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. Once this occurs, the ball has an unbalanced force acting upon it, which causes it to leave its resting state. It only starts to move when the racquet comes into contact with the ball. Newton's First Law, also known as the law of inertia, states that an object at rest will stay at rest, and an object in motion will stay in motion with the same speed and in the same direction unless acted upon by an external force.

This is why when the ball is first dropped from the hand it has no horizontal velocity. According to the second part of the law, an object at rest tends to stay at rest unless acted upon by an unbalanced force. Then, when it hits the ground, friction acts as an unbalanced force, and that is why it slows down. 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. This law occurs during a tennis shot when the ball is in the air. 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.

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