# Homework Help: A question on gravity

1. Oct 26, 2015

### Sobi

• Moved from technical forums. Did not use template.
I have seen a question about two bodies left in a free space and the question ask when shall they collapse.
For solving this I reached this step:(GM)*r^-2)*dr=(M*(m+M)^-1)*v*dv
For the left side,the lower bound of the integral is R and the upper bound is 0.after calculating it the answer of the left side is:1*0^-1-(GM*R^-2),what can we do with it?

2. Oct 26, 2015

### BvU

Hi Sobi, welcome to PF !

Interesting question, but assume that I don't know how you got here. Could you tell a little more about what you started with, what steps you did to get here ?

3. Oct 26, 2015

### Sobi

a=(GM*r^-2)=(dv*dt^-1)*M*(m+M)^-1

4. Oct 26, 2015

Staff Emeritus
Are we going to have to wring this out of you one line at a time? "Could you tell a little more about what you started with, what steps you did to get here?" was a very good question, and pretty much what we need to know to help you.

5. Oct 26, 2015

### Sobi

Continuing the question:I then multiplex the right side of it with dr*dr^-1.then bringing dt to the left side of the equation then it became the integral I talked about first.

6. Oct 27, 2015