Junior science by Dr . Naomi Lavelle
Experiments you can try
What is Friction ?
Frction is the force that acts on two objects that rub off each other .
Dynamic friction
Static friction is the force that holds something still and prevents it from sliding .
is the force that acts on an object as it moves over a surface .
Lets learn more !
Friction works on a moving object to slow it down or stop it .
When an object slows down it loses energy - this energy is convered into heat or sound .
When you rub your hands together friction causes heat and your hands feel warm .
When you pull hard on the brakes of your bicycle it is friction that stops the bike .
Sometimes we hear a screech when we pull the breaks - some of the energy lost due to friction is converted to sound !
There are three types of friction :
1 . DRY or SLIDING friction ... where two solid objects rub off each other
2 . FLUID friction ... friction formed when an object moves through a liquid or a gas
3 . ROLLING friction ... friction produced between a rolling object ( like a wheel ) and a surface
Lift a jar of rice with a pencil !
Fill the jar with the dry rice and insert the pencil ( point side down ).
Dr . How ' s Science Wows !
Friction
You will need ... an empty plastic jar , a pencil , a bag of dry rice
Raise the pencil slowly up towards the top of the jar but do not remove it completely . Now slowly push the pencil back into the jar again and repeat several times . You should feel it getting harder to push the pencil down . If the level of rice in the jar drops then top it up with more rice . After a few goes you should find that when you try to lift the pencil up out of the jar , the whole jar lifts ! So what is happening ?
... exploring Friction !
Friction works on an object in the opposite direction to which the object is moving !
Factors that influence friction include : the roughness of the surfaces , the weight of the object and surface area
Each time you push down the pencil the rice in the jar is compressed . When this is done repeatedly the grains of rice are squashed tightly together and cannot move . This increases the friction between the rice and the pencil until eventually the pencil can lift the jar of rice .
Friction can often be a help to us ... if there was no friction between our shoes and the surface beneath us then we would not be able to stay on our feet .
It is friction between our food and the fork or spoon that allows us lift the food to our mouth .
Brakes in cars use friction to allow us to control the speed of the car .
Some musicians use friction to make music . Violin players rub rosin on their bow to help it stick better to the strings .
The CF is always a value between zero and one .
The coefficient of friction ( CF ) is a measurement of the amount of interaction between two surfaces . The CF is the ration of the friction to the weight of the object .
Metal moving over metal has a CF of 0.74
Skis on snow have a low CF value of about
Sometimes friction works against us ... such as slowing down an object that we want to move faster or wearing down the moving parts of a machine .
To prevent friction we might use an oil to lubricate moving parts to reduce wear and tear .
We also use small balls called “ ball bearings ” within machines and other moving parts . The ball bearings move more easily on a surface - reducing friction .
If we reduce the surface area of an object we can rduce the friction acting on that object - this is how ice skates work !
If you want to know HOW something works why not write to Dr . How and ask ? Send your e-mail to naomi @ sciencespin . com
Experiments you can try
You will need .. a balloon , an old CD , a drinking bottle top , and glue
Make a hovercraft !
0.15
Friction
Glue a bottle top over the whole in the CD and leave it until the glue has dried . Make sure that the drinking bottle top is closed . Inflate a balloon and pinch the neck of it while you place it over the bottle top . Place the CD on a smooth surface and open the bottle top . Give the CD a little push and see what happens !
So what is happening ?
The air from the balloon gets released and forms a cushion of air under the surface of the CD . This allows the CD to travel across the surface with a lot less friction - just like a real hovercraft !