A simple balance 1
A brief introduction to measuring mass by balancing.
Apparatus and materials
Crude balance (see technical notes)
Retort stand, boss, and clamp
Hanger with slotted masses, 10 g
Hanger with slotted masses, 100 g
Health & Safety and Technical Notes
This simple lever balance is made out of a lath of wood or a metre rule (with the graduations ignored) drilled with holes at the ends and centre, together with three hooks. It is not meant to produce accurate results nor is it meant to be an accurate part of teaching the lever law, though it could be an introduction. Suspend the simple lever by one of the hooks positioned centrally. It is convenient to support the hook from a clamp attached to a retort stand by a boss. Position the other two hooks near the ends of the lever at equal distances from the centre.
The sensitivity of the balance can be changed by adding a small load (e.g. Blu-Tac or a screw) on the lower edge of the beam, at the centre, to bring down the centre of gravity. Better still, drill another hole nearer to the top of the lath so that the lath is suspended with a lot of it below the suspension point.
a Weigh a small parcel (about 500 g), using a 100 g hanger and 100 g slotted masses on one side, the parcel on the other. Since this is only a demonstration to introduce a problem do not labour this experiment. Do not add smaller masses to make the weighing more 'accurate'. It is only a rough measurement.
b Weigh a letter to the nearest 10 g by the same method, using the 10 g slotted masses.
c Ask whether the beam will weigh a single hair. It fails, suggesting the need for a more sensitive balance. This could lead on to Making a microbalance.
1 You could introduce this experiment to students in the form of a problem-solving task, to be designed and perhaps built at home. One context which has been used successfully is that of finding out how much a parcel and a letter cost to send by post. The current postal rate leaflets, obtainable from a Post Office or related website, make this task more realistic.
2 A parcel and a letter can be weighed relative to the mass of laboratory masses or substances found at home such as a bag of sweets, bag of sugar, slab of butter and so on. All the masses are marked on these products. However, you might find yourself debating whether to sort out the mass/weight problem.
Many teachers tend to try to use the correct words without penalizing students for getting the wrong ones at the introductory stage. That is, mass is measured in kilograms relating it to 'massive' objects. A discussion of units and the standard kilogram in Sèvres may also come into the discussion when comparing slabs of butter, bags of sugar and bags of sweets as fractions of a standard kilogram. How would a slab of butter stand up to reliability over time?
This experiment was safety-checked in July 2007