Types of Forces Pressure Buoyant Force Friction Normal Force
Pressure Ratio of Force Per Unit Area p = F A P = N/m 2 = 1 pascal (very small) P= lbs/in 2 = psi = pounds per square inch Example: Snow Shoes increase surface area, reduce pressure, reduces the amount you sink.
Skis Determine the Pressure on each of the following skis Area = 165cm *7.5cm Mass of skier =70 kg Area = 180cm *12.5cm Mass of skier =70 kg
Atmospheric Pressure 14.7 psi = 101.3 kpa = 29.3 inhg = 1 atm Ears popping
Scuba Diving Every (10m) 33 feet of water (10m) = 1 atm The Bends: Rapid Decrease in Pressure The Squeeze: Rapid Increase in Pressure
Tire Pressure Contact patch supports weight (F = PA) Lower Pressure requires larger contact patch
Determine the area of the contact patch for each bike below if they belong to a 160 lb rider with mad skills! 120 psi 25 psi 8 psi
Pressure is transmitted evenly throughout a fluid Hydraulic Brakes for cars Jacks or changing tire
Pressure is transmitted evenly throughout a fluid P 1 = F 1 A 1 = F 2 A 2 = P 2 R= 3.5cm F=? R= 1.25cm F= 25N
Buoyant Force Upward Force exerted by a fluid Equal to the weight of the fluid displaced
Buoyant Force Equal to the weight exerted by the fluid Mass = Volume x Density Weight = mass x g Bouyant Force = Volume * Density * g F b = V ρ g Density of water = 1 g/cm 3 = 1000 kg/m 3 Density of sea water = 1 g/cm 3 = 1025 kg/m 3 Density of air = 1.29 kg/m 3 Density table on pg. 256 of text book
A spherical balloon filled with helium has a radius of 0.20m is floating in air. What is the buoyant force acting on the balloon? What is the net force on the balloon?
Scientific Balloon: Volume expands as altitude increases because density and pressure of air decreases. Volume expands until pressure inside balloon equals pressure outside balloon Buoyant Force = Force of Gravity
An Aluminum Rod is submerged in Fresh Water. r= 0.025m L= 0.50m ρ=2.7*10 3 kg/m 3 What is the buoyant force acting on the rod? What is the net force on the rod? What is the apparent weight of the rod?
Apparent Weight mg = Fb + Wa W a = mg Fb
NASA Neutral Buoyancy Lab Apparent weight = zero
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 = mu) Normal Force (N) Force exerted by a surface perpendicular to and away from surface. Pg. 90 f = μn μ = f N
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
Kinetic Friction (sliding friction) A 50 kg skateboarder moving at 5 m/s skids on his face and comes to a stop in a distance of 1.2 m. What is the Coefficient of Friction (μ) between his face and the pavement?
Kinetic Friction (sliding friction) 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?
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
STATIC FRICTION: Rolling Tires Tires roll due to static friction No motion between contact patch and the ground. Anti Lock Brakes Rolling tires exert greater force than skidding tires
STATIC FRICTION 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?
STATIC FRICTION 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)
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 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 θ A 75kg skier accelerates down a 25 o slope. What is the Normal force on the skier? What is the acceleration of the skier? F g = mg N = mgcosθ F x = mgsinθ
FORCES ON AN INCLINE (friction) A 10kg puppy slides down a 30 o slide that has a coefficient of kinetic friction, µ k =.20 What is the normal Force on the puppy? What is the force of friction on the puppy? What is the acceleration of the puppy?
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?
FORCES ON AN INCLINE (friction) A 50 kg snowboarder sits on a 20 o slope. What is the minimum coefficient of static friction required to keep the snow boarder at rest?
By rule, speed suits for ski racing must have a minimum coefficient of friction of 0.90. If a skier crashes at 30 m/s on a 30 degree slope, what distance is required to stop? Ski Crashes
Initial speed at bottom of ramp = 15 m/s angle of the 2 nd ramp = 20 o, If zero friction, how far up the ramp does he slide? If μ k = 0.70, how much time is required to stop?
Average coefficient of kinetic Friction: μ k =.85 Length of skid = 20m What is the initial speed of the car?
Mass of person = 60kg μ s hands =.60 μ s shoes =.80 With what force is the person pressing on the wall?