Once a bubble of mass m and radius r and no internal pressure has become much denser than its environment, it collapses as if it were an isolated object. Treat it as strictly spherical Show that the equation of motion for a point on the outer edge of the bubble is given by Gm. i" = r3 Solve this equation for the radius of the outer edge as a function of time for a bubble edge initially at rest wrt. its centre and nd the exact expression for the time at which the bubble reaches radius zero