Here are the questions: If a ball is thrown upwards with a initial velocity (u) from the hand, the ball throws to the highest point and after bouncing the floor, it bounces back to its initial position. In this case, it is said that air resistance is not negligible. Okay, I am confused by the followings: 1. The velocity after bouncing back is less than or larger than the initial velocity? 2. Is the energy of the ball and conserved? 3. Is there any net force acting on the ball when it is at its highest point? THX everyone!!