Posted by **Skyler** on Sunday, August 15, 2010 at 10:53am.

A 8.5 kg steel ball, travelling left to right at 12 m/s, undergoes an elastic collision with a 5 kg steel ball at rest. To the nearest Joule, what is the Total Kinetic Energy of the two balls after collision?

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If the first ball in the question above has a mass of 10 kg and the second ball is travelling at it with a speed of 3 m/s, what is the velocity, to one decimal place, of the first ball after collision?

- physics -
**Skyler**, Sunday, August 15, 2010 at 10:54am
What is the velocity of the second ball in problem the last problem above to one decimal place?

- physics -
**drwls**, Sunday, August 15, 2010 at 11:29am
For your first question:

In an elastic collision, the total kinetic energy does not change. In this case it equals the initial KE of the ball travelling 12/m/s.

In the second question, both momentum and total KE remain constant. You have to solve two equations in two unknowns. It gets quite messy when the masses are unequal, as in this case. You should try this exercise yourself.

There is a shortcut if you do the problem in the center-of-mass coordinate system (in which case both masses simply reverse direction at the same speed), and then transform back to laboratory coordinates.

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