# Contact force problem?

1. Oct 23, 2008

### papi

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

As shown in Figure 5-19, a force of magnitude 7.50 N pushes three boxes with masses m1 = 1.30 kg, m2 = 3.10 kg, and m3 = 5.80 kg.

Figure 5-19
the figure depicts 3 boxes, in order m1, m2, m3 with a force pushing directly on m1 to the right on a flat level surface.

(a) Find the contact force between boxes 1 and 2.
N
(b) Find the contact force between between boxes 2 and 3.
N

2. Relevant equations

I know you solve for acceleration by taking the fnet of 7.5 N and dividing it by the total mass to get .735 N.

i then was able to get part b by multiplying a, or .735, by the m3, or 5.8 which was the correct answer. however, when i did the same thing for part one, by multiplying a by m2 to get the contact foirce, i was incorrect. please help and explain

2. Oct 24, 2008

### LowlyPion

The contact force between the blocks will come from the blocks that the force is not yet acting through, that is the blocks that are being pushed.

That means your acceleration applied to the masses it is still pushing ahead of the contact point is the source of your contact force.

In part b) the only mass it is pushing is m3. But in part a) you are pushing both m2 and m3.