I've been thinking about basic momentum problems lately I'm having a surprising amount of trouble with one that didn't seem so bad at first glance. Here's what I'm imagining: We've all heard of the simple momentum problem where a particle moving in the x direction at speed v1 runs into some unmoving object of a much much greater mass. Much like a basketball hitting the Earth, the particle simply bounces off in the negative x direction with the same speed v1. Now here's the twist that's messing me up. Lets say the giant mass the particle is hitting is moving, but the movement is perpendicular to the particles initial velocity. So say the particle still has initial velocity v1 in the x direction but the giant mass has velocity v2 in the y direction. After the collision, is the particle going to have some velocity in the y direction or will it have the same result as the example I mentioned above? Another way I've tried to think about it is what would happen if I dropped a basketball on the Earth from a car moving parallel to the Earth's surface? I mean I know what will happen in real life, but if there was no wind or ground friction, and the ball could not deform or rotate, then would it travel at the same speed as the car indefinitely? Also when I try to do this on paper I find I'm one equation short, I have 2 conversation of momentum equations (for x and y) and conversation of energy, but I have 4 unknowns, the x and y components of speed for both objects (i.e. the basketball and Earth) after the collision.