- #1
DoobleD
- 259
- 20
Let's say a tennis ball with velocity with only an horizontal component hits a vertical wall at rest.
After collision, conservation of momentum tells that :
[tex]m_{wall}v_{wall} = 2m_{ball}v_{ball}[/tex]
Thus, the wall has now a (tiny) velocity and kinetic energy :
[tex]v_{wall} = \frac{2m_{ball}v_{ball}}{m_{wall}} , K_{wall} = \frac{2m_{ball}^2v_{ball}^2}{m_{wall}}[/tex]
while the kinetic energy of the ball didn't change. Where does the "new" energy in the wall come from ?
After collision, conservation of momentum tells that :
[tex]m_{wall}v_{wall} = 2m_{ball}v_{ball}[/tex]
Thus, the wall has now a (tiny) velocity and kinetic energy :
[tex]v_{wall} = \frac{2m_{ball}v_{ball}}{m_{wall}} , K_{wall} = \frac{2m_{ball}^2v_{ball}^2}{m_{wall}}[/tex]
while the kinetic energy of the ball didn't change. Where does the "new" energy in the wall come from ?