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A railroad car with a mass of 2.31*10^4kg moving at 3.59m/s collides

and joins with two railroad cars already joined together, each with

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?