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Can someone please explain the following problem to me?

Assuming a 3kg ball travelling at 4m/s hits and joins with a static 1kg ball (a perfect join with no loss of energy).

Momentum before collision = p = mv = 12kgm/s

Using conservation of momentum final velocity of total 4kg mass is 3m/s

The problem I am trying to resolve is the total kinetic energy before the collision = Ke = 1/2 m(v)2 = 24J. I do not understand how the total kinetic energy after the collision calculates to be 18J if no energy is lost when the balls join.

Are my calculations incorrect, or is a perfect collision impossible?

Thanks heaps,

Someone