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We use Professor Leigh's favorite model: the car rolling on an incline.
Let us suppose that Michael is sitting in this car, which has started to roll
backwards, downhill. He wants to proceed slowly astern, so he applies a little
throttle while he is in a forward gear. This prevents the car from
careering away.
The work done by the car, we say, is negative. There is no discussion,
no debate.
This is the convention.