If P is the momentum of an object and m is its mass, then its kinetic energy is:

Explanation

Kinetic energy is half mv squared and momentum is mv, so substituting v equal to P over m gives half m multiplied by P squared over m squared, which simplifies to P squared over 2m. A quick check is that energy must rise with the square of momentum, which only the second option does. This relation is used constantly in collision and de Broglie problems.