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# Momentum - Separating Velocity Watch

1. For the M2 paper on collisions how do I know how to set out the equation with
E= Speed of Separation/Speed of Approach, if I dont know the directions of the particles after, because it changes from Va + Vb to Va - Vb etc and I never know what to write as when I need to use it in another equation (conservation of momentum) i never know what to do. I doubt many people will understand what I mean it's hard to explain...
2. it's going to be even harder if I don't know what M2 is haha!

Can you write out your problem in full?
3. I know exactly what you mean

The coefficient of restitution is the ratio of the separation speed to the approach speed. And speed is a scalar quantity. It doesn't have direction. So is always positive.

If two bodies are going in the same direction, one at 70 mph and the other at 30 mph then the difference in their speeds is 40 mph.
But if they are going in opposite directions, one at 70 mph and the other at 30 mph, then they are moving apart at a speeed of 100 mph. And that is the speed difference between them.

Don't bother with signs. Just think about what is happening, and see, for example, what speed they are moving apart from each other, whether they are moving in the same direction, or opposite to each other.

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Updated: December 8, 2010
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