Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Measuring the density of water 2


This method uses an equal-arm balance rather than a domestic spring balance.

Apparatus and materials

Equal-arm balance

Perspex boxes with internal dimensions 10 cm x 10 cm x 11 cm, 2

Mass, 1 kg

Health & Safety and Technical notes

If the pan size is such that the 1 kg mass might fall off, place an open cardboard box below the pan to catch it.

The boxes should have horizontal marks, on one face, 10 cm above their internal bases.



Equal-arm balance with cubes

a Place a box on each scale pan to show that they balance. 
b Add the kilogram mass to one pan and carefully pour water into the box on the other pan until the beam balances. The water will then be seen to have a depth of 10 cm, and so a volume of 1000 cubic centimetres and a mass of 1 kg.

Teaching notes

1 An alternative is to measure out a kilogram of water and show it has a volume of 1000 cubic centimetres (1 litre). 

2 A metre cube could be used to help students see that the mass of a cubic metre of water would be 1000 times greater. 
This experiment was safety-checked in July 2006


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