Types of ForcesPressure

Buoyant Force

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!

110 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

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 fluidMass = Volume x DensityWeight = mass x g

Bouyant Force = Volume * Density * g

𝑭𝒃 = 𝑽 ∗ 𝝆 ∗ 𝒈

Density of water = 1 g/cm3 = 1000 kg/m3

Density of sea water = 1 g/cm3 = 1025 kg/m3

Density of air = 1.29 kg/m3

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= 0.50mρ=2.7*103 kg/m3

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

𝑊𝑎 = 𝑚𝑔 − 𝐹𝑏

NASA Neutral Buoyancy Lab• Apparent weight = zero

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)

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 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?

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?

Intitial speed at bottom of ramp = 15 m/sangle of the 2nd ramp = 15o, If zero friction, how far up the ramp does he slide?

If μk = .50, how far up the ramp will he slide?

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?