A45.0 kg skater is skating at a speed of 12.0 m/s. the skater coasts to a speed of 6.00 m/s. how much kinetic energy has the skater lost?

Kinetic energy is the energy of an object that is moving. It is calculated from one-half the product of the mass and the change in square of the velocity of the object. It is the opposite of potential energy which the energy possessed by an object at rest. We calculate as follows:
KE = mΔv^2 / 2 = 45 ( 6^2 - 12^2 ) = -4860 J had been lost by the skater