In the absence of air resistance, a ball thrown vertically upward with a certain initial KE will return to its original level with the same KE. When air resistance is a factor affecting the ball, will it return to its original level with the same, less, or more KE? Does your answer contradict the law of energy conservation?The KE of the ball with air resistance would be less since the net force acting upon the ball would be smaller due to the increase in normal force. The answer does not contradict the law of conservation of energy, because you need to remember that there are more types of energies out there than just KE and PE. In this case due to the interactions with the air molecules there is an increase in the force of friction which leads to a less quantity foe KE