Mechanics: Friction Box

PIRA Class: 1K20.12

Purpose:
To demonstrate the dependence of friction force on normal force and static vs kinetic friction force.

Description: 

1. Attach a spring scale to the box and pull until the box starts to move then continue pulling the box at constant speed. You should see the spring reading drop down to a smaller value once the box starts sliding at constant speed.

2. Add weights to the box and repeat to see larger force values.

3. Optional: Have a volunteer push the box against the wall and measure the spring force when the volunteer pushes light vs pushes hard to illustrate it is normal force not simply mass that is important.

Equipment:
Friction box, 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