October 14th, 2008

Problem with energy conservation law?

Pythagor asked:


Hello,

I have got a problem which is described below.

A heavy metal ball starts to fall from 1 meter height, ant this ball hits a light tennis-ball on the table. After the hit, metall ball rises nearly to 1 meter height. To what height does a light tennis-ball rise?

In my opinion, energy conservation law should be used here. But then it means that the tennis-ball nearly don’t rise, and as I suppose, it is not a logical answer.

Could anyone help me with this problem?

Thanks in advance.
This is a problem where masses can’t be measured.

Powered by Yahoo Answers

Share and Enjoy:
  • Digg
  • Sphinn
  • del.icio.us
  • Facebook
  • Mixx
  • Google
This entry was posted on Tuesday, October 14th, 2008 at 7:00 pm and is filed under Creative Energy Ideas. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.

3 Responses to “Problem with energy conservation law?”

  1. misoma5 Says:
    October 15th, 2008 at 3:55 pm

    You have to take into account the weight of both the heavy metal ball and the light tennis ball. As long as the combined energy does not surpass the initial potential energy of the metal ball, no law has been violated.

  2. Bekki B Says:
    October 17th, 2008 at 10:00 am

    You are correct. It depends on what nearly 1 meter means. If the heavy metal ball doesn’t bounce all the way back up, there’s some energy that could be turned into heat or tennis ball energy. You need more info to numerically handle this problem.

  3. C Anderson Says:
    October 17th, 2008 at 11:35 am

    Mass differential is assumed in the forming of the problem / question by the use of metal vs. tennis balls. You don’t need to know the specific mass — they’re trying to get you to assume it, and also to assume the steel ball isn’t a tiny little thing, else this problem wouldn’t make any sense.

    It requires only a tiny fraction of the kinetic energy of the steel ball, imparted to the tennis ball, to get the tennis ball to bounce (due to its rebound from compression) quite high. The steel ball as a great deal of kinetic energy since it is of high mass. The tennis ball requires very little new KE to move it quite a distance since it is so light.

    In short, while the steel ball may bounce back to nearly its original height, it takes only a tiny fraction of the steel ball’s huge kinetic energy to really move that tennis ball… so little that the results would hardly be noticed in the rebound of the steel ball.