2 Block Push


Start

Description

Two blocks are pushed by identical forces, each starting at rest at the first vertical rectangle (start). The mass m1=2m2.

Question

Which object has the greater kinetic energy when it reaches the second vertical rectangle (finish)? 


Instructor Resources

Reference: See Giancoli-PA: 6-3, Giancoli-SE: 7-4.
Answer: The kinetic energies are identical. This is a good problem to ask in the middle of discussing the work-energy theorem. Typical student reaction is that the second mass has a greater velocity and therefore has the greater kinetic energy. While the second mass does have the greater velocity at the finish, it is not double the velocity of m1. Students can see this from Newton’s second law and kinematics, although it is a little tedious. The correct rationale here is to recognize that the work done by the force on each mass is identical as the displacements are identical.
Script Author: Mario Belloni