If an object is mass m with a velocity u collides with a stationary object of mass m, how much kinetic energy is lost?
There will be no loss of kinetic if the collision is elastic and in all cases momentum will be conserved. If the collision is inelastic some energy is converted to heat.
Energy is always conserved, so the kinetic energy of objects which collide is conserved unless some of the energy was converted into another form during the collision. If the collision is inelastic some energy is converted to heat but momentum is conserved.
So, before the collision a mass
This means that
The change in kinetic energy is:
So, the loss of kinetic energy is