A person pushes a box from left to right and then lets the box slide freely across the floor. The box slows down as it slides across the floor.

When the box is sliding freely, what is the direction of the net external force? Why?

Towards the opposite direction of the motion of the box.

Explanation:

The reason is that the net external force acting on the box when sliding freely across the floor is the frictional force.

Frictional force is a resistant force. It always acts against the direction of motion as it seeks to work against motion to resist it.

This is the reason why frictional force always carries a negative sign.