In a cases where frictional forces can be neglected (nearly every student laboratory on this subject), and the object is dropped from rest onto a horizontal surface, the above is equivalent to a comparison between the E p of the object at the drop height with that at the bounce height. In this case, the change in E k is zero (the object is essentially at rest during the course of the impact and is also at rest at the apex of the bounce); thus: e = E p, (at bounce height) E p, (at drop height) = m g h m g H = h H