How Do Newton's Laws Explain Motion with Two Carts and a Medicine Ball?

[shayboy4]

Hey guys, I got this project to do for physics. basically we each sit on one cart each (with wheels), and we attach the carts together. then one person throws the medecine ball to the other and we notice the there is no net movement. so how does this relate to Newtons law?

i know it has to do with law of momentum conservation.. but that is only when ONE person is on the cart and the other one throws the medicine ball. the person in the cart will move back because he gains the balls momentum.

but how do i explain it when the other person is also on the cart and the cart is attached?

 

[PhanthomJay]

If the boy throws the ball to the girl, then provided that she catches it, all forces are internal to the system and there is no net external force acting , hence, no acceleartion of the system and thus no movement. That's Newton 1. If the girl missed catching the ball completely, then the attached carts would move in the opposite direction of the throw. That's Newton 2, due to the loss of mass of the initial system. From a momentum viewpoint, if the girl catches the ball, the mass of the system remains unchanged, so if it was at rest initially, it remains at rest. All this assumes that the throw takes place over a rather short time period...if it was a long throw, then the carts would move opposite the direction of the throw initially, while the ball is in the air, until the girl catches the ball, then it moves back to where they were after the catch, so there is still no net movement.