Mechanics:
Area & Surface Dependence of Friction

PIRA Class: 1K20.20

Purpose:
To demonstrate the dependence of friction force on surface roughness and (independence) on surface area.

Description: 

1. The flat block has sandpaper on one side and not on the other. Attach a spring scale to the string and compare the force required to start/keep the block moving when each surface is facing down.

2. Compare the spring force when the large wooden side of the block is down to when the block is turned on its side significantly reducing surface area.

Note: The friction force depends on contact area which is only a small fraction of the surface area "in contact". When you change to a smaller surface area there is a larger pressure which increases the amount of contact resulting in nearly the same friction force.

Equipment:
Friction block, spring scale, weights

Location:
Shelf 123 (friction box), shelf 116/124 (spring scale), shelf 151 (weights)

Extras:
Nice discussion of why static friction > kinetic friction
Friction force only increases linearly with normal force up to a point
Hyperphysics discussion of limitations of this model of friction