Introduction to Friction
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D) Friction
The force between two surfaces that opposes motion.
Friction 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
Spoilers push down on the car, increasing the force between the tire and the road.
Two Kinds of Friction
Static Friction (fs)
The force of friction that must be overcome to move an object at rest.
Kinetic Friction (fk)
The force of friction that opposes a moving object.
Ice and skate - low μk
Fundamental Rule
Kinetic friction is generally weaker than static friction.
Friction Equations
General Formulas
Static Friction
fs = μsFn
Kinetic Friction
fk = μkFn
Friction on a Ramp
(in terms of mass and angle)
f = μ[mg cos(θ)]
Friction on a Horizontal Surface
Level Force
f = μFn = μmg
Tilted Push
Fn = mg + F sin(θ)
f = μ[mg + F sin(θ)]
Tilted Pull
What if the object was pulled instead of pushed?
Fn = mg - F sin(θ)
f = μ[mg - F sin(θ)]