# Measure static and kinetic coefficients of friction

Today, we'll use one of the friction blocks with mass bars sitting on it as the object labelled "A" above, and cylindrical weights as the hanging mass labelled "B".

There is definitely a friction force which pulls object "A" to the left. We can make a table showing all the forces on A:

And another table for all forces acting on the hanging weights.

#### Two equations, two unknowns

You should end up with a set of two interesting equations:

• static case: if the friction block doesn't move at all, then the acceleration a = 0. Solve for the coefficient of static friction μs
• kinetic case: if the friction block is sliding and accelerating, then the acceleration a is NOT zero. Solve for the coefficient of kinetic friction μk (it will be a bit messy)

#### The experiment

Set up a track, pulley, string, etc., as shown in the diagram above. Wipe the track clean with paper towels, and wipe the wooden side of the friction block, too. Use a friction block and two mass bars as object "A".

1. Add mass to the hanging weight "B" until the block ju-u-u-st barely starts to slide. You can use the masses to figure out the coefficient of static friction.
2. Add a enough mass (maybe 60-100 grams) to the object "B", so that the block accelerates smoothly along the track, without sticking. Measure the time it takes for the block to slide a distance of 40 or 50 cm. Compute the acceleration of the block. Use this acceleration, and the masses, to determine the coefficient of kinetic friction.

In theory, the kinetic coefficient should be smaller than the static coefficient. Is it?

Make separate measurements for the wood side of the block and the fabric side of the block. Which is larger?

If you have time, make a test to see if the area of contact between block and track affects the coefficient of friction.