# Thread: centre of mass help proof

1. ## centre of mass help proof

can someone please guide/ show me how to do this question, ?

thanks

2. ## Re: centre of mass help proof

It is impossible to do that without knowing $v_1$, $v_2$, $v_1*$, and $v_2*$ mean! We might guess that $v_1$ and $v_2$ are the speeds of the two objects but that should be given in the problem.

3. ## Re: centre of mass help proof

Originally Posted by HallsofIvy
We might guess that $v_1$ and $v_2$ are the speeds of the two objects but that should be given in the problem.
If $v_1$ and $v_2$ are indeed speeds in the lab frame and $v_1^*$, $v_2^*$ are speeds in the center-of-mass frame, then use the definitions $v_i^*=\frac{dr^*_i}{dt}$, $v_i=\frac{dr_i}{dt}$ and the definition of $R$ to rewrite the left-hand side $m_1v_1^*$.