**PURPOSE: **To show that the friction force depends linearly on
the normal component of the applied force.

**DESCRIPTION: **1. Put 5-kg mass on left pan of balance and the
blue block on the right pan. Add to the right pan 1 kg plus additional
masses from the weight set to achieve balance. 2. Place block on
lecture table with the additional right-pan weights on top of it.
(Together they have a mass of 5 kg.) 3. Determine the force of
friction by pulling the box with its weights across the lecture table
using twine and the spring scale. Place additional 5 kg mass on block
and repeat. Repeat with block on edge. Typically about 13 Newtons is
required to pull the 5 kg combination. This gives a coefficient of
friction mu=13/(5X9.8)=0.26, which is roughly what is expected for
wood on wood. The difference between static an sliding friction is
readily visible.

**SUGGESTIONS: **This demonstration is bigger, and therefore may
be more accurate and easier to use than some of the smaller versions.

**REFERENCES: **PIRA
1K20.00.

**EQUIPMENT: **Large blue block with eye and twine, 40 Newton
spring scale, one 1 kg mass, two 5 kg masses, weight set, pan balance.

**SETUP TIME: **10 minutes