Three boxes are pushed across a frictionless horizontal surface as shown. When we compare the normal force N2,5 that mass 2m exerts on mass 5m with the normal force N5,10 that mass 5m exerts on mass 10m, we find that:
A)N2,5 = N5,10 = F.
B)N2,5 = F N5,10.
C)F N2,5 = N5,10.
D)F N2,5 N5,10.
E)F N5,10 N2,5.

Question

Three boxes are pushed across a frictionless horizontal surface as shown. When we compare the normal force N2,5 that mass 2m exerts on mass 5m with the normal force N5,10 that mass 5m exerts on mass 10m, we find that: 
A)N2,5 = N5,10 = F. 
B)N2,5 = F > N5,10. 
C)F > N2,5 = N5,10. 
D)F > N2,5 > N5,10. 
E)F > N5,10 > N2,5.

Quizplus · Accepted Answer

According to Newton's Third Law, the forces between the boxes must be equal and opposite. This means that the force F pushing on mass 2m is also pushing on mass 5m. Therefore, N2,5 must be equal to F. 
However, when mass 5m pushes on mass 10m, the force is spread out over a larger area, resulting in a smaller normal force. Therefore, N5,10 must be less than N2,5, but both are less than F. This leads us to choice D, where F > N2,5 > N5,10.

Three Boxes Are Pushed Across a Frictionless Horizontal Surface as Shown