Indirectly Measuring Friction

This is a demonstration of a method of indirectly measuring the friction force between the bottom of a box and the table surface, and for comparing static friction and kinetic friction.
A box is being pulled across a table top using a force sensor connected to a computer, which measures the pulling force.
The horizontal pulling force is gradually increased until the box begins to move, at which point it is kept moving at a fairly constant velocity. Since the motion is not changing most of the time, the friction force is equal in magnitude to the pulling force.
The resulting graph can be used to discuss differences between static friction and kinetic friction.