﻿ Article Title | Interact | physics.org

This is a good visual demonstration of forces, and in particular vortices. This demonstration does require a specialised adapter, but it is readily available throughout the country and the demonstration is striking enough to warrant its inclusion in the list.

## Ingredients:

• two large (~2 litre) empty soft drink bottles
• water

## Instructions

1. Fill one of the soft drink bottles with water until it is mostly full.
3. Invert the empty bottle and screw the opposite thread of the tornado adapter into the water-filled bottle.
4. Turn the whole contraption upside-down.
5. Grasp the top and middle of the contraption and spin it in a circular motion – either clockwise or counter-clockwise.
6. Once a vortex (whirlpool) forms in the upper bottle, stop spinning – you should see the vortex form throughout the liquid, and continue as long as there is liquid in the upper bottle.

## How Does it Work?

This demonstration produces a 'vortex' such as those observed in cyclones, tornadoes and whirlpools. As the water spins around the bottle there is a downward pull formed due to the water passing through the opening into the empty bottle below. The initial small rotation caused by spinning the bottles gains speed as the water is sucked through the opening. As the rotation speeds up the vortex forms.

## Tips for Success

Try adding food colouring to the water for a more colourful visual effect. Practice making the vortex until you're sure how to do it. If it doesn't work straight away, try reducing the size and increasing the speed of the circles you are making. Make sure that the bottles are held vertically and your circles are centred about the long axis of the contraption.

Replace the plastic bottles if they get too bent out of shape – they need to be round to make a proper vortex.

## Serving Suggestions

This is a good visual trick suitable for all ages. It works best with small groups, where each audience member can have a chance to create the vortex. It's also a good demonstration to leave the audience to perform on their own while the demonstrator has a well-earned break!

## Did You Know?

It is well known that water always go down the plughole in a clockwise direction in the northern hemisphere and counterclockwise in the southern hemisphere. Unfortunately this is not scientifically valid! What is more interesting is that the whirlpool was first mechanically induced in a bathtub in 1968 by Roy Jacuzzi. Now, what ever became of him?