Friction is a force that opposes motion betweentwo surfaces that touch each other.

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You will find friction everywhere that objectscome into contact with each other.

The force acts in the opposite direction to theway an object wants to move.

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1. The type of surfaces in contact.

2. The amount of force pressing the surfacestogether.

The amount of friction between two surfaces depends on:

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What Creates More Friction?• Rough Surfaces

– Rough surfaces have more microscopic hills & valleys.

– The rougher the surface, the greater the friction will be.

• Greater Force– The amount of force exerted increases the amount of

friction– Changing the amounts of the surfaces that touch

does NOT change the amount of friction.

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Friction is greater when surfaces are pushed together with more force.

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To the naked eye a material may seem smooth, suchas the polished mica pane shown below.

At the microscopic level, even the smoothest surfaces are covered with dips and bumps.

Picture of extremely smooth mica with a scan probed microscope.

If you look at it at mica at the microscopic level, it will appear rough.

The tips of the bumps are the only points that touch theother material. Only a very small portion of the apparentsurface area is in contact with the other surface.

This causes extremely high pressure to form on the parts that touch. The microscope ridges between two surfaces can get stuck in the grooves of the other, effectively creating a type of mechanical bond, or microweld, between the surfaces. 6

A force must be applied to break these welds.

Friction is independent of surface area. Changing the amounts of the surfaces that touch does NOT change the amount of friction

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Friction converts kinetic (motion) energy into thermal (heat) energy.

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There are two kinds of friction: static friction and kinetic friction:

Static friction is the force between two stationary objects.static friction is always greater than kinetic friction.

Static friction prevents an object from moving against a surface. It is the force that keeps a book from sliding off a desk, even when the desk is slightly tilted, and that allows you to pick up an object without the object slipping through your fingers.

In order to move something, you must first overcome the force of static friction between the object and the surface on which it is resting.

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Once two objects are in motion it is easier to keep them in motion.

When two surfaces are in motion with respect to each other there is Kinetic friction.

There are several types of kinetic friction, including sliding friction and rolling friction.

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Sliding friction resists the motion of an object as it moves along a surface.

Sliding friction, is created by flat objects moving against each other.

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Sliding FrictionShawn White

Sliding Friction

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Rolling friction hinders the motion of an object rolling along a surface.

Another force must be present to keep an object rolling. For example, a pedaling bicyclist provides the force necessary to the keep a bike in motion.

When one surface is rolling over another surface there is rolling friction.

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When the Great Pyramids were being build in ancient Egypt, they used logs as rollers under the giant blocks of granite instead of trying to slide the rocks along the ground.

Friction Can Be Harmful or Helpful

• Harmful friction– Holes in socks or knees of jeans– Wind & water cause erosion

• Helpful friction– Friction between pencil & paper– Without friction, you would just slip &

fall when you tried to walk

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Rotors and brake shoes on a Porsche.

Brake pads in your car help produce friction to slow you down

We have contours in our skin's surface called friction ridges that help the hand to grasp by increasing friction. Friction ridges occur in patterns (fingerprint).

The skateboard has a layer of grip tape on its surface for the sole purpose of creating friction between the skateboarder's shoe and the skateboard.

This is what keeps the board under the skateboarder's feet as he jumps into the air. This friction allows skateboarders to flip the board, jump with the board, and stay on the board at all times.

In much the same way, skateboarders turn in midair by twisting their arms and legs in opposite directions. Upon landing, a skater can use the friction between his or her feet and the skateboard to twist the upper body back into alignment.

Sometimes lack of friction can be harmful!

A cracked and worn disc brake pad

In machines with moving parts, friction increases wear,produces heat and reduces efficiency.

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While friction allows us to convert one form of motion to another, it also converts some energy into heat, noise, and wear and tear on material.

Losing energy to these effects often reduces the efficiency of a machine.

In automobiles and airplanes, friction converts some of the energy in the fuel into heat, noise, and wear and tear on the engine’s parts.

Excess frictional heat can damage an engine and braking system.

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Averaged over urban and highway conditions, the average mechanical efficiency is around 52% for typical automobiles.

Much of the energy is lost in overcoming friction in operating the engine.

The types of friction include energy necessary for pumping (moving the intake and exhaust valves), energy to overcome rubbing friction, and energy required to operate accessories such as the air conditioner.

http://www.parcon.uci.edu/paper/Conservation/energy.htm

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More friction means more heat and less motion.

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Kick Start Gear in a motorcycle engine

Friction Reduction

• Ways to reduce friction– Lubricants- Substances applied to

surfaces to reduce the friction between them

– Switching from sliding friction to rolling friction by the use of ball bearings

– Make surfaces that rub together smoother

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Remember:Friction is a force that opposes motion.

Standard friction equation

Friction = coefficient of friction x normal force.

Measures of friction are based on the type of materials that are in contact.

Each material has what is known as a coefficient of friction. The coefficientcoefficient is a measure of how easily one object moves in relationship to another.

When you have a high coefficient of friction, you have a lot of friction between the materials.

Concrete on concrete has a very high coefficient of friction.

A material such as Teflon (on most things) has a very low coefficient. Teflon is used on surfaces where we don't want things to stick; such as pots and pans.

How is friction measured?

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= coefficient of kinetic friction

= coefficient of static friction

FN = the normal force

μkμs

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μ= Symbol for the Greek letter mu, used in the formula to represent the coefficient.

The coefficient of kinetic friction between a block and the level surface it slides on is 0.45.

If the mass of the block is 10.0 kg, what is the minimum force needed to keep the block moving with uniform motion?

The applied force need only balance the kinetic frictional force in order to maintain uniform motion.

Fk = µ kFN

Fk = µ kFN

Fk = (0.45)(10.0)(9.8 m/s2

Fk = 44 N

A student pulls a 5.00 kg object and discovers that sheneeds to exert 30.0 N of force before the object moves.

What is the coefficient of static friction between theobject and the surface on which it rests?

Since the applied force isEqual to 30 N just beforeMoving, the friction forceMust be equal in size (30 N).Therefore:

A 7.30 kg box is at rest on a level table. The coefficientOf static friction between the box and table is 1.03.

How big is the static frictional force?

There is no static frictional force acting since there is no net force attempting to move the box.

The static frictional force could, at most be equal to μstaticFN