Acceleration with force of gravity and friction

1. Jul 17, 2011

djokoman95

1. The problem statement, all variables and given/known data

A block is placed against the vertical front of a cart. The coefficient of static frction between the block and the cart is $\mu$s. What acceleration must the cart have in order that the block does not fall? How would an observer on the cart describe the behavior of the block?

2. Relevant equations
F=$\mu$s*n
F=ma

3. The attempt at a solution
I have no idea how to start this problem. I know that the force pushing up on the block is equal to mg. I tried drawing acceleration as a horizontal vector, then setting a=mg, then trying to solve, but that didn't quite work out..

2. Jul 17, 2011

Staff: Mentor

Good. What other forces act on the block?
'mg' is a force, which can't be set equal to an acceleration.

Identify all the forces and apply Newton's 2nd law.

3. Jul 17, 2011

djokoman95

so after trying a few things, i got$\mu$s /g for my answer. I think that should be right.

4. Jul 17, 2011

Almost.