Three boxes rest side-by-side on a smooth, horizontal floor. Their masses are 5.0 kg, 3.0 kg, and 2.0 kg, with the 3.0-kg mass in the center. A force of 50 N pushes on the 5.0-kg mass, which pushes against the other two masses. What is the contact force between the 5.0-kg mass and the 3.0-kg mass?
A) 0 N
B) 10 N
C) 25 N
D) 40 N
E) 50 N

Question

Quizplus · Accepted Answer

Since the 5.0 kg mass is pushing against both the 3.0 kg mass and the 2.0 kg mass, we need to consider the forces on both masses. The force of 50 N is applied to the 5.0 kg mass, causing it to accelerate to the right. This creates a contact force between the 5.0 kg mass and the 3.0 kg mass to the left. 
By Newton's third law, the contact force on the 3.0 kg mass is equal in magnitude but opposite in direction to the contact force on the 5.0 kg mass. Therefore, the contact force on the 3.0 kg mass is also 25 N to the right. 
There is no net external force acting on the 3.0 kg mass in the horizontal direction, so it remains at rest.

Three Boxes Rest Side-By-Side on a Smooth, Horizontal Floor