In an ideal system with no friction, what happens to the total mechanical energy as it converts between kinetic and potential energy?

• A. Total energy increases
• B. Total energy stays constant
• C. Total energy decreases
• D. Total energy fluctuates

Answer: (B)

Energy can switch between kinetic energy and gravitational potential energy, but in an ideal frictionless system the total mechanical energy stays constant. As an object moves, energy freely trades between how fast it is moving and how high it is, yet the sum of kinetic and potential energies remains the same because no energy is lost to heat, sound, or deformation. When kinetic energy increases, potential energy decreases by the same amount, and when potential energy increases, kinetic energy decreases by the same amount. Because only conservative forces are doing work, the total mechanical energy does not grow, shrink, or fluctuate.