# Direction of friction in rotational motion

## Main Question or Discussion Point

hi everybody!I am to find the direction in which friction acts in rotational moton. In some cases we say friction supports rotational motion and in some others we say it doesn't.Plz help me to sort it out
thanks

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Doc Al
Mentor
Do you have a specific situation in mind? Not sure what you mean by "support" rotational motion. Maybe this statement will help: In order to change the rate at which something "rolls", a torque is required; friction can provide that torque.

Yes.All confusion was due to this problem.Here is the qn:"a force is applied on a cylinder of mass m and there is no slipping anywhere.The ratio of Rotational KE provided by F to rotational KE provided by friction is"
The answer calculated by me is 3 but the suggested answer is -3.the explanation given by my teacher is that F and friction act in opposite directions but I was not convinced as both support rotational motion

Doc Al
Mentor
Please give the complete statement of the problem. There's mention of "no slipping anywhere" yet they ask about friction.

Depending on the situation, friction can increase or decrease rotational speed.

Friction is needed in rolling and pulleys etc. to make sure that there is no slipping. But, friction is usually ignored in any problem where you see a statement like "A cylinder is rolling without slipping ..."

Doc Al
Mentor
But, friction is usually ignored in any problem where you see a statement like "A cylinder is rolling without slipping ..."
That depends on the particular problem (which is why I asked for details). It's certainly true that there's no loss of mechanical energy to friction when there's rolling without slipping, which I suspect is what you meant.

Say a cylinder is rolling without slippling down an incline. A perfectly reasonable question to ask is: What is the value of the static friction acting on the cylinder? In what direction does it act?

I agree, Doc. My statement was directed at harini. We need the exact problem statement before suggesting a solution.

I posed the same question to my teacher"When there is no slipping why consider friction?" He asked me to imagine a practical situation and told me that slipping is prevented by friction and that there is no loss of energy due to friction.This is why I was more confused with the MINUS SIGN in the answer.

Doc Al
Mentor
If you would like your question answered, post the complete problem exactly as it was given to you.

Actually the question is, ”A uniform cylinder of mass m is placed on a horizontal surface with its axis parallel to the plane. The surface offers necessary friction to prevent slipping. The radius of the cylinder is r. Now a force F is applied tangential to the upper surface. The cylinder starts rolling and slipping is prevented by friction. Find the ratio of the rotational K.E provided by F to that provided by friction.

Doc Al
Mentor
Actually the question is, ”A uniform cylinder of mass m is placed on a horizontal surface with its axis parallel to the plane. The surface offers necessary friction to prevent slipping. The radius of the cylinder is r. Now a force F is applied tangential to the upper surface. The cylinder starts rolling and slipping is prevented by friction. Find the ratio of the rotational K.E provided by F to that provided by friction.
Excellent--now we're getting somewhere. Start by figuring out the magnitude and direction of the friction force by using Newton's 2nd law (for translation and rotation). Which way must the friction force point? To determine the direction of the friction force, consider how the surfaces would slip if there were no friction to prevent it.

Once you've determined the friction force, compare its effect on rotational KE by comparing its torque to that of the applied force F.