Answer:
GRAVITY
Gravitational force, or the force of attraction between an object and the Earth, has an impact on every element of Volleyball. Whether you are serving, bumping, or spiking, gravity will affect every interaction you have with the ball.
Serving: When a volleyball is served, the server exerts an upward, and forward, force on the ball. Meanwhile, gravity is exerting a downward force on the volleyball. This downward force is what causes the ball to fall down on the opponent's side, after clearing the net. To assist gravity, you can snap your wrist which adds top spin the skids over the ball as you serve. This spin crates pressure difference above and below the ball. According to researcher Dr. Marion Alexander, this top spin causes the ball to fall quickly on the opponent's side after clearing the net.
Passing: When a player performs a forearm pass, or bump, an upward and forward force is exerted on the ball toward a target player. Gravity exerts a downward force on the ball, and if you do not compensate for this, your target player will not have time to set underneath the ball before it hits the ground. To account for the force of gravity, simply follow through with your forearm when bumping, to exert force on the ball, over a longer period of time. This will cause the ball to go higher and ensure that the target player has time to prepare herself beneath the ball.
Spiking: When you spike a volleyball, you have the opportunity to deliver a crushing offensive blow to your opponent. When spiking, you exert a downward force on the ball so that it falls rapidly on the opponent's side of the court, making it very difficult for your opponent to return the ball. Gravity works in your favor when you spike, because it also exerts a downward force that makes the ball fall to the court floor. For this reason you do not necessarily have to exert tremendous downward force to spike effectively, because gravitational force is also acting on the ball in the same direction.
