Answer:
Newton's First Law of Motion.
Step-by-step explanation:
Inertia can be defined as the tendency of an object or a body to continue in its state of motion or remain at rest unless acted upon by an external force.
In physics, Sir Isaac Newton's First Law of Motion is known as Law of Inertia and it states that, an object or a physical body in motion will continue in its state of motion at continuous velocity (the same speed and direction) or, if at rest, will remain at rest unless acted upon by an external force.
Hence, once a ball is hit and no longer touches the mallet, it continues moving because of its inertia, as explained by Newton's First Law of Motion.