Homework Help: Conservation of momentum and baseball throw

1. Nov 23, 2008

bkl4life

While standing still on the baseball field, you throw the ball to a teammate. Why do you not move backward as a result? Is the law of conservation of momentum violated?

2. Nov 23, 2008

PhanthomJay

Look up the description of conservation of momentum. Is it always conserved?

3. Nov 23, 2008

bkl4life

Maybe I am making this harder than it really is.

I know the of conservation of momentum will remain constant unless acted upon by an outside force. The reason I am not moving backwards is because my arm is moving forward?

4. Nov 23, 2008

PhanthomJay

NO. Any forces acting on the player and ball while they are in contact with each other are internal to the man-baseball system (Newton 3). What is the external force? HINT: What would happen if the thrower was standing on ice?

5. Nov 23, 2008

bkl4life

The normal force pushing up on the player is equal to the force the player is exerting on the ground. This is why the player does not move backwards?

If the thrower were standing on ice and threw the ball he would slide because there would be friction between the player and the ice.

6. Nov 23, 2008

PhanthomJay

Yes, that's true, but it is an outside external force, acting up, but the man doesn't move down , because another external force , gravity, balances it. But let's concentrare on the horizontal x direction.
NO.
No, there would ideally be no friction or little friction between the player and ice, that is, no external force in the x direction, and he would move backwards due to conservation of momentum. So what's the answer as to why the player does NOT move backwards?

7. Nov 24, 2008

bkl4life

Do you have to add the mass of the whole system (ball and player)? The mass of the ball will not be great enough to make a an impact of moving the person. The external force in the x direction is stopping the person from moving backwards.