types of forces - mmstc physics · 2020. 11. 30. · forces on an incline weight: force of gravity...
TRANSCRIPT
Types of ForcesPressure
Friction
Normal Force
Pressure• Ratio of Force Per Unit Area
• 𝑝 =𝐹
𝐴
• P = N/m2 = 1 pascal (very small)
• P= lbs/in2 = psi = pounds per square inch
• Example: Snow Shoes increase surface area, reduce pressure, reduces the amount you sink.
Skis
Area = 165cm *7.5cmMass of skier =70 kg
Area = 180cm *12.5cmMass of skier =70 kg
Determine the Pressure on each of the following skis
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 Squeeze: Rapid Increase in Pressure
• The Bends: Rapid Decrease in Pressure
Tire PressureContact 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!
90 psi 25 psi 8 psi
Pressure is transmitted evenly throughout a fluid
Hydraulic Brakes for cars
Jacks or changing tire
R= 1.25cmF= 25N
R= 3.5cmF= ?
Pressure is transmitted evenly throughout a fluid 𝑃1 =𝐹1𝐴1
=𝐹2𝐴2
= 𝑃2
FrictionForce 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 : 𝒇𝒌 = 𝝁𝒌𝑵
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 FRICTIONOccurs 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
• 𝑓𝑠 ≤ μ𝑠𝑁
EXAMPLES
Snowboarder Sitting on hill Unsuccessfully pushing on Couch
Coffee cup sitting on dashboard
Walking
STATIC FRICTION: Rolling TiresTires 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 μs = 0.30?
• If the same force is exerted after it starts moving and the coefficient of kinetic friction is μk = 0.20, what is the acceleration of the couch?
STATIC FRICTIONWhat 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?(μs = 0.40)
Sample: A Force of 75 N is used to push a crate (m=150kg) along the ground. If the coefficient of kinetic friction is (µk = 0.20) What is the acceleration of the crate?
FORCES ON AN INCLINEWeight: 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
mg
N
mg
N
θ θ
FORCES ON AN INCLINEWeight can be resolved into Components
• Perpendicular to the surface (Normal Force) (y-component: Fy)(N)
• Parallel to the Surface (Parallel Force)( x-component ) (Fx )
Fx
N
N
Fx
mg
FORCES ON AN INCLINE
𝑭𝒈 = 𝒎𝒈𝑵 = 𝒎𝒈𝒄𝒐𝒔𝜽𝑭𝒙 = 𝒎𝒈𝒔𝒊𝒏𝜽
A 75kg skier accelerates down a 25o slope.• What is the Normal force on the skier?• What is the acceleration of the skier?
θ
FORCES ON AN INCLINE (friction)• A 10kg puppy slides down a 30o slide that has a coefficient of kinetic friction, µk = .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 40o slope with an acceleration of 4.0m/s2. What is the coefficient of kinetic friction between the panda and the slide?
FORCES ON AN INCLINE (friction)• A 50 kg snowboarder sits on a 20o slope. What is the minimum
coefficient of static friction required to keep the snow boarder at rest?
FORCES ON AN INCLINE (friction)
A worker pushes a 250kg piano up a 10o slope that has a coefficient of friction of µk = .15, What force is required to push it up the ramp at a constant speed?
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/sangle of the 2nd ramp = 20o, If zero friction, how far up the ramp does he slide?
If μk = 0.70, how much time is required to stop?
Average coefficient of kinetic Friction: μk = .85
Length of skid = 20m
What is the initial speed of the car?
• Mass of person = 60kg
• μs hands = .60
• μs shoes = .80
• With what force is the person pressing on the wall?