Conservation of mechanical energy when a ball is released from a height An isolated system means that no energy should be transferred or  taken away from it. Let us consider a ball- earth system. Mostly the law of conservation energy is proved  when the ball is released from some height. When we throw a ball from the ground, we are providing external energy to the ball, still the law is proved valid. Why?
 A: Notice that the system you consider depends on what objects and from when you start observing it if you consider energy conservation in that system. If you consider observing the situation just after you've provided the ball kinetic energy then its kinetic energy is simply its own so from then conservation of mechanical energy applies(provided there is no air-resistance). This is exactly what is considered or implied in most books, we consider observing the system just after the ball has been given the kinetic energy.
Now, if you consider starting to observe as you are providing kinetic energy to the ball by pushing it upwards suppose with your hands. Then you are now also part of the system. Hence your body's chemical energy etc. would also be now involved but still energy conservation would hold for this system. 
A: This perfectly obeys the law of conservation of energy too.  We are assuming that there is no air resistance here.  Then, you and the ball are a closed system.  
Your chemical potential energy in your body from the food you eat is converted to the kinetic energy of the ball, which is then converted to the gravitational potential energy of the ball, allowing the ball to go up.  Consequently, this is then converted to kinetic energy of the ball coming down, and when you catch it, it is released as sound energy from the impact.
Hence, your scenario definitely obeys the law of conservation of energy.
