1. The problem statement, all variables and given/known data Two solid copper spheres of radii 1 and 2 cm are released from rest in free space, their centers being 20 cm apart. Estimate the velocity with which they collide 2. Relevant equations density =mass/volume F=GMm/r^2 3. The attempt at a solution Find mass of each from density of copper and volume. With mass and separation I can find force and initial acceleration towards each other. This force changes with distance, so how would I take account of this in my solution? Many thanks for any prompts.