# Centripetal force and rotation

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## Main Question or Discussion Point

I noticed that if you swing a ball around attached to a string with a centripetal force. The ball also rotates. What causes this rotation? Is there always rotation when there's a centripetal force? does an object orbiting the earth always face the same way as it orbits?

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jbriggs444
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The string normally attaches so that it is effectively tied to a point on the ball's surface. Naturally, the attachment point is pulled by the tension in the string. The side of the ball where the string comes out will be pulled to face the center of the circle. If the ball did not rotate to begin with, it would start to rotate because of this torque. So the ball rotates.

If the string were not tied to the surface of the ball but were connected to an axle through the ball's center which had some sort of magical frictionless bearings then no such torque would exist and no such rotation would need to start. This is essentially the situation for satellites in orbit. The effect of Earth's gravity is effectively applied on an orbitting object's center and is almost completely frictionless, so orbitting objects experience negligible net torque from gravity and are free to spin independently of their orbits.

As it turns out, gravity is not entirely frictionless if you consider tidal effects. So satellites such as the moon can become "tidally locked" to the earth and rotate once per revolution, just like a ball on a string.

Dale
Mentor
For a ball on the end of a string which is not rotating once per "orbit" the string exerts a torque about the ball's center. This torque causes the rotation you mention and disappears once the ball is rotating at the correct speed.

CWatters
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What others have said plus...

What do you mean by "rotate"? Rotate in which plane?

As you swing the ball around it will try to twist up the rope. If you hold the string in your hand you stop that end rotating so the ball must rotate once per revolution to stop the rope twisting up.

Edit: Ah I see Dale is referring to the same thing.