Law of Inertia
Newton’s first law states that an object in motion tends to stay in motion, and an object at rest tends to stay at rest, unless acted upon by an unbalanced force. This law occurs during a tennis shot when the ball is in the air. There is no force being applied to it horizontally, and therefore it continues to move horizontally with a constant velocity until it hits the ground. Then, when it hits the ground, friction acts as an unbalanced force, and that is why it slows down. According to the second part of the law, an object at rest tends to stay at rest unless acted upon by an unbalanced force. This is why when the ball is first dropped from the hand it has no horizontal velocity. It only starts to move when the racquet comes into contact with the ball. Once this occurs, the ball has an unbalanced force acting upon it, which causes it to leave its resting state. We know this law occurs because we see the ball start to move as soon as it touches the racquet, meaning that as soon as an unbalanced force is applied, the ball changes from resting to moving.
Free Body Diagrams
Finally, once the ball has traveled through the air to the other side of the court, it hits the ground. While it is touching the ground, there are three different forces acting on it. The first is the force of gravity, which is pushing it downwards toward the ground. However, since it is no longer moving downward, we know that there must be an equal force pushing back against gravity and causing it to remain at the same height. This is the normal force. The third force acting on the ball here is force of friction. Friction is caused by the ball rubbing against the rough surface of the court. This force causes the ball to slow down slightly, so we know that it is pushing in the direction opposite of the ball's motion, which would be backwards.