[Physics] Does friction oppose rotational or translational motion

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My textbook (University Physics by Young and Freedman, 13E) showed that if a ball rolled down an inclined plane, friction would point uphill. Below are the relevant diagrams:

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It then says that "If the ball were rolling uphill without slipping, the force of friction would still be directed uphill.” I’m presuming that the ball spins with clockwise rotation looking from the side. By this token, friction will point upward along the incline because friction resists motion and will exert a torque about the center of mass that counters the clockwise rotation.

However, the ball is also translating uphill, and shouldn’t friction want to counter that translation by pointing downhill? How do I know which type of motion friction wants to oppose?

And also, I had assumed that the ball was rolling with clockwise rotation. What if it’s counterclockwise? In that case, friction would point downhill, would it not? Yet my book seems to imply that friction will point up no matter which direction the ball rotates in.

Best Answer

The book is assuming that the ball is rolling without sliding, so the direction of rotation is fixed by that constraint. Also, if there is no sliding, the problem is completely time reversible. When you time reverse the forces, they point in the same direction as before, essentially because there is a t^2 in the accelerations, so the signs are unchanged. What static friction is doing here is simply transforming some of the translational kinetic energy into rotational kinetic energy when the ball is accelerating down the hill, and the opposite when the ball is decelerating up the hill. In that sense, static friction is always impeding what gravity is trying to do to the translational kinetic energy, but when the ball rolls uphill, static friction yields more translational kinetic energy than you would have had at that same height if you turned off the static friction. Surprisingly, this means that when a ball rolls toward an upward ramp, at will go higher up that ramp if the ramp's surface is rough than if the ramp's surface is perfectly smooth.

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