# Homework Help: Where is the friction for the following situations?

1. May 5, 2013

### jeevankasin

Hi All,

It would be great if you guys can help me out with this attached question.

My colleague says that when the trolley is being pushed and when it is not being pushed, the friction should be backwards against the direction of motion. Ans : Option (D)

Another of my colleague says that when the trolley is being pushed, the frictional force acting on the wheels should be forward in the direction of motion and when it is released, the frictional force on the wheels should be backwards against the direction of motion as the trolley slows down. Ans : Option (B)

What are your views on this?

Jeevan

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2. May 5, 2013

### Andrew Mason

You can easily figure this out if you do a free body diagram. A free body diagram shows all the forces (vectors) acting on the body, so the sum of the vectors is equal to the mass x acceleration of the body.

In the first case (being pushed), assume there is no acceleration (constant velocity). So the forces sum to zero. There are two forces, the pushing and the friction.

In the second case, there is acceleration. There is no pushing, just friction.

That should enable you to work out the directions of each force.

AM