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!