Let's say that 2 objects inelastically collided in outer space. One has a Mass of 2kg, and velocity 5m/s. The other has a Mass of 4kg, and velocity is 0m/s. ( at rest) 1) Energy cannot be created or destroyed. (Energy is conserved) 2) Momentum is conserved. Total Momentum before was (2x5) + (4x0) = 10kg m/s Total Momentum after, therefore, also = 10kg m/s Hence, velocity of the 2 objects moving is 'v' in the equation : 10 = (v x [4+2]) so v = 10/6 m/s Now from Kinetic Energy perspective: Before -- k.e = 1/2 x 2 x 25 = 25J After -- k.e = 1/2 x [4+2] x 25/9 = 25/3 J So what happened to the K.E after the Collision for it to go from 25 to 25/3??