How to determine the direction of friction acting on a purely rolling body like a cylinder when it is being accelerated? Is it in the directino of acceleration or not? I found out -

- Direction of friction aciting on a cylinder rolling down an incline is upward.
- Direction of friction acting on a cylinder roling up an incline is upward.
- Direction of firction acting on a cylinder being accelerated on a flat surface is in the direction of acceleration.

What is criteria that determines the direction of friction? Is it true friction acts so as to oppose relative motion? Further, I am not sure about statement 2. It might be wrong.

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## Comments

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TopNewestRolling Motion. Two cases. Slipping and non-slipping. In the slipping case, kinetic friction acts on the body. Consider the point in the body that is in contact with the surface. The direction of friction is opposite the motion of this point relative to the surface. In the non-slipping case, static friction acts on the body. To determine the direction of friction, follow the suggestion of Saurabh D.

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Just assume some arbitary direction (Either up the incline or down the incline). Now you solve the problem with your assumption. If the friction force comes out to be negative, Then the direction is opposite to the assumed direction else it is same as assumed.

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agreed..:)

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My teacher said the same thing but what if you are not given any numerical problem but just to find the direction of friction.

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it works opposite to the motion of an object,plain and simple

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The following link helped me in the past and I had it bookmarked as I refer to it frequently, hope this helps you: http://cnx.org/content/m14385/latest/

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Thanks, it explains everything clearly and this is exactly what I was looking for.

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Direction of friction is always opposite to the direction of motion. That's all. If body is moving forward friction is backwards. etc

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And even so in case of a rolling body. The friction is still opposing the motion, so is acting in the opposite direction. There's nothing to get confused about.

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friction acts against motion of the object, 1 statement: friction works against gravity,because gravity is causing the motion, 2 statement: friction works with gravity,but against the force applied to move it upward.because force applied is causing the motion. take note,weather the object is slipping or not

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