CARS And Forces - Friction

Friction To be able to: AllMostSome 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 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 smooth

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 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 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 Friction

OOPS!

Starter Is 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 bearing pedal bearing links in chain Click for answers Air resistance or “Drag” Friction

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. Press the car Reducing Friction

Placing BALL BEARINGS between two surfaces allows them to move more easily past one another. Reducing Friction

Our 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 Friction 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 Friction Sliding friction Rolling friction Fluid friction Static friction Air Resistance

Sliding Friction Moving surfaces slide against each other.

Rolling Friction Using wheels or spheres between two surfaces.

Fluid Friction Opposes motion between objects travelling through fluids like air and water.

Static Friction Occurs when a force is applied to an object, but the object does not move.

Air Resistance A 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 force INVESTIGATION We 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…………..

MaterialForce, 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 Affecting Forces

Friction Revision

Investigating grip in shoes Ruth 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 Forces