# What is the decrease in kinetic energy during the collision?

Problem 25.
A railroad car with a mass of 2.31*10^4kg moving at 3.59m/s collides
the same mass as the single car and initially noving in the same
direction at 1.34m/s.
b.What is the decrease in kinetic energy during the collision? Answer
in units of J.
Note: I find out that the final speed of the joined cars were 2.09m/s.
When I found the difference of kinetic energy I found the final kinetic energy by multiplying .5(2.31*10^4)(2.09)^2 by 3 to get 151,354.665. As a result my answer was -38,981.25 which I found out was wrong. What did I do wrong?

Problem 15.
Given: g=9.81m/s^2.
A 2.22 kg ball is attached to a ceiling by a 1.39m long string. The
height of the room is 3.60m. What is the gravational potential energy associated with the ball relative to
a. the ceiling?
Note:Would you use mgh=PE and have m=2.22 g=9.81 and h=3.60?
c. a point at the same elevation as the ball?
Note: How would you do it?

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For the first problem, your solution looks OK to me. You found the change in total KE; perhaps they wanted change in KE for the third car alone.

For the second problem, use PE = mgh, where h is measured from your reference point. Relative to the ceiling, h = - 1.39; relative to the height of the ball, h = 0.0.