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# Friction on a rolling body

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 -

1. Direction of friction aciting on a cylinder rolling down an incline is upward.
2. Direction of friction acting on a cylinder roling up an incline is upward.
3. 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.

Note by Lokesh Sharma
3 years, 5 months ago

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Rolling 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. · 3 years, 5 months ago

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. · 3 years, 5 months ago

agreed..:) · 3 years, 5 months ago

My teacher said the same thing but what if you are not given any numerical problem but just to find the direction of friction. · 3 years, 5 months ago

it works opposite to the motion of an object,plain and simple · 3 years, 5 months ago

The thing you are saying can explain the direction of friction when body is being accelerated by some force on a plane without any incline. In this case the friction acts in the direction of acceleration so as to oppose the relative motion at the point of contact. But what about direction of friction when a body moves down the incline? The confusion is because it looks weird that in both cases the body is being accelerated though the direction of fiction is determined by a different rule. In another way, how does nature distinguishes if the body is being accelerated by gravity or some other force. This is hard to perceive that accleration due to gravity is different than acceleration due to some other force! · 3 years, 5 months ago

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/ · 3 years, 5 months ago

Thanks, it explains everything clearly and this is exactly what I was looking for. · 3 years, 5 months ago

Direction of friction is always opposite to the direction of motion. That's all. If body is moving forward friction is backwards. etc · 3 years, 5 months ago

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. · 3 years, 5 months ago