Confused about P.E.

Kevin Willis

Yeah, the example with dropping the ball sure is. Although it posseses PE until the ball has hit the ground. If the ball lost all it PE at halfway through the fall it would stop in mid air. There would be no potential for it to fall further.

sophiecentaur

You still seem to be under the misapprehension that KE is relevant here because you are not using the terms in an accepted way. Gravitational Potential Energy is defined as Weight times Height above a reference level. This is not an appropriate use of the word "potential". If the ball is arrested in mid air, it will still have the PE, associated with its new height relative to the ground. You have to stick to the same reference height throughout your description of the situation. The only way of changing PE is to alter the vertical position.

The speed at which this ball is moving down is irrelevant to the energy situation. What counts is the work done on the way down, which is the Force times the Vertical Distance. Of course, in order to get to the bottom, it must be travelling (and still possess a finite amount of KE) but its vertical velocity when it finally makes contact with the ground can be arbitrarily low. The weight inside a grandfather clock will have transferred virtually all of its original PE in driving the mechanism, once it hits the bottom of its (very slow) movement - compare this with the same weight, dropped through the air from the same height, where its KE will be almost exactly equal to the PE value at the top and virtually no work will have been done on the way down.