Physics

Two coupled boxcars are rolling along at 4.0 when they collide with and couple to a third, stationary boxcar.

1.) What is the final speed of the three coupled boxcars?

2.) What fraction of the cars' initial kinetic energy is transformed into thermal energy?

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  1. Assume that the boxcars all have the same mass.

    Use conservation of momentum:

    2*m*vi = 3*m*vf

    where m is the mass of one boxcar, vi is the initial speed, vf is the final speed.

    2*m*4 = 3*m*vf

    vf = 8/3

    Initial kinetic energy is ki

    1/2*(2*m)*vi^2 = m*4^2 = 16*m

    Final kinetic energy kf is

    1/2*(3*m)*(8/3)^2 = 1/2*(3*m)*(64/9) = (32/3)*m

    The fraction of the car's initial kinetic energy that is transformed into thermal energy is (ki - kf)/ki

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