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Re: [Phys-l] definitions ... purely operational, or not




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I don't like the forward rocket engine, reverse rocket engine analogy
recently given. If the rocket is moving forward and the reverse engine is
fired, the rocket initially slows down, but if the reverse engine continues
to fire, the rocket will slow to zero, and will then speed up in the reverse
direction. We have the similar situation if the rocket is originally
traveling "backwards" and the forward engine is fired.

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This exactly what I DO like about the analogy! If you can get your students to the point where they agree that a negative acceleration will slow a forward moving sled down, instantaneously stop it, and then continue to DECREASE the velocity as the SPEED increases in the negative direction, then all they have to do is turn their heads sideways and they are looking at precisely what happens every time you throw a ball straight up. I'm not saying it is the miracle cure, but it seems to help with the "what is the acceleration when the ball is at its peak" issue.