Biomechanics with dissipation and conservation of momentum of ball travelling upwards

jhale18

I hav just begun a biomechanics unit in Year 9 Phys ed, and have recieved my assignment for the term. As part of the assignment we must consider the momentum of the ball and its forward trajectory. I didnt place this thread in homework, as there is no actual problem, just considerations. On aspect of biomechanics we must explain is throwing a softball, and the angle at which it is released. The main determining factor as to the distance which can be achieved by the throw, is gravity. My question is, if you threw the softball straight up, like 90 degrees to the horizontal, its momentum will be conserved moving the air around it with air resistance and all, but does the ball lose momentum to gravity? If so, how, as gravity does not move away from the ball, although the earth does?

Doc Al

The momentum of the ball is not conserved since there are forces acting on it. Both air resistance and gravity.

I think you are saying that as the ball collides with the air, the total momentum of air and ball is conserved. Good! The same thing happens with gravity and the earth. The ball and the earth exert equal and opposite gravitational forces on each other. So as the ball loses momentum, the earth gains momentum. (Since the earth is so vastly bigger than the ball, its tiny change in velocity is unmeasurably small.) The net change in momentum of ball and earth due to gravity is zero.

jhale18

Cool, I can understand that. So as the ball falls back towards the earth, the earth can be seen to be gaining momentum, but due its extreme mass, acclerates at an infinitesimal rate. I hope I got that right. Thanks very much.

Doc Al

Exactly.

jhale18

Cool, thanks.