Dynamics

Friction (AP)

AP Physics

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Introduction to Friction

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D) Friction — the force between two surfaces that opposes motion.
Caveman rubbing sticks together to start a fire Raccoon climbing a tree Skier going downhill

Friction Depends Only On:

1. Texture of Each Surface

Snowplow scooping snow
Question mark character

What do plows have on their wheels to increase friction?

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Chains

2. Force Holding Objects Together

Cartoon villain plotting
Question mark character

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

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Spoilers

Spoilers push down on the car, increasing the force between the tire and the road.

Rear-engine dragster with a rear spoiler

Two Kinds of Friction

1) Static Friction (fs)

The force of friction that must be overcome to move an object at rest.

Child pulling another child in a wagon

2) Kinetic Friction (fk)

The force of friction that opposes a moving object.

Hockey player skating on ice
Ice and skate — low μk

Fundamental Rule

Kinetic friction is generally weaker than static friction.

fk < fs

Friction Equations

General Formulas

Static Friction: fs = μsFn

Kinetic Friction: fk = μkFn

Friction on a Ramp (in terms of mass and angle)

Box on a ramp diagram showing the normal force

f = μ[mg cos(θ)]

Friction on a Horizontal Surface

Level Force

Level push diagram

f = μFn = μmg

Tilted Push

Tilted push diagram

Fn = mg + F sin(θ)

f = μ[mg + F sin(θ)]

Tilted Pull

What if the object was pulled instead of pushed?

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Fn = mg − F sin(θ)

f = μ[mg − F sin(θ)]

Next: Friction Relationships →