All liquids exert an upthrust on objects placed in it because the pressure increases as the object gets deeper. Therefore the pressure on the bottom of the object is greater than on the top providing an upwards force. This force is called upthrust and explains how boats can float on water.

An object is weighed in air and then in water. The difference in weight represents the resultant force of upthrust.

This is usually a lower school lesson in which pupils use newtonmeters to weigh objects in water. One at a time the objects are weighed in air and then lowered into the water. Objects that float will create a reading of 0N when in the water showing that the forces of gravity (pulling the object downwards) and the upthrust of the water (pushing the object upwards) are equal and balanced.

Objects which sink will create a +N reading in both air and water showing that the forces involved (gravity and upthrust) are unbalanced.

A selection of objects may be required for this lesson. Good examples to include are listed below:

• Rocks
• Polystyrene chips or blocks
• Wood pieces
• Cork
• Bone
• Metal cubes
• Plasticine
• Rubber bungs
• Golf balls
• Lego pieces
• Wax blocks