Finding Magnitude of Contact Forces in Boxes 1, 2 and 3

[matt@USA]

Homework Statement

A force of magnitude 7.50N pushes three boxes with masses m1=1.3kg, m2=3.2kg, and m3= 4.9kg as shown in the figure. I want to find the magnitude of the contact forces between boxes 1 and 2, and between boxes 2, and 3. Now, the force, 7.5N is being pushed from right to left on box m3. We are assuming that friction is irrelevant in this problem.

Homework Equations

I know to find the acceleration, I add the three masses together, and divide the total into the force. To find the magnitude of the contact force, f12, I would simply say f2-f1. For contact force f23, I would do the same, except I would say f3-f2. I solved the problem, but when I submitted the answers, they were incorrect. CAN SOMEONE PLEASE DOUBLECHECK TO SEE IF I DID SOMETHING WRONG?

The Attempt at a Solution

I came up with f1=-1.04N, f2=2.56N, and f3=3.92N.
f2-f1=2.56N+1.04N?

 

[cepheid]

If you found F1, F2, and F3 just by using F = ma, then be warned that these are the NET forces on each mass. However, each of these net forces is the sum of all the individual forces acting on that mass. The contact force with an adjacent box is one of these individual forces. For example, for a box in between two other boxes, there will be contact force acting on it from the left, and another acting on it from the right. These two sum together to make the net force. So, in order to determine all the forces acting on each mass, start by drawing a free body diagram for each mass and solve for all the forces on each mass.