# A see-saw weighing device

##### Demonstration

Principles of physics are not just abstract. They have practical value.

#### Apparatus and materials

Plank, wooden

Brick or wood block

Metre rule

Masses, 1 kg to be used as 10 newton weights, 16

#### Health & Safety and Technical notes

If the weighing device is used to find the weights of students then a steadying rail must be available to avoid falls.

Students should be disciplined to avoid misbehaving if an attempt is made to find the mass of a student or teacher.

The wooden plank should be 2 - 3 m long, at least 20 cm wide and 2.5 cm or more thick.

#### Procedure

a Place the plank on the brick as the pivot or fulcrum.

b Place the body that is to be measured at a distance of, say, 0.5 m from the brick.

c Add 10 newton weights (1 kg masses) at the other end until balance is achieved.

#### Teaching notes

1 Encourage students to design and build a weighing machine to find weights of everyday objects such as parcels or letters.

2 You could highlight distinctions between mass and weight here. Weight is a force, a force due to gravity, and is measured in newtons. The weight of a body can vary from place to place, such as on the Earth and on the Moon, and in deep space it is zero. The mass of a body is a measure of the quantity of material, and is measured in kilograms.

The mass of a body doesn't change unless material is added to it or taken away. The distinction is less important in everyday life provided that this takes place exclusively on the surface of the Earth, where the weight of a body does not change too significantly from place to place. In a more universal context, and in science, the difference is very real.

This experiment was safety-checked in October 2004