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Frictional Forces
Chapter 4, Section 4
Pg. 141-149
Friction
There are two types of frictional forces in the physical world that effect us.
- Static/Kinetic Friction
- Air Resistance
Air ResistanceAir ResistanceIs a resistance force that acts in the
opposite direction of gravitational forces.
Force of Gravity
Air Resistance
Help!!
SPLAT!!
Force of Gravity
Air Resistance
v1 > v2
Static and Kinetic FrictionStatic and Kinetic Friction
Forces that oppose motion between two surfaces that are touching each other.
Fx
Fr
Fr (non-moving) = Static Friction (Fs)Fr (moving) = Kinetic
Friction (Fk)
Frictional force depends on the size and mass of the object moving across a surface.
Fk
Fx
Fx
Fk
The amount of friction occurring between an object and the surface depends on the surface type.
Tile Floor
Carpet
Fk
Fk
Fx
Fx
The value that expresses the dependence of frictional forces on the surface type the object is in contact with is called the coefficient of friction (µ).
It is the ratio between the force of friction and the normal force.
Coefficient of Kinetic FrictionCoefficient of Kinetic Friction
μk = Fk/Fn coefficient of kinetic friction
μs = Fs,max/Fn coefficient of static friction
Ff = μFn frictional force
Sample problem
While redecorating her apartment, Suzy slowly pushes an 82 kg cabinet across a wooden dining room floor, which resists the motion with a force of friction of 320 N. What is the coefficient of kinetic friction between the cabinet and the floor?
FappliedFk = 320 N
m = 82 kgg = 9.81 m/s²
w = mg
w = (82 kg) (9.81 m/s²)w = 804 N w = Fn = 804 N
μk = Fk/Fn
μk = 320 N/804 N
μk = 0.40
Fn
mg
Sample Problem 2 (Pg. 146)
A student moves a box of books by attaching a rope to the box by pulling with a force of 90 N at an angle 30° to the horizontal. The box of books has a mass of 20.0 kg and the coefficient of kinetic friction between the bottom of the box and the sidewalk is 0.50. What is the acceleration of the box?
Fapplied = 90 N
Fk
mg
Fn
30°
g = 9.81 m/s²m = 20.0 kg
μk = 0.50
w = mg = 196 N
Fapp,y30°
90 N
Fapp, x
Step 1: Solve for Weight and X & Y components of applied force
Fapp, y = (90N) (sin 30°) = 45.0 N
Fapp, x = (90N) (cos 30°) = 77.9 N
Step 2: Find the Kinetic Friction
∑Fy = Fn + w + Fapp, y = 0
0 = Fn – 196 N + 45.0 N
Fn = 196 N – 45 N = 151 N
μk = Fk/Fn
Fk = (μk)(Fn) = (0.50)(151 N) = 75.5 N
Fapp, y = 45.0 N Fapp, x = 77.9 Nμk = 0.50
w = 196 NFapp,y
30°
90 N
Fapp, x
Step 3: Solve for acceleration
∑F = ma
a = (Fapp, x – Fk) / m
a = (77.9 N – 75.5 N) / 20.0 kg
a = 0.12 m/s² to the right
a = ∑F/m
Fapp,y30°
90 N
Fapp, x
m = 20.0 kg
Fk = 75.5 N
Fapp, x = 77.9 N