You are here:

# Physics/conservation of momentum

Question
Hi!

How could we explain law of conservation of momentum for a rigid ball colliding with a wall?

Believe it or not, the second object in such a collision is the Earth itself. When the ball bounces against the wall attached to the earth, th Earth and the ball end up with a final change in momentum equal to the momentum change ofthe ball.
For example, if a 1.0 kg ball moving at 20m/s bounces off of a wall with a new velocity of -15m/s, the change in the momentum of he ball will be:
Dp=Pf-Po=m*Vf-m*Vo=1*(-15)-1*(20)=-35kgm/s
Because momentum is conserved the change in the momentum of the Earth will need to be equal but opposite the change in momentum of the ball. Therefore, the change in momentum of th Earth will be:
DPearth=-(35)=+35kgm/s
Of course we never notice this change since the mass of the Earth is so much larger than that of the ball and so the change in the velocity of the Earth will be infinitesimal.

Physics

Volunteer

#### James J. Kovalcin

##### Expertise

I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.