Friction

Do Now

Left or right. Which experiences the most friction? Why?

School Blocks YouTube? Download the file below

D) Friction — force between 2 surfaces that opposes motion Fill In the Blank Notes

What do plows have on their wheels to increase friction?

↓

Chains

What do race cars have mounted on their rear hood to increase their friction?

↓

Spoilers increase the force between tire and road

Why do some people put barbells in their car trunk on a snowy day?

↓

Weights in a car truck
increase the tires force on the road

(f_s) 1) Static Friction — force of friction that must be overcome to move an object at rest.

(f_k) 2) Kinetic Friction — force of friction that opposes a moving object.

Kinetic friction is weaker than static friction

f_k < f_s

Static friction: f_s = u_sF_n

Kinetic friction: f_k = u_kF_n

f_s = force of static friction

f_k = force of kinetic friction

F_n = normal force (supporting force)

u_s = coefficient of static friction — “the roughness number”

u_k = coefficient of kinetic friction — “the roughness number”

1. Relates to texture of surfaces in contact

2. Calculated in lab

ice and skate - low uk

What do skiers do to their skis to lower the coefficient of friction between their skis and the snow?

↓

Skiers put wax on the bottom
of their skis to lower the coefficient of friction

Try it

What is the direction of the friction at P?