PHYSICS OF A JUMPING BASKETBALL PLAYER
A basketball player, before jumping is subject to two forces. The normal force applied up on his feet from the ground and gravity pushing him downwards. From Newton’s Third Law, we find that the ground pushes up on his feet with a force of the same magnitude as him pushing down on the ground through his feet as seen in figure 1.



Figure 1. Player before jumping
Fn = mg, m = mass of player, g = gravity

As the player is jumping, his torso is moving upward with some velocity. The velocity increases as he straightens his legs in order for him to jump higher. As a result, his center of mass is accelerating upward, indicating that he is experiencing a net force upwards, allowing him to elevate because the normal force is greater than the gravitations force.
The instant the player leaves the ground, the normal force vanishes and the only force acting on the player is gravity. If no gravity acted on the player, he would elevate until another external force acted on him, but because of gravity, he is pulled back to earth. Figure 2 shows the free body diagram of him in the air.

Figure 2. Forces on player in air

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