Suppose you jump straight up as high as you can. Estimate how much initial velocity you had. Potential energy increases, and kinetic energy decreases. Potential energy decreases, and kinetic energy is constant. Potential energy decreases, and kinetic energy is constant.
Potential energy is constant, and kinetic energy increases.
Potential energy is constant, and kinetic energy is constant. An oxygen molecule of mass M is sitting at rest minding its own business. Suddenly, a cosmic ray photon passes by and adds an energy D to the molecule. The molecule sits and wiggles for a while, and the breaks up into two equal mass oxygen atoms traveling in opposite directions. Suppose you throw your physics book out of your window after a long night of studying.