1. The problem statement, all variables and given/known data Under some circumstances, a star can collapse into an extremely dense object made mostly of neutrons and called a neutron star. The density of a neutron star is roughly 10^14 times as great as that of ordinary solid matter. Suppose we represent the star as a uniform, solid, rigid sphere, both before and after the collapse. The star's initial radius was 8.0×10 5km (comparable to our sun); its final radius is 18km. 2. Relevant equations Conservation of momentum 3. The attempt at a solution The solution to this problem is obtained using conservation of angular momentum. I understand how to solve it, but I'm not sure how I would have figured out to use conservation of angular momentum without looking at the solution. For linear momentum it was easy because usually whenever objects collided, the problems usually dealt with conservation of linear momentum. In this problem, one star collapses with no interaction with any other objects.