A block of mass m is released from rest from the top of a frictionless incline a vertical distance h from the table underneath. The incline is fixed to the tabletop. Obviously the block speeds up as it moves to the bottom of the incline.
In AP Physics 1, students will be asked carefully about energy conversion. Depending on the system we consider, what are the "external" and "internal" forces? How should we articulate the energy conversion that allows the block to speed up?
1. On one hand, consider the block alone. It has zero kinetic energy to start, and some KE to end. Thus, some force must have done work on the block. (Since the we are considering the block alone, all forces acting on the block must be "external.")
The incline can't do work on the block -- the force of the incline on the block is perpendicular to the block's motion, so does no work. The only work done on the block is from the force of the earth on the block -- gravity. The amount of that work is the block's weight mg times the displacement parallel to the force's direction, i.e. the vertical displacement of the block h. That work mgh is converted to the block's kinetic energy.
2. Consider the block-earth system. Now the earth cannot do work on the block-earth system because the earth is part of the system. But the block-earth interaction produces a potential energy mgh at the beginning. No work is done by external forces -- the only force external to the system is the force of the incline on the block, and that's perpendicular to the block's motion, and so can do no work. The system loses mgh of potential energy, which is converted to kinetic energy of the block.