High School Physics Practice Test

When only conservative forces act, energy can shift between kinetic and potential, but the total mechanical energy stays the same. The work done by conservative forces equals the negative change in potential energy, W_conservative = -ΔU. The change in kinetic energy equals the total work done, so ΔK = W_conservative. Combining these gives ΔK + ΔU = 0, meaning K + U does not change. In practice, energy moves between kinetic and potential—like a pendulum or a mass on a spring—while their sum remains constant. If non-conservative forces were present, mechanical energy would decrease as some energy is (for example) dissipated as heat.