A football player weighing 75 kg running at 2 m/s toward west tackles a 70 kg player running 1.5 m/s in the opposite direction. What is the decrease in kinetic energy during the collision?

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  1. First use conservation of momentum to calculate the final velocity of both, V.
    75*2 - 70*1.5 = 145 V
    V = 0.31 m/s (west)

    Now add up the initial and final kinetic energies.

    KE1 = (1/2)*75*2^2 + (1/2)*70*(1.5)^2
    = 150+ 78.75 = 228.75 J
    KE2 = (1/2)*145*(0.31)^2 = 6.97 J

    Subtract for the KE Loss. About 97% of the initial KE is lost.

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  2. A football player weighing 75 kg running at 2 m/s toward west tackles a 70 kg player running at 1.5 m/s in the opposite direction. What is the final velocity of the players if they both fall together

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