# Homework Help: Acceleration of upper block

1. Sep 20, 2010

1. The problem statement, all variables and given/known data
A block M1 of mass 15.0 kg sits on top of a larger block M2 of mass 23.7 kg which sits on a flat surface. The kinetic friction coefficient between the upper and lower block is 0.549. The kinetic friction coefficient between the lower block and the flat surface is 0.122. A horizontal force $F$ = 156 N pushes against the upper block, causing it to slide. The friction force between the blocks then causes the lower block to slide also. Find the magnitude of the acceleration of the upper block.

2. Relevant equations

F=ma

3. The attempt at a solution

I don't even know... very confusing

2. Sep 20, 2010

### cepheid

Staff Emeritus
Have you drawn a diagram of the forces acting on each block? Can you figure out the force of friction that arises between the blocks because the upper block is sliding relative to the lower one? As the problem explains, it is this friction force that tries to move the lower block.

3. Sep 22, 2010

Okay so I have an attempt. I drew a free body diagram of M1. it has a gravitational force down of (15.0)(9.8)=147N so therefore has the same force being exerted back up. Then I have the 156N given going right. Then I used .549(147)=80.703N to be the frictional force going left.

So I did Fnet=ma

156N-80.703N=(15.0kg)a

and a came out to be 5.0198m/s^2

I dont know if that is right because I didnt even take the lower block into consideration, not really sure if it would affect it.