Acceleration with force of gravity and friction

[djokoman95]

Homework Statement

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?

Homework Equations

F=\mu_s*n
F=ma

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..

 

[Doc Al]

I know that the force pushing up on the block is equal to mg.

Good. What other forces act on the block?

I tried drawing acceleration as a horizontal vector, then setting a=mg, then trying to solve, but that didn't quite work out..

'mg' is a force, which can't be set equal to an acceleration.

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

 

[djokoman95]

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

 

[Doc Al]

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

Almost.