CARSAnd Forces - Friction

FrictionFriction

To be able to:

All Most Some

Say what is meant by friction

(MYP 1/2)

Explain why we sometimes slip

(MYP 3/4)

Analyse different surfaces to find the best for trainers

(MYP 5/6)

Starter – Say what you know about forces

Would it be really good if the world was

really smooth

A force between two surfaces that are sliding, or trying to slide across one another

Friction always works in the direction opposite to the direction the object is moving (or trying to move).

Moving

Friction

It always slows down a moving object.

What is What is Friction?Friction?

Friction occurs when any two surfaces move against each other and tries to stop the movement.

Heat is generated as a result of friction.

Which type of surface generates more friction,a rough surface or a smooth surface?

Put your palms together and rub hard. What happens?

You are creating the force called .

Friction – Take the rough with the smoothFriction – Take the rough with the smooth

How strong is friction?How strong is friction?• Watch the video• When the teacher pauses the video (1min 50 sec), write

down why it is difficult to pull the books apart• Then show your explanation to the person next to you …• … and look at their explanation• Do you agree?

Friction depends on two Friction depends on two things:things:

• the type of surfaces that are touching (e.g. waxed kitchen floor versus rocky pavement)

The rougher the surface, the more friction.

• the force pressing the surfaces together (e.g., pulling an empty wagon versus one filled with bricks).

The heavier the object, the more friction.

?

?

Friction prevents our shoes slipping on the pavement when we walk and stops car tyres skidding on the road.

Useful FrictionUseful Friction

Ice causes very little friction, which is why it is easy to slip over on an icy day. But this is a good thing for ice

skating and sledging.

Useful FrictionUseful Friction

OOPS!OOPS!

StarterStarterIs it important to oil a bike? Why? Are there any parts you don’t oil?

Why?

Make notes in your book

One more? Probably the most important…

On the diagram label all sources of friction.

tyre and road

brake pad and rim

wheel bearingwheel bearing

pedal bearing

links in chain

Click for answers

Air resistance or “Drag”

FrictionFriction

Moving parts inside a car engine are lubricated with OIL, to reduce friction

between them.

The oil holds the surfaces apart, and can flow between them.

The reduced friction means there is less wear on the metal, and less heat

produced.

Reducing FrictionReducing Friction

Placing BALL BEARINGS between two surfaces allows them to move

more easily past one another.

Reducing FrictionReducing Friction

Reducing FrictionReducing FrictionOur bodies, like machines, also have

moving parts.

Where bones slide over each other at joints it is important to reduce friction.

Because of this there is a lubricating fluid between them to make them slide

more easily

If this lubricating system doesn’t work properly, you get swelling and pain in

your joints called “arthritis”

Ways to Increase Ways to Increase FrictionFriction

Make surface rougher. (Buy new tyres)

Increase the force pushing surfaces together.

(Push harder when scrubbing.)

Increase the surface area between surfaces. (Use snow shoes.)

5 Types of Friction5 Types of FrictionSliding friction

Rolling friction

Fluid friction

Static friction

Air Resistance

Sliding FrictionSliding FrictionMoving surfaces slide

against each other.

Rolling FrictionRolling FrictionUsing wheels or spheres between two surfaces.

Fluid FrictionFluid FrictionOpposes motion between objects travelling

through fluids like air and water.

Static FrictionStatic FrictionOccurs when a force is applied to an object, but the object

does not move.

Air ResistanceAir ResistanceA force which opposes a moving object. This

can be easily seen with falling objects.

When a piece of paper and a crumple piece of the same type of paper are dropped, they fall

at the same rate.

A penny and a feather dropped in air will not fall at the same rate, but a penny and feather dropped in a vacuum (no air) will.

Measure the friction forceMeasure the friction force

INVESTIGATIONWe will investigate friction and surfaces. We will change the ………….. ( Independent variable)

We will measure the ……………….. using a …………………..(Dependent variable)

We will keep these things the same : ( Control variables)The ob…….… The N………………….. The s………… The angle of the s…………..

Material Force, N

Bench top  

Carpet  

Paper  

Floor  

Foam  

Rubber  

Vinyl  

Plastic  

Plastic ( with oil)  

These surfaces are examples only – write down the surfaces you try

Can you trust your results? How can you test if they are reliable?

Draw a bar chart of your results

Practical: Factors Practical: Factors Affecting ForcesAffecting Forces

Friction RevisionFriction Revision

Investigating grip in shoesInvestigating grip in shoesRuth wanted to investigate which of three different shoes had the most grip. Ruth put bags of sand in

each shoe to hold them down. She then pulled them over a surface with a force meter.

a) What variables would Ruth have to keep the same to make the investigation fair?

The shoes must be pulled over the same surface, with the same mass inside each of

them.

b) Using clues from the diagrams of the shoes, predict which shoe you think will have the greatest grip.

The trainer – has many large ridges so would be able to catch onto rough surfaces easily.

The flat shoe – has a large area of the sole in contact with the surface, which might make the levels of friction high.

The stiletto heeled shoe – the heel might dig down into certain surfaces and give a very good grip.

Frictional ForcesFrictional Forces