During inelastic collision of two particles, K.E after collision is not equal to kinetic energy before collision. Here, K.E appears in other forms. In some cases such as when initial K.E is converted into internal energy of the product (as heat, elastic or excitation energy) while in other cases . Such as when internal energy stored in the colliding particles is released.

For example, when we fire a pistol, the hammer hits the bullet with relatively small initial kinetic energy but the kinetic energy of the hammer and bullet after collision is considerably greater than the initial kinetic energy.