# Homework Help: Newtons Theory of Gravity

1. Oct 22, 2007

### oreostar90

1. The problem statement, all variables and given/known data

Two spherical asteroids have the same radius R. Asteroid 1 has mass M and asteroid 2 has mass 2.58·M. The two asteroids are released from rest with distance 12.35·R between their centers. What is the speed of second asteroid just before they collide? Give answer in units of (G·M/R)1/2.

2. Relevant equations

Ki + Ui = Kf + Uf

U= -GMm/r

K= 1/2 m v^2

3. The attempt at a solution

My Ki is 0. So I get

0 -GMm/r1 = 1/2 m vf^2 + -GMm/r2

r1= 12.35R
r2= 2R

so i solved for Vf = sqrt( 2GM (-1/ r1 + 1/r2) )

and then I get Vf = .915 (GM/R)^(1/2)

What am I doing wrong, as that is the wrong anwser.

2. Oct 22, 2007

### G01

EDIT: ERROR IN THIS POST, see post 6.

Last edited: Oct 22, 2007
3. Oct 22, 2007

### oreostar90

hmm are you recommending me to work with kinematic equations?

I do not see where you would go with knowing that information.

i) o ----------------- o
f) oo

I got a hint: You will need to use two conservation laws.

So I was thinking now I would use momentum and what I had earlier...but not sure if I should. I might need this one spelled out to me...

4. Oct 22, 2007

### G01

EDIT: ERROR IN THIS POST, see post 6.

Last edited: Oct 22, 2007
5. Oct 22, 2007

### oreostar90

umm, 2 times the radius of the mass?

Which I had as 2R for r2...

6. Oct 22, 2007

### G01

SORRY! I was tired and thought you were making a mistake that you weren't! Your values of R are correct! Very sorry!

I think your problem is in the following fact:

Remember that the kinetic energy on the right hand side of the equation will be split evenly between BOTH asteroids. What does this tell you?

Again, very sorry for misleading you. I must have really been out of it then!

Last edited: Oct 22, 2007