Friction, air resistance

How important is friction?

• Think of the mousetrap powered vehicles

• Think of your life

• What would happen if there was no friction in the world?

What would change in your life if you lived in a world with no

friction?

The Evil Mr. Baker

And the Switch

FRICTION PARAGRAPH

Friction Paragraph

• Identify a specific place and action you would be doing when all friction of the world is turned off.

• Describe at least 4 things that would be altered related to what you are doing as a result of the loss of friction

• Write a paragraph with opening and closing sentence, plus the above description of alterations

• Due tomorrow

Next topic…

• Write about what would be different in the way our species evolved over time if there was no friction on Earth.

• Think of at least 4 things that would change about us if there was no friction

EFFECTS OF FRICTION ON YOUR LIFE

Dangers of friction

• Engines

Reduce or modify friction by introduction of lubricants

Smoothes out surfaces

Where does friction help in our lives?

Some Helpful examples

• Cleaning baked on food from pans/dishes

• Placing a book on papers in front off a fan

• Car tires gripping the road

• Keeping mucus inside of the nose

Can you think of any more examples?

Hydroplaning

• When a layer of water separates the tire from the rough road surface

• Instance of no friction, results in no control over vehicle

DEFINITION OF FRICTION

Friction definition

• A force that resists the motion of one material in sliding past another

• Works with all states of matter

Friction

• Force applied opposite to the direction of motion

• Amount of force depends on:– The roughness of the surfaces being put

together– The force used to push surfaces together

Friction forceApplied force

HOW DOES FRICTION OCCUR?

What makes friction?

• Force is a combination of physical interaction between the 2 surfaces– The meshing of peaks and valleys

• The temporary bonds formed between molecules on the 2 surfaces

TYPES OF FRICTION

STATIC VS KINETIC VS ROLLING FRICTION

Static friction

• When an object is on a rough surface, an applied force is needed to overcome the frictional force keeping the object at rest

• This frictional force is called static friction

Maximum static friction

• Occurs at the point when the object is just about to move.

Depends on the normal friction requirements (surface conditions and amount of force pushing surfaces together)

Plus

The amount of force applied to object trying to make it move

Kinetic friction

• The frictional force trying to slow down a sliding object in motion is called kinetic friction

• Kinetic friction is always less than max. static friction (less bonding forces to overcome)

Rolling friction

• Friction that occurs between surfaces as one or both move past each other

• Rolling friction is much less than either of the other types

• Tires and Road

DIRECTION OF THE FRICTIONAL FORCE

Direction of friction

• Parallel to the surface contact

• In a direction that opposes slipping between the two surfaces

• Generally one surface does move, so friction opposes the motion of the other

DETERMINATION OF THE AMOUNT OF FRICTION

Calculation of friction

Ff = µ Fn

Ff = Force of frictionµ = Coefficient of friction

Fn = Normal force

How to calculate the frictional force

• Calculate static and kinetic friction the same way using the same equation

Coefficient of friction

• A constant that indicates the ability of the two surfaces to interact with each other

• Usually a number between 0 and 1

• The higher the #, the more interaction (rougher surfaces)

• No units, a dimensionless value

Normal force

• Reactive force of the “ground” pushing the surfaces together

• Normal means perpendicular

• Force’s direction is away from and perpendicular to surface

Determination of Normal Force

Fground (Normal force)

Fgravity

Weight

Foot

Normal forceGround pushes back with enough force to match both weight and foot

Calculating normal force

• In this case the force of gravity = the reactive force of the ground pushing up (?)

• The force of the ground pushing up = the normal force (it is = to the force pushing the surfaces together)

FORCES AND CONSTANT VELOCITY

Forces and constant velocity

• What can be said about the forces on a block being pulled along rough ground at constant velocity?

Fapplied

The balance of forces

• For something to move at constant velocity, the forces must be balanced.

• Balanced means the net force on the object must = 0

• Fapplied = Ffriction

• Fgravity = ?

Fapplied

Ffriction

Fgravity

?

WORDS IN PROBLEMS AND FRICTION

Words that indicate amount of friction

• Smooth surface indicates a frictionless surface

• Any thing else indicates a surface that generates a friction force

• Friction related values

The box problem

• A 13.8 kg box is sliding horizontally at a constant velocity over a rough surface. It is being pushed by a hand with 59 N of force.

• Draw a free-body diagram of the box• Determine the force of friction• Determine the normal force on the box• Determine the coefficient of friction

between box and surface

Example question

• A wooden block is placed on a smooth wooden tabletop. You find that you must exert a force of 14 N to keep the 40 N block moving at a constant velocity.

a) What is the coefficient of sliding friction for the block and table?

b) If a 20 N brick is placed on the block, what force will be required to keep block and brick moving at a constant velocity?

ADD FRICTION FORCE TO ALL PREVIOUS TYPES OF FORCE PROBLEMS

2nd Law, Third Law, Equilibrium…

Homework in textbook

• Pg 130 22-24, 26

• Pg 142 90-94

AIR RESISTENCE

Air creates a force

• Each air molecule that hits the body creates an upward force

• The combined effort of all these molecules pushing against the body creates a force opposing gravitational pull of Earth

Air Resistance

• Air resistance can play a significant role

at high speeds for all objects

at low speeds only for lightweight objects with a lot of surface area

On Earth, are objects that fall through the sky in freefall?

Air and fluids act the same way

• Greater density of molecules in the fluid, therefore greater friction

• Fluid friction differs from air resistance in whether the air molecules are impacting on the object or trying to slide past

TERMINAL VELOCITY

Is this man in free-fall?

Sky Diving

Terminal velocity

• The maximum velocity reached by a dropped object

• Occurs when the downward gravitational force (weight) is matched by the upward force of air resistance

Factors that determine air resistance

• Air resistance depends on mass, surface area and speed of object

• Air resistance is more significant with:– Less mass– More surface area– At greater speeds

Terminal velocity

• At a high enough speeds,

the press of air pushing up on the falling object = the pull of gravity

The object will then no longer accelerate

If you dropped a spider, a penny and you off a tall building, which will land first?

Gliding ants, balloon spiders