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Science Fair Project: Get on Your Bike and Learn All About Centrifugal Force

This project is good for wannabe physicists and you can actually watch as constituents are separated on your own homemade centrifuge.

Salad dressing made from oil and vinegar will separate out even if it stays in the refrigerator for a while. This is because vinegar is denser than oil, so each cubic cm of vinegar feels a larger force from gravity than a cubic cm of oil. The vinegar is thereby pulled down harder and it gravitates towards the bottom of the receptacle.

Mustard acts to stop the oil droplets from sticking together and growing. Large drops float more quickly than small ones so it makes the mixture last longer.

When things are spun around on a bike or a cetrifuge, there is a force which throws everything away from the centre of rotation. We call this centrifugal force, which like gravity is proportional to something's mass. So dense liquids like vinegar will feel more centrifugal force than less dense ones like oil, which separates them out.

The size of the centrifugal force quadruples every time you double the rate of rotation, and for an adult bike if you rotate the back wheel about 6 times a second, the forces will be equivalent to about ten times normal gravity.

Some mixtures, called colloids, don't separate under normal gravity no matter how long you wait. This is because the small vibrations caused by impacts with water molecules are strong enough to keep the colloid mixed. One such colloid is blood, the blood cells are so small that they will never separate out on their own, so when you donate blood your blood is spun in a centrifuge to separate out the components. The centrifugal force is strong enough to overcome the thermal mixing and the cells sink to the bottom.

Similarly nuclear power stations work by splitting U-235 atoms but most uranium is U-238, which is slightly denser. The fuel is enriched in U-235 by spinning a uranium solution in a very fast centrifuge and collecting the lightest fractions.

To do this science fair project, you will need a bicycle. A two wheeler or a three wheeler will be fine. You will also need two small clear plastic bottles that will fit between your spokes,. One of these bottles will be used as a control. The other will fit between the spokes of the bike.

For some salad dressing, you can use oil and vinegar and you will need a little mustard.

You will also require tape and cardboard, or other tube to insert the plastic bottles.

Make some salad dressing using 2/3 oil , 1/3 vinegar and a little mustard to act as an emulsifier and to stabilize the emulsion. Make a card tube to fit around the plastic bottle.

Attach the tube to the rear wheel of a bike between two spokes or sitting against the rim. Use all the tape you need to make certain it is very secure. Now you should turn the bike upside down, and make certain that the bike is very stable. Shake up your salad dressing really well, and split it into two halves, put half in one bottle, and half in another one to act as a control. The bottle will move very fast, so if your tube fails, it could fly off very rapidly. So make sure that no-one is standing in front of or behind the bike, and the person winding the pedal should wear goggles. Put the bottle of salad dressing into the tube and spin it for five seconds. Try to stop the wheel at the bottom. Do not put your fingers or anything else near the spokes. Next time try spinning for ten seconds. Then for fifteen seconds and so on. Record what you see in terms of separation each time.

Compare the two bottles, is there a difference? You should find that the bottle that has spun around has separated far more than the one that did not.

READ ON for more information about this "forceful" project.


 



 

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