Kinetic energy is the energy that is associated with motion. It is the energy possessed by an object that is moving. It is calculated from one-half the product of the mass and the square of the velocity of the object. From this, we can calculate the mass of the object. We do as follows:
KE = mv^2/2
500 J = m(5 m/s)^2 / 2
m = 40 kg