Pumping Straw - a centrifugal pump

19 April 2009

Ingredients

A straw A straw A skewer A wooden skewer
Sticky Tape A small amount of tape A Bowl of Water A bowl of water

Instructions

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Centrifugal pump

Half cut the straw a third and two thirds of the way along its length.

Push a wooden skewer through the centre of the straw.

Bend the two ends of the straw together and tape them to the skewer.

Place the pointy end fo the straw into some water and spin it about the skewer.

Result

You should find that if you spin the skewer that water will start to lift up the straws, and if you spin the skewer fast enough water will start to spray out of the sides. You are pumping water upwards.

Explanation

When you spin the straw it forces the water inside to spin. If an object is spinning anything on that object appears to feel a force pushing it outward. This pseudo-force is called centrifugal force.

This means that in your straw the water is pushed outward and the only way it can move outward is to move up the the straw - it is pumped upwards. If you spin the straw fast enough then water will exit the straw and fly outwards as it is now moving quite fast. 

Centrifugal pump spinning Centrifugal pump pumping
As the straw is spun centrifugal force pushes the liquid outwards, and the only way it can move outward is to rise. Eventually the water rises above the end of the straw and the water escapes.

How is this used in real life?

Many pumps use the same principle, they consist of a fluid inlet in the middle, and then an impeller spins the fluid, and centrifugal force pushes the fluid outwards and out through an outlet.

A centrifgal pump with impellor

This is a very simple pump to build, so most low pressure pumps such as vacuum cleaner pumps, washing machine pumps etc work on this principle.

Show Us Your Kitchen Science!

Randy Heisch, from Georgetown in Texas, very kindly sent us these incredible pictures.

Straw Pump 1 Straw Pump 2
Straw Pump 3 Straw Pump 4

Thanks Randy!

If you would like to show us your kitchen science experiments, send them to
Chris@thenakedscientists.com

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