St. Mary's H.S. Physics - Intro to Friction Notes

Friction

School Blocks YouTube? Download the file below

D) Friction - force between 2 surfaces that opposes motion

Depends only on:

1) texture of each surface

What do plows have on

their wheels to increase

friction?

Chains

2) force holding objects together

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

Two Kinds of Friction

(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

Friction Equations

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