Inverted Bucket

27 January 2008


Bucket of Water A small bucket of water with a strong handle Some Space Some space with nothing breakable in it.


Fill your bucket with water, it is probably best to start with a little rather than a lot.

Stand so you can swing the bucket really easily, but make sure that there is no-one and nothing breakable in line with where you are about the swing the bucket (in case you let go or the handle breaks)

Swing the bucket back and forth with bigger and bigger swings.

When you think you will make it swing the bucket all the way over your head in a smooth motion.

Did the water fall out?


With any luck you should have stayed entirely dry.


The experiment works due to some very fundamental physics worked out by Isaac Newton. If an object is not pushed by anything it will continue moving at a constant speed and direction, so to make it travel in a circle you have to push or pull it towards the centre of the circle. If you have ever played on a roundabout in a playground this is the force you have to provide my hanging on tight to avoid being thrown off.

Moving in a circle
The natural state for an object is to go in a straight line at a constant speed. To make it go in a circle you have to pull it towards the centre with a centripetal force.

This means that you are pulling the bucket downwards while it is over your head. If you are pulling the bucket downwards faster than the water is being pulled down by gravity, the water will get left behind towards the base of the bucket and so, stay inside and not fall out, keeping you nice and dry.

Upside Down Bucket



Bucket above your head
If you just turn a bucket of water upside down you are holding the bucket up, so the water is pulled out from it by gravity. If it is going in a circle fast enough then you will be pulling the bucket down faster than gravity is pulling the water down so the water is left behind in the bucket and doesn't fall out.

A laboratory centrifuge © Magnus Manske


This centripetal force which is needed to keep something going in a circle increases the faster you spin something, So if you build a machine called a centrifuge that spins incredibly fast the forces can be immense.   Astronaughts and fighter pilots are trained to be able cope with large accelerations in a centrifuge. They are also used to separate things of different density, as dense things will try harder to keep going in a straight line than less dense things, this means that you can separate red blood cells from plasma, or different isotopes of uranium.

So what is centrifugal force then?

There is another way of looking at this effect, if you were standing on a huge roundabout, and you didn't know it was spinning it would seem that objects on the roundabout were pulled to the outside of it by a mysterious force we call centrifugal force.  From the outside of course the objects would just be trying to move in a straight line, but sometimes it is easier to think of a problem in terms of centrifugal force and if it makes the maths easier, why not.

If you are standing on a spinning object there is a second mysterious pseudo force called the Coriolis force, On earth this has the effect of making areas of air rushing towards a point spin, like cyclones or hurricanes.

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