Science Experiments

Hurricanes in a Lemonade Bottle

Sun, 19th Mar 2006

Part of the show Invasive Species, Conservation and the Last Giant Tortoise

What you Need

A lemonade bottle

Some water

A sink

What to do

1- Fill the lemonade bottle with water.

2- Turn it upside down empty the bottle - what happens?

3- Fill the bottle again

4- Empty the bottle again, this time swirling it - what happens?

What may happen

You should find that if you just empty the bottle it empties quite slowly but if you swirl it, the water creates a whirlpool and empties much quicker.

Why does it happen?

For the water to get out of the lemonade bottle something has to take its place, and the most convenient thing to do so is air. So the air is trying to move up whilst the water moves down. There is obviously a conflict here, ther normal way is for some water to fall out then a bubble moves up through the neck, allowing more water to fall out. The water comes out in big lumps, which is actually quite a slow process, because it has to keep speeding up and slowing down.

Emptying a BottleAn upended bottle

Upended swirling bottleHowever if you give the water a bit of a swirl as you empty it, something entirely different happens. Although the water is only spinning quite slowly to begin with, as it moves inwards towards the neck it speeds up to the point where it is thrown out to the sides of the bottle by centrifugal force* so strongly it forms a hollow tube with an airspace in the centre. This allows the air to get into the bottle much more quickly than as bubbles because it is a continuous process, so even though the water is flowing out through less of the neck it will flow faster, and the bottle will empty more quickly.

Why does the the water spin faster in the neck?

There are two effects which contribute to this. Even if the water was going at the same speed at the neck of the bottle as the outside, because the circle it has to travel in is much smaller it would take far less time for it to do a rotation, so it would be spinning faster.

Also when the water moves into the centre it has to fight against centrifugal force this uses energy, and this actually goes into spinning the water even faster.

What has this got to do with hurricanes?

A hurricane starts to form over some particularly warm water, this heats up the air making it lighter and the water evaporates producing water vapour which is also lighter than air. This warm wet air begins to rise sucking cooler air in at the sides. Because the world is spinning the air being sucked in is spinning slowly to start with, as it is pulled in it gets faster and faster (just like the water in your bottle). If the hurricane is large enough this can produce winds of up to 225kph (140mph), quite capapble of hurling trees, cars and houses all over the place.

* Centrifugal force is a pseudo (pretend) force that can be useful in thinking about rotating objects - it is the force that is applied to something else (a string, the ground etc) that is needed to keep an object (which would naturally go in a straight line) travelling in a circle. Although it is not real it is often a useful way of thinking about things.

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