2 Block Push

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