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Equipment

Guinea and Feather Tube
AKA: Freefall Apparatus

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Fig 1: Commercially available tube with hose and clip
Fig 1
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Summary

A glass tube containing a guinea and feather, used to show how air resistance affects falling objects.

Operation

This piece of equipment consists of a glass or plastic tube of around 5cm in width and 60cm or so long. At each end of the tube is a bung, with one end having a metal tube going through it, this is connected to a length of pressure tubing and then to a vacuum pump. Somewhere on the pressure tubing will be a Hoffman clip. Inside the tube is a guinea (usually a small coin but for safety reasons, a thick rubber washer is sometimes preferred) and a feather (or small piece of tissue paper).

In air, the guinea and the feather fall at different speeds due to air resistance. To show this, turn the tube upside down so both items fall from one end of the tube to the other. As you will see, the guinea will fall much faster than the feather because it is affected less by air resistance. (The feather has a greater surface area which comes in contact with the surrounding air).

Turn the vacuum pump on and expel as much air from the tube as is safely possible. When you have done this, tighten the Hoffman clip and turn off the pump. Inside the tube you should now have a near vacuum. Turn the tube upside down once again and you will see that the guinea and feather arrive at the other end at the same time. With no air in the tube, both objects fall at the same speed as there is no air present (therefore no or negligible resistance) to affect their descent.

Safety

 CautionOnly use tubing specifically designed to withstand the pressure of a vacuum.

When finished, carefully unscrew the Hoffman clip fully to let air into the tube again, only then will it be safe to remove eye protection.

The contents of this page are for information only. Please refer to CLEAPSS or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.

Notes

The Guinea and Feather tube was originally invented by Isaac Newton. The Guinea used in the tubes was an English coin, minted until 1813 and named after the area of Africa from where its metal was mined.