Solve 2 Jupiter-Size Planets Homework Problem

[jorn0807]

Homework Statement

two Jupiter-size planets are released from rest 1.50×1011m apart.
What is their speed before crash

Homework Equations

GMm/r=U
KE=-GMm/2r

The Attempt at a Solution

I tried setting U+KE=Uf+.5mv^2 and solve for V but it doesn't work :(

 

[ehild]

Hi jorn0807, welcome to PF.

Show your work in detail - where does the size of the planets come in? What is the final state ? How far are the planets then? And what is the initial kinetic energy, if the planets are released from rest? (just imagine that you hold a Jupiter-size planet and then you release it ...)

ehild