Answer
13.1 J
Work Step by Step
In Problem 7b, we saw that the change in the potential energy of the ball was $\Delta$U = –13.1 J. If the gravitational potential energy at the lowest point is zero, and the ball had to lose 13.1 J to get there, then the original potential energy at the release point had to be U = 13.1 J.
We can also work it out as follows:
$\Delta$U = $U_{final}$ – $U_{initial}$ = 0 – $U_{initial}$ = –13.1 J
So $U_{initial}$ = 13.1 J
