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Assume that we have two disks in space.

If we act same force F on each disk like above figure, how they react?

I first thought that since $F_{ext}=\frac{dP_{cm}}{dt}$, the CM(center of mass) momentum will become same.

But what about angular momentum thus energy?

In first case, there will be no torque, so no angular momentum. But for second case, there exists external torque about CM, so the ball will rotate. Then, it will have spin motion, so there will be additional kinetic energy.

Is this correct? How same force can give different total energy change?

user42298
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  • You are correct. And if the radius is $r$ there is a point the body is rotating about located a distance $\frac{I}{m r}$ distance from the center. This is the instant center of rotation. Now if you apply the force at the instant center of rotation, the disk will appear to "roll" about it's edge. – John Alexiou Aug 08 '14 at 16:45

1 Answers1

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For definiteness, I'll consider the following two systems:

System 1: a rigid disk, with a constant force $F$ to the right applied at the centre.

System 2: a similar rigid disk, with the same constant force $F$ to the right, applied always at the lower rim of the disk (a point which will change as it rotates). Perhaps you could achieve this by tightly wrapping a light string round the disk and pulling it horizontally.

Most of your deductions are spot on: the centre of mass motion will be identical in each case. System 2 will start to rotate, whereas system 1 will not; it will therefore have more energy than system 1. Your confusion stems from the following:

How same force can give different total energy change?

The energy change (AKA work done) depends on both the force, and the distance over which it is applied. System two will be spinning, so the force is applied over a greater distance, hence more work is done, and the resulting energy is greater, consistent with your deductions. Using the mechanism of the wrapped string, you have to pull in much more string than the distance travelled by the disk as it unwinds.

Holographer
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