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# M2 Collisions Help please. watch

1. Hi, when working out Newtons law of restitution you use the relative speeds of separation and approach. However, for this question, on the last line of working in the image, how come the speed of separation is v2 - v1 and not v2 + v1 as they are moving away from each other?Do you always take the two speeds away when you have unknown values for the velocities?
Thanks for the help!
2. It shouldn't matter. If you use - , then v1 will have a positive value and vice versa, I think!
(Well, I'm screwed for M2 this year!)
3. (Original post by Glavien)

Hi, when working out Newtons law of restitution you use the relative speeds of separation and approach. However, for this question, on the last line of working in the image, how come the speed of separation is v2 - v1 and not v2 + v1 as they are moving away from each other?Do you always take the two speeds away when you have unknown values for the velocities?
Thanks for the help!
Judging by the arrows, they've chosen to take both v1 and v2 to the right, since you don't know when you start which way they'll be going after the impact (mind you, it doesn't help that they've put their arrow for the 8 going the wrong way).
4. (Original post by tiny hobbit)
Judging by the arrows, they've chosen to take both v1 and v2 to the right, since you don't know when you start which way they'll be going after the impact (mind you, it doesn't help that they've put their arrow for the 8 going the wrong way).
Thanks man!

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