Posted by Lauren on .
Three blocks rest on a frictionless, horizontal table (see figure below), with m1 = 8 kg and m3 = 20 kg. A horizontal force F = 101 N is applied to block 1, and the acceleration of all three blocks is found to be 3.5 m/s2.
Find m2.
What is the normal force between blocks 2 and 3?

Physics 
drwls,
F = (m1 + m2 + m3) a
You already know m1 and m3, a and F.
Solve for m2.
The force between blocks 2 and 3 is what acclerates m3. That force is m3*a