We derived two equations in class.
- The work done between two points $A$, $B$ is equal to the difference between the kinetic energy at the last point and the one at the first point.
- The work done between two points $A$, $B$ is equal to the difference between the potential energy at the first point and the one at the last point.
Now the thing is the following: if I have a car driving with constant velocity on the street and I use some work to accelerate it, then it is driving with a higher speed, so the kinetic energy will have been changed the way we said. But the potential energy is left unchanged, so I was wondering: when are these two equations true and when are they not applicable?