You are going to:

1. Find out about Friction
2. Compare two different surfaces and work out how many times greater the friction is on one surface compared to the other
3. Investigate some floor coverings and find out how safe they are
4. Devise your own friction investigations

Task 1
What is Friction?
Study these pictures and discuss what is happening. Use the words Force, Motion and Friction in your discussion.

The Egyptians used rollers to help them move heavy stone blocks. A very large force is needed to move a heavy object simply by pushing it along the ground. The bottom of the block would scrape along the ground and would resist any pushing force trying to move the block. This resistance to motion by one surface rubbing on another is called Friction.

Friction is a Force which opposes Motion

The rollers keep the block off the ground so it can’t scrape and because the rollers can turn they don’t scrape as much on the block. The end result is that it is much easier to move the block on the rollers because the Friction has been reduced. When there is less friction it takes less Force to make something move.

The racing car needs lots of grip on the road surface or it will skid off at high speed. The grip comes from the high frictional force between the very wide rubber tyres and the road. Thinner tyres would have less contact with the road so there would be less friction and so the car would skid too easily. Another way to increase the frictional force would be to make the car heavier so the rubber tyres press on the road with greater force but this would make the car slower so it could never win a race! The picture shows smoke coming from the tyres because the engine is so powerful that it can force the wheels to go round and overcome the frictional force causing the tyres to lose grip on the road and to spin - ‘burning rubber’.

The spacecraft is glowing on the outside with heat made by friction. But what is rubbing against a spacecraft on the outside? It is the gas molecules (tiny particles) in the atmosphere of a planet. This is a landing vehicle on the way to Mars and the friction between the outer surface of the spacecraft and the atmosphere of the planet is so great that part of the spaceraft burns away on entry.

Friction causes heat

People knew this a long time ago and used friction to make fire. How?

Curling is a popular sport. The player crouching on the ice in the middle is pushing a heavy ‘stone’ and letting it slide along the ice to a target. Friction is very important in curling. Both low friction and high friction are needed or you can’t play! Ice is slippy so there is very little friction. This means that you don’t need much force to make the stone slide along the ice and it will keep on going a long way once you have started it off with a push. But how do the players manage to stand up and give the stone a push without slipping themselves and why are they carrying sweeping brushes?
This is the Olympic Gold medal winning team from Scotland - masters of friction!

Task 2
How can we measure Friction and compare the Frictional force on two different surfaces?

For this investigation we need a way of placing different surfaces together and measuring how much force it takes to make them slide over one another. Here are some items which can be used.

1. wooden block with rubber sheet or lino etc. glued on one side and hook fitted
2. string
3. plastic cup or bag
4. marbles

Demonstration
Try lying the block on the desk, take hold of the hook gently and pull the block towards you.
Now try the other side of the block.
What difference do you notice?

You can probably tell that there is a difference but how can it be measured?
The block has to be made to slide but if you push or pull it you don’t have anything to measure!

Work with a partner or small group to make a plan showing how you are going to carry out the investigation using only the equipment provided. Keep reminding yourself of what you are trying to find out! Draw and label a rough sketch of what you intend to do with the block etc.

The difficult part to appreciate is that you have to do the practical experiment with the block but you cannot provide the Force with your own muscles to move the block and make it slide since your own Force cannot be measured! How else could a Force be applied to move the block? Can the marbles be of help?

Here is a sketch drawn by a pupil.

Plan
My plan is to count how many marbles it takes to make the block slide on the wood side then try the same thing on the rubber side and compare the results.
step 1   Attach string to block and place wooden side down on the desk.
step 2   Hang string over the end of the desk and attach plastic cup to the end.
step 3   Add 1 marble at a time to the cup until the block slides across the desk.
step 4   Write down the number of marbles it took.
step 5   Repeat for the rubber side of the block.

Results
Number of marbles to make the wooden side slide    4
Number of marbles to make the rubber side slide      12

At this point we should look back at what we were trying to find out then we will be clearer about what to do with the results.

We are trying to find out How many times greater the frictional force on one side of the block is compared to the other.

Discussion of results
As the marbles are added the force pulling down on the string increases.
The block remains still and this tells us that the force of friction between table and block is greater than the pulling force of the marbles. The greatest force wins!
As more marbles are added the pulling force on the string increases until it equals the frictional force trying to keep the block still on the desk. One more marble will be enough to make the pulling force greater than the frictional force and the block begins to slide.

What do we do with the numbers?
Many people would simply take 4 away from 12 and say that the frictional force on the rubber side is 8 times greater than on the wooden side but this is wrong!

You have not been asked to find the difference between the numbers! You have been asked to find out how many times greater one is than the other. In other words

What number do we have to multiply 4 by to make 12 ?

4 x ? = 12

The answer involves a product rather than a difference.

Finally we arrive at the answer
The frictional force on the rubber side is 3 times greater than on the wooden side.

Questions

1. What have we learned about Friction so far?
2. Why is it useful to know about friction between different surfaces?
3. Could the practical part of the investigation be improved so that it gives better results?
4. What other investigations could we carry out?

Answers

1. Friction is a Force which acts at the surfaces between materials and tries to stop them from moving.
   Without friction we could not walk because our shoes would slip on the floor
   Car tyres and shoes are made from rubber because it produces a large frictional force on roads and floors etc.
   Ice skating only works because ice offers very little friction to the ice skates sliding over it.
   Friction produces heat and can even cause fire.
2. If we understand friction and can measure it we can choose the correct materials for different situations. For example ice shoes worn by ‘curlers’ which have small spikes or rubber points to increase the friction and stop them from falling over. The heat shield on a spacecraft has to resist very high temperatures caused by the friction of entering the atmosphere of a planet so special ceramic tiles are used as insulators to prevent the spacecraft melting. Goalkepers gloves have rubber grip pads to help hold on to the ball and some modern football boots have rubber pads at the toe to help ‘bend it like Beckham’!
3. There are all sorts of improvements which could be made. The desk top should really be cleaned before each experiment since dust or finger marks could alter the amount of friction. The experiment should be repeated many times rather than just once to check that similar values are obtained each time and an average of the results should be worked out. The string hanging over the table edge could cause a problem since friction exists here also! Since it would be the same each time we tried the experiment then this would make it ‘fair’ when comparing values but unfair if the individual values were considered. The string might also catch in a groove at the table edge during one experiment then not in the next and this would be unfair. When the marbles are dropped into the cup there is a sudden jolt which might start the block sliding. How could this be prevented? - there are several ways!
4. Further investigations could involve blocks with different types of surface material eg. different types of rubber sole material from training shoes to find out which has the most grip for a particular sport.
   How could you compare the grip of various training shoes without having to cut pieces if rubber from the soles?
    

Task 3
Floor coverings Investigation

Introduction
Qn. Where do most accidents involving people being injured by slipping take place in the home?
Ans. In the kitchen and bathroom.
There is more than one reason for this but we are going to concentrate on the type of flooring used in kitchens and bathrooms and see what we can find out. Investigations like this could help keep accidents down!

Your aim is
To find out which type of flooring is safest for use in a kitchen or bathroom.

You are provided with samples of different types of floor coverings as well as the blocks and marbles etc. used in the previous investigation.
You must:
make a fully labelled drawing of what you intend to do in your experiment.
Describe your plan in steps.
Say what you are going to measure.
Discuss the results you obtain and state which type of floor covering is best.
Say what parts of the experiment might not be accurate and could be improved.

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