Squashing Bottles

02 March 2008

Ingredients

A bottle A plastic bottle, 500ml smooth sided fizzy drinks bottles work well A jug A container nearly as deep as your bottle is tall

Instructions

Blow across the bottle, remember the note it makes.

Squash the bottle

Blow across it again.  Is there a difference in the note?

Fill your container about 3/4 full of water

See if immersing the bottle with water affects the note it makes, when it is both squashed and unsquashed.

Result

You should find that squashing the bottle makes the note go down.

When the bottle is immersed it will increase the pitch of the squashed bottle, but not the unsquashed one.

 

Explanation

VibrationWhen you blow over the top of the bottle the stream of air you produce can either be deflected into or out of the bottle.  If it starts off going into the bottle the pressure will build up until the stream is pushed out, and then as the bottle empties the pressure drops and the stream can move back in.  This produces a vibration you hear as sound.  As you can find from the
musical bottles experiment, if the bottle is large it takes a long time for the pressure to build up so the vibration is slow and you hear a low note.

If a plastic bottle is flattened the changes in pressure cause the bottle can change shape slightly, if you touch the flattened area while you are blowing you can feel this as a strong vibration.  This change in shape allows more air to flow in for the same change in pressure, so the speed of the vibration and therefore the pitch is lower than it should be for the amount of air available.

If you surround the bottle with water by immersing it, the water effectively stops the walls from moving.  This means that the pitch is correct for the amount of air in the bottle - which is of course reduced from its unsquashed volume so the pitch is higher than normal.

Flat Bottle Flat bottle in water
Cross section through a squashed bottle. When the pressure rises or falls it will push outwards or inwards making the bottle more or less circular. The dense water stops the sides moving.

If the bottle is unsquashed on the other hand, because it is circular it can't get any larger without more plastic, and because there are no weak spots it is hard for it to get smaller.  This means that the walls do not vibrate in the same way as in the squashed bottle, and putting the bottle in water will make no difference to the sound.

A round bottle A round bottle in water
In air a round bottle can't change shape if the pressure increases or decreases So when you move it into water it still can't change shape so nothing has changed

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