On the rebound
A cubic block of metal of mass M is hanging from a long thread. You throw a perfectly elastic rubber ball of mass m at speed u, perpendicularly to one of the block's vertical faces. Immediately after the ball bounces off the block, what is the speed of the ball and the block?
Labels: funphysics
posted by Chris at
11:59 AM
8 Comments:
Hi Chris,
it's (M-m)u/(M+m) for ball and 2mu/(M+m) for block.
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Hi quantense, you got it right (of course). Did you use conservation of energy, or did you view the problem in the centre of momentum frame?
I must be getting old, it took me ages to work it out.
Chris, I've used standard method of energy-momentum conservation.
Mv-mu'=mu;
Mv^2+mu'^2=mu^2.
Rewrite
Mv=m(u+u');
Mv^2=m(u-u')(u+u').
Divide one by another, get v=u-u', substitute in the first eq. and so on.
Hi quantense. I'd subconsciously pre-supposed conservation of momentum. I've recently become mistrustful of CoE for these mechanics problems. Even though I said the ball was perfectly elastic, I can still hear it hitting the block in my imagination.
I just took a look at your profile - I can see why this problem is easy for you. How long have you been studying theoretical physics? Would you like some relativity or quantum mechanical problems?
Hi Chris, of course we can't perform such ball collision which is absolutely elastic. I very enjoy you know that sound of collision means loose of energy (from ball and block to sound).
Yes, I'm a theoretical physicist. Namely, I'm fifth year student of Moscow Institute of Physics and Technology. As you've noticed, this problem was really easy for me. But I like to visit this site and solve these problems, because I enjoy conversation and I can practise my english (which is not very good, as you may notice).
Of course, there are questions in quantum mechanics and general relativity, for which I don't know answers. They are very deep and fundamental and I like to think about them.
Hi quantense. I wasn't intending deep questions; only "off-beat" ones.
Okay, Chris, if so I'll be able to answer.
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