This is one area of basic physics that I feel I'm never sure what is wanted in the question: what is the work done by a force decelerating an object? I mean work is F*d, but in this case the distance moved by the object is in the opposite direction of the force. I mean I would think the work done by the force would then be the change in kinetic energy, which makes sense to me, but the problem is that in a similar question that came up in an exam I was marked wrong for that, here's what the question was more or less: A ball of mass m is thrown in the air(negligible air resistance) with velocity v from height h1 off the ground. It reaches height h2 then falls back down to h1. What is the work done by gravity on the ball? What I figured is that the work done would be the gain in GPE (or loss in KE) to get it to its maximum height, plus the gain in KE (or loss of GPE) for it to fall back to its original height. Thus I would have given my answer as 2mgh. However, apparently the answer was zero. I mean I guess we could say it's because work is the conversion of energy from one form to another, and at the end of the sequence of events the ball has the same KE and GPE as it had at the start. But looking at it from that perspective in my mind has a very vector-like nature to it (eg the difference between distance and displacement), and work is a scalar property. Additionally(though not entirely relevant in my original question), the accompanying diagram of the events in the question showed that the ball wasn't thrown straight up, but at an angle, thus there was even a horizontal distance moved. Personally I think the question was flawed, but you try telling my teacher that!