how does an object in motion stay in motion unless acted upon by an unbalanced force

This concept is explained by Newton's First Law of Motion, which states:

"An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced external force."

This principle is often summarized as the law of inertia. Inertia is the tendency of an object to resist changes in its state of motion. The key points to understand are:

1. At Rest: If an object is at rest, it will remain at rest unless an external force acts on it.
2. In Motion: If an object is in motion, it will continue to move at a constant velocity (speed and direction) unless an external force acts on it.

This means that no force is needed to keep an object in motion once it's moving. Instead, a force is required to change the object's state of motion – to stop it, slow it down, speed it up, or change its direction.