AP PHYSICS Chapter 5 Friction Inclines Circular Motion
Friction Force that opposes motion due to contact between surfaces. Depends on: Composition and Qualities of the two surfaces in contact (μ) Roughness, Smoothness, softness, etc. Coefficient of Friction (μ ) (Table 5.1 pg. 173) Normal Force (N) Force exerted by a surface perpendicular to and away from surface. μ = f N f = μn
Kinetic Friction (sliding friction) Occurs when there is relative motion between the two surfaces in contact. Acts in the opposite direction of velocity Force of Kinetic Friction : f k = μ k N Examples: Skiing, Sledding, Water Slides, Skidding, Skiing, Ice Skating
STATIC FRICTION Occurs when there is no motion between the surfaces. Must exceed maximum static friction to start an object sliding. Once in motion kinetic friction act on the object. Static Friction is greater than kinetic friction f s μ s N Coffee cup sitting on dashboard EXAMPLES Snowboarder Sitting on hill Unsuccessfully pushing on Couch Walking
Problem Solving Strategies List Terms Draw picture and/or Force Diagram (LABELS!!) Determine Net Force Determine which equations(s) to use Solve equation Plug in Terms EXAMPLE: What is the maximum acceleration a car slow down if you do not want the coffee you left on the roof of your car to slide?(μ s = 0.40)
What is the minimum force required to start a 75kg couch in motion if the coefficient of static friction is μ s = 0.30? If the same force is exerted after it starts moving and the coefficient of kinetic friction is μ k = 0.20, what is the acceleration of the couch?
A force of 75N is required to push a 70 kg snowboarder along a flat trail with a constant speed. What is the Coefficient of Friction (μ) between the boarder and the snow? A 70 kg Bike rider moving at 12 m/s skids to a stop and comes to a stop in a distance of 8 m. What is the Coefficient of Friction (μ) between the tires and the pavement?
FORCES ON AN INCLINE Weight can be resolved into Components Perpendicular to the surface (Normal Force) (y-component: F y )(N) Parallel to the Surface (Parallel Force)( x-component ) (F x ) Fx F x mg N N
FORCES ON AN INCLINE Weight: Force of gravity exerted straight down Normal Force: Force exerted by a surface perpendicular to and away from surface The resultant of Weight and Normal Force is parallel to the incline N mg mg N θ θ
FORCES ON AN INCLINE (friction) A panda slides down a 40 o slope with an acceleration of 4.0m/s 2. What is the coefficient of kinetic friction between the panda and the slide?
FORCES ON AN INCLINE (friction) A worker pushes a 250kg piano up a 10 o slope that has a coefficient of friction of µ k =.15, What force is required to push it up the ramp at a constant speed?
Initial speed at bottom of ramp = 15 m/s angle of the 2 nd ramp = 15 o, If zero friction, how far up the ramp does he slide? If μ k =.50, how far up the ramp will he slide?
A kid pulls his lazy little brother in a sled (m=25kg) with a force of 40N at an angle of 35 o at a constant speed through the snow. What is the coefficient of friction between the snow?
A man wastes his Saturday pushing a 20kg lawn mower along the ground with a force of 100N at angle of 40 o to the ground. If the wheels are broken and slide along the grass and the mower accelerates 1.5 m/s was the coefficient of friction?
A bug hit a racers number plate just as the rider accelerates from the start. What is the minimum acceleration is the coefficient of friction between the bug and the plate if 0.50?
Centripetal Force For an object to accelerate, a force must be applied. F = ma, ac = v2 r, mv2 Fc = ma = r. Centripetal Force is a NET resultant Force It can be provided by: Tension Gravity (moon) Friction (Car, motorcycles) Normal Force (Gravitron, Nascar) Or any combination of these
FRICTION AS CENTRIPETAL FORCE A 300 kg motor cycle and rider rounds a curve (r=50m) with a velocity of 20 m/s. What is the force of friction between the tires and the track? What is the coefficient of friction between the tires and the track?
M = 0.75 r = 3.0 m Minimum speed =?
Components as Centripetal Force Windseeker at Cedar Point Maximum Diameter = 110 ft = 33.5 m Velocity = 10 rpm What is the angle of the swing arm?
Mass of dog = 20kg Length of chain = 2.5m Velocity of dog = 3.0 m/s Tension on each chain =?
Force diagram at the top? Force diagram at the bottom?
Components as Centripetal Force Ted Ligity m=95kg V=15m/s ϴ above hz = 20 What is the radius of the turn?
Components as Centripetal Force Horizontal Component of Normal Force is directed toward the center of turn. Horizontal Component of Normal Force contributes to Centripetal Force