# Determine the coefficient of static friction

If a track is perfectly horizontal, and you place a block on it, the block doesn't move. That's obvious.

If you now tilt the track by just 1 degree, the block still doesn't move. Why not? There is a (small) component of the gravitational force pulling the block downwards along the ramp -- so why doesn't it accelerate downwards along the ramp?

The answer is --- because there's also a force of friction, holding it back. When the track is tilted just a tiny amount, the force of friction is equal in size to the component of the gravitational force along the ramp. The net force is zero, so the block doesn't move.

As you tilt the track to a greater and greater angle, the component of the gravitational force along the track grows and grows. The force of friction must grow and grow, too, to prevent the block from sliding.

At a critical angle, the block starts to slide.

1. Determine this critical angle for your equipment -- what is it?
2. What is the component of the gravitational force pulling the block downward at this angle?
3. What is the static friction force holding the block at this angle?
4. What is the coefficient of static friction between block and track?

If you have time ... It's not easy to find the exact angle at which the block doesn't slide. Place your track flat on the table, then prop it up again from scratch and find the critical angle again. What is the uncertainty in the critical angle? What is the uncertainty in the coefficient of static friction?