1. The problem statement, all variables and given/known data Suppose that two elastic balls moving through three dimensional space collide and rebound, conserving both kinetic energy and momentum. The balls cast shadows, and these shadows moving over a flat two-dimensional surface come together, collide and rebound. Now pretend the shadows have mass. Suppose that the shadow of a call has a mass proportional to the mass of the ball. Then, the colliding shadows A) conserve kinetic energy B) conserve momentum C) conserve both kinetic energy and momentum D) conserve neither kinetic energy nor momentum The attempt at a solution This was a problem on a test in my physics class at a community college. I think my professor is fairly bad and doesn't know what he's talking about most of the time. This problem is also the only problem that I haven't been able to find online, so I assume he created it. I originally picked D, and found out later that my professor marked this as wrong. My reasoning for picking D was that I thought of a counter example to C. I imagined two spheres traveling towards each other completely horizontally (w/o gravity and parallel to the shadow plane) with one slightly higher than the other. When they both collide, both of the spheres will have some vertical velocity from the impact (which the shadows would not pick up). So in this collision between two elastic balls, the shadows are picking up both velocities perfectly before impact since the motion of the balls is parallel to the shadow plane, but after the impact some of this velocity is lost from both balls in the shadow plane even though in 3D it isn't. The answer is of course not A or B, since both must be right if one were right. So, my professor implying that the answer is C is causing me some trouble. Is my counter example not really a counter example? If so where did I go wrong? Or am I right?