different than if a ball rolls and accelerates down an incline, then a frictional force acts upon whatever material point of the ball actually touching the incline at that particular moment.
This frictional force produces a torque about the center of mass.

5. Jun 9, 2005

Nylex

Eh? Doesn't smooth imply no friction? It's the force, F that produces a torque, surely?

6. Jun 9, 2005

arildno

Yes, what I referred to was his puzzlement to why a force acting on successively different points of the ball still produces a torque.

The example with the friction was meant to convey that the actual contact point is not the same material point on the ball over time in the case of rolling.

I did not mean that there is any friction in the original case; F is the force producing the torque.

7. Jun 9, 2005

C0nfused

Yes, smooth impies no friction. Sorry guys for the bad use of language but it's the first time i am writing about Physics in English so I don't know many words.