A girl pushes her little brother down a snowy hill on a sled and he rides quickly to the bottom of the hill which one of Newton's Laws is this

Newton's First Law of Motion.

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.

In this scenario, a girl pushes her little brother down a snowy hill on a sled and he rides quickly to the bottom of the hill. The push from the little boy's sister is the external force acting on him and consequently, causing him to be in motion down the snowy hill.