Because energy is conserved, we can calculate kinetic or potential energy at all times if we know one or the other at different
times. For example, if a car on a roller coaster has 1000J of potential energy when its parked at the top of the ride, and later it only has 100J of potential energy, we know that it must have 1000-100=900 joules of Potential energy at that later time. If the car has 250J of potential energy at some point in the ride, how much kinetic energy does it have?
