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2. P = mv, KE=1/2mv

^{2}

3. Initial KE of the system is the sum of the kinetic energies of the two masses.

K1 = (1/2)Mv

^{2}, K2 = (1/2)m(-3v)

^{2}= 9mv

^{2}

sum: 10mv

^{2}

assuming momentum is conserved, velocity of the two masses after the collision is -V.

Then calculate final KE = 1/2(2m)(-V)

^{2}= mv

^{2}

subtract this from the initial and I get 9mv

^{2}Joules are lost in the collision.

My key says the answer is 4mv

^{2}