What’s happening?

Before Isaac Newton, nobody gave too much thought to what made us fall back to Earth after we jump, or why we can’t float into the sky. This 17th century English philosopher had an interesting thought – he wondered if it had something to do with the reason planets stayed in orbits around the Sun. He came up with some rules about gravity and forces, explaining how all things moved.
Gravity is the force that pulls masses together. More mass means more gravity. The Earth’s huge mass is bigger than ours, but we still pull on the Earth just a tiny bit. The Earth also pulls on the water in the bottle, as well as all of the dirt particles in it. Newton came up with a rule that said there was a relationship between the mass of an object, how fast it was accelerated and its force. Dense particles of dirt – when accelerated (made to move) by gravity – will push harder than smaller, lighter bits of dirt. This means the heaviest, most dense bits of mud will have enough force to push to the bottom of the bottle, while the light water particles will remain on top. You can see this gradually happen if you leave a container of muddy water sitting still for a few days. This is usually called ‘settling’.
Your bicycle did exactly the same thing, only it used a phenomenon called ‘inertia’ in place of gravity, which sped up the process. In some ways, they are very similar forces and have similar effects on objects. Inertia can even be measured in units called ‘G’, where 1G is the same as the force of gravity here on Earth. To learn more about how inertia works, see this Science by Email activity.

Applications

Centrifuges are machines that use inertia to create a force similar to gravity. Sometimes the Earth’s gravity isn’t strong enough to separate dense particles out of liquids, so a centrifuge is used to create the same force as a stronger gravitational pull.
Medical scientists in pathology laboratories often need to separate your blood cells from the liquid plasma that surrounds them. They do this by spinning them in a centrifuge, however if the tube is shaken or tipped upside down, the blood is mixed again. To prevent this from happening, jelly-like goo is put into some tubes to act like a plug. Once the blood is spun, the red blood cells are pulled under the goo and trapped in place, leaving the less-dense plasma to sit on top of the plug for the scientists to test.
Next time you get your blood taken, ask if you can have a look at the different collection tubes and spot any that have the goo in them.

You will need

• 350 ml clear plastic bottle (or smaller)
• Muddy water – the muddier, the better
• 2 zip ties (cable ties)
• Scissors
• A bicycle (and helmet!)

What to do

Note: The bottle must be small enough to sit between the spokes of a bicycle wheel. Small water bottles or fruit juice bottles work well.

1. Clean the labels from your bottle.
2. Fill your bottle with the muddiest, dirtiest water you can find. Add some extra dirt and muck if you don’t think it’s muddy enough.
3. Place the bottle in between the spokes of the front wheel of your bicycle. Make sure the wheel will turn completely without the bottle touching the ‘fork’ of the frame as it passes through. If it does touch the fork, you might need a smaller bottle.
4. Secure the bottle in place with one or two zip ties, clipping the ends from the ties to make sure they don’t interfere with the wheel’s rotation.
5. Go for a ride (don’t forget to wear your helmet!). Find a long stretch where you can peddle fast.
6. Make sure your bottle is pointing to the ‘six o’clock’ position (closest to the ground) in your wheel when you stop. Take a look at the material inside. Is the water much clearer? Can you see layers in the mud?
   

   	A bottle of muddy water.
   	Secure it in the front wheel of your bike.
   	After a ride, see how much clearer the water is.