1. The problem statement, all variables and given/known data 2. Relevant equations KE=1/2 mv2 3. The attempt at a solution To calculate the kinetic energy of the ball just after it leaves the surface, I use the ratio of the centre of gravities of the ball at the two different heights. My working is (0.41/0.76)×0.75 which gives me the answer C. But the answer is B. And according to the solution, I must take the ratio of the bottom of the balls instead. Why is it so? I thought that COG is always used in calculating change in potential energy.