In a totally inelastic collision between two equal masses, with one initially at rest, what fraction of the initial kinetic energy is lost

50%

Explanation:

Assuming no external forces acting during the collision, total momentum must be conserved.As one of the mass was initially at rest, the initial momentum can be written as follows:

       

If the collision is totally inelastic, both masses must move as one after the collision, so we can write the following equation: (2)As po = pf, we can solve for vf  in terms of v1o, as follows:

       

The initial kinetic energy will be:

       

The final kinetic energy of both masses will be as follows:

       

Rearranging in (2) we get:

       

So, the fraction of the initial kinetic energy lost is just the half of the initial value.

Rain is pulled by gravity which is a non contact force