# Does torque only cause rotation and does centripetal force only cause revolution?

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1. Nov 2, 2014

### PPERERA

Torque causes a system to rotate about its axis while centripetal force causes a system to rotate about the axis of the separate system that exerted the centripetal force. So does this mean that torque specifically causes rotation and centripetal force causes revolution?

2. Nov 2, 2014

### Staff: Mentor

I don't think that statement makes sense. For a circular motion, there has to be a centripetal force, but that can come from anywhere.

What does "cause rotation" or "cause revolution" mean?

3. Nov 2, 2014

### PPERERA

If it is true that centripetal force must be present for an object to undergo circular motion, then what I asked is not possible.

What I was trying to do was to see if there was a distinction between revolution (an object is in circular motion around another object that is not in circular motion) and rotation (an object in circular motion about an axis within itself) that could be explained by rotational dynamics.

The way that I was thinking about this was by imagining a uniform-mass rod in space (no forces are acting on it initially). A force is applied to the end of the rod. In this situation, I would expect the rod to being to rotate about the axis going through its center of mass as a result of the applied torque only. This would indicate that torque does cause rotation by itself.

So would the force cause the rod to rotate or would it only produce translational motion? And if the rod begins to rotate, is there a centripetal force involved?

Now clear your mind and imagine a ball attached to a string at a fixed position. A person swings this ball around, and the tension in the string acting as the centripetal force causes the ball to undergo circular motion about the stationary person (revolving around the person). Because there is no torque applied on the ball, the ball does not rotate about its center of mass. So the centripetal force specifically causes revolution but not rotation.

Thanks for the response.

4. Nov 3, 2014

### Staff: Mentor

The difference is just the axis of rotation.

It would rotate around its center of mass, but it would also produce translational motion.
Internally in the rod, sure, as the outer parts do not move in a straight line.
Oh, it would, at the same angular frequency as the rotation around the other end of the string, otherwise the string starts to roll up and that provides a torque.

5. Nov 3, 2014

### rcgldr

The change in linear momentum of the rod equals the impulse (force x time) applied to the rod. The applied torque equals force x radius (distance from point of application to center of mass of the rod), and the change in angular momentum = torque x time.

In order to get the ball moving or to change speed, a force in the direction of the velocity of the ball is required. The person does this by initially pulling on his / her end of the string, then twirling his / her end of the string in a circular path that is "ahead" of the path of the ball.

6. Nov 3, 2014

### A.T.

If think the difference the OP means is between rotation (changing orientation) and circular motion (translation along
a circular path without changing orientation)

7. Nov 3, 2014

### A.T.

You can view any rotating rigid body as a collection point masses in circular translatory motion.

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