Two Asteroids Colliding- Find the Initial Velocities

[jessicak]

1. Homework Statement
Two asteroids of 1.0x107kg and 6.0x107kg respectively, are initially at rest in interstellar space separated by a large distance. Thier mutual gravitational attraction then causes then to fall toward each other on a straight line. Assume the asteroids are spheres of radius 130m and 420m respectively.

A) what is the velocity of each asteroid before they hit?
130m, v=? 420m asteroid, v=
What is the kinetic energy of each?
What is the total kinetic energy?
B) The collision is totally inelastic. What is the velocity of the joined asteroids after they hit?
2. Homework Equations
K=1/2mv2
vf = (m1v1 + m2v2)/(m1 + m2)



3. The Attempt at a Solution
I have absolutely no idea about how to get the initial velocities, but I know where to go from there.]

 

[Pagan Harpoon]

You can do it with conservation of energy and conservation of linear momentum. Assume the asteroids start at infinity distance from each other. Find the change in potential energy as they go from infinity distance to 0 diastance, this has to be equal to their kinetic energy just before the collission. Also, the total momentum at the start was 0, so the momentum before the collission is 0.