action and reaction force

1. Since the external force is applied to m1, m1 would push m2 to move, there is a reaction focre acting on m1 by m2, in a direction opposite to the motion of the blocks.

For m2, there is a pushing force acting on it be m1, and because it also pushes m3, there is a reaction force acting on it (m2), which is given by m3. The reaction force is in the opposite direction to its motion.

For m3, it is acted by the pushing force given by m2, in the direction along with motion direction.

Given this information, you could be able to draw the free body diagrams for the three blocks.

2. .Let F1 be the force between m1 and m2, and F2 be the force between m2 and m3.

By the formula: [net force] = mass x acceleration
For m1, 18 - F1 = 2a ------------------ (1)
where a is the acceleration of the blocks
For m2: F1 - F2 = 3a ------------------ (2)
For m3: F2 = 4a ----------------- (3)

Substitute (3) into (2)
F1 - 4a = 3a
F1 = 7a
Then (1) becomes 18 - 7a = 2a
this gives a = 18/9 m/s2 = 2 m/s2

Therefore, F1 = 7x2 N = 14 N
and F2 = 4 x2 N = 8 N