Suppose i have a ball of mass m and if i left the ball at a particular height 'h', then i have done some work on it or by the law of conservation of energy i have transferred some energy to the ball. Now suppose i release the ball isn't the energy that i have transferred changing because it moves with a particular acceleration and hence a changing velocity and also the potential energy keeps on decreasing as the height decreases, then doesn't the energy necessarily change? So how is energy constant. The ball falls down and is then at rest and hence has no potential energy or kinetic energy (mgh=0; 1/2mv^2=0), hence the potential energy given to the ball mgh now becomes 0 (mgh-->0). Thus energy has been destroyed isn't it??