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# Mechanics 2 - coefficient of restitution

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1. Hi,

With the newtons experimental law equations (which I presume I've got right), are you taking the speed s(so they're always positive regardless of direction, or the velocities? (So vector nature would come into account, and they could be negative).

And what values would I put into the formula for the example given?
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2. Neither.

Say if A and B were travelling towards eachother at speeds a and b respectively. Their approach speed would be a + b, because that is the speed that they are ultimately approaching one another at.
If A is heading towards B but B away from A, such that a > b, the approach speed would be a - b.

For separation it's the same but backwards. Let the speed of A after the collision be c and B to be d.
If they are headed away from eachother, speed of separation will be c + d.
If A is heading towards B and B away from A, such that d > c, the speed of separation will be d - c.

I hope I've explained this well enough. The velocities aren't relative to the direction of particles independently but more or less as a system, relative to one another.
3. (Original post by JLegion)
Neither.

Say if A and B were travelling towards eachother at speeds a and b respectively. Their approach speed would be a + b, because that is the speed that they are ultimately approaching one another at.
If A is heading towards B but B away from A, such that a > b, the approach speed would be a - b.

For separation it's the same but backwards. Let the speed of A after the collision be c and B to be d.
If they are headed away from eachother, speed of separation will be c + d.
If A is heading towards B and B away from A, such that d > c, the speed of separation will be d - c.

I hope I've explained this well enough. The velocities aren't relative to the direction of particles independently but more or less as a system, relative to one another.
Ah that's good thanks, though it's more complicated than I thought haha. so it's a bit like their relative speeds?

So it's more thinking through each situation independently, and working out whether they're added or subtracted?

In terms of the speed of separation (after collision) what would you do if it wasn't known if they are moving in opposite directions or not?
4. (Original post by JohnnyDavidson)
Ah that's good thanks, though it's more complicated than I thought haha. so it's a bit like their relative speeds?

So it's more thinking through each situation independently, and working out whether they're added or subtracted?

In terms of the speed of separation (after collision) what would you do if it wasn't known if they are moving in opposite directions or not?
Take a guess.

You'll also need to use momentum, and using your guessed directions you'll formulate a new equation which you can substitute to find the speed. So long as your directions in the restitution equation and directions in the momentum equation are the same, you will have no problems.

The best way to prove this to you is for you to do a question two different ways, and notice you'll still get the same answer.

And yes, you are considering each particle relative to the other.

Hope this helps.

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