Potential energy decreases, and kinetic energy is constant. An elevator going down at a constant speed. Potential energy decreases, and kinetic energy is constant. Potential energy is constant, and kinetic energy increases. A car accelerating on a flat highway.
Potential energy is constant, and kinetic energy is constant.
A car going 55 mph on a flat highway. 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. We did this one in discussion.