# Conservation of momentum & inelastic collisions

1. Sep 14, 2009

### chelseaalyssa

1. The problem statement, all variables and given/known data
This question is related to an in-class experiment. The lab involves bouncing a ball and using a motion sensor which creates a graph representing the motion.
The question is: Is the principle of conservation of momentum violated in this collision (the ball colliding with the ground)?

2. Relevant equations
m1v1i + m2v2i = m1v1i + m2v2i

3. The attempt at a solution
I don't think the collision violates the conservation of linear momentum law because the law applies to closed systems, and this system (when the ball collides with the earth) is not closed.
However I'm not sure if my theory on this is correct ...

2. Sep 14, 2009

### kuruman

What does your system consist of? Is it just the ball? What have you learned about momentum conservation, namely when is momentum not conserved?

3. Sep 14, 2009

### chelseaalyssa

The experiment only involves bouncing a ball on the ground (and detecting motion). Therefore, I am not sure if the system involves both the ball and the ground, or just the ball.
Regarding momentum, I know that momentum is always conserved in a closed system, but not a system which is affected by external forces.

thanks for helping :)

4. Sep 14, 2009

### kuruman

In a collision there are always at least two parties that participate, an object cannot collide with itself. One party is the ball so it is reasonable to assume that the Earth (or floor which is attached to the Earth) is the other party. Is the Earth-ball a closed system? Are there external forces that affect it?

5. Sep 14, 2009

### chelseaalyssa

Okay, so if the system is the earth-ball (and the collision between the two)... I'm not sure if the gravitational force from the earth on the ball is included in the system, or is counted as an external force?

6. Sep 14, 2009

### kuruman

It is not external to the Earth-ball system. The force that the Earth exerts on the ball is equal and opposite to the force that the ball exerts on the Earth. The Sun's force on the ball and Earth would be an external force, but we pretend that it is negligible.