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