A simple balance 2
A balance which introduces the principle of reading masses against a scale.
Apparatus and materials
Crude balance (see technical notes)
Retort stand, boss, and clamp
Mass with hook (mass approx 25 g)
Hanger with slotted masses, 10 g
Another mass, approx 25 g
Long strip of card (to act as scale)
Triangle of card (to act as pointer)
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. 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 screw) on the lower edge of the beam, at the centre, to bring down the centre of gravity. Or, drill another hole nearer to the top of the lath so that the lath is suspended with a lot of its mass below the suspension point.
a Set up the simple lath balance (see the experiment A simple balance 1) and discuss how it measures the mass of a small parcel.
b Position the scale so that the pointer moves up and down next to it. Introduce a new balance which uses a counterpoise and the angle of the beam to measure mass.
c Attach the 'anonymous' counterpoise mass on the left.
d Try 10 g, 20 g, 30 g on the right, and mark the pointer positions on the card scale. The third weighing shows excess mass by the tilt of the beam
e Hang another mass, approx 25 g, on the right. Read its value from the scale.
1 This balance is a lever balance using a counterpoise weight with the suspension point in the middle or off-centre depending on the sensitivity required. This is a useful design for finding the mass of lighter things such as a letter.
2 To make a success of this experiment, you will need to prepare the beam quite carefully so that it is rather insensitive, bringing the 10-, 20-, 30-g scale into a reasonable range. That simply needs a move of the central pivot to a higher hole, or the adding of a considerable load under the beam at the centre. A large bulldog clip with a piece of metal anchored to it will make the latter change easy.
3 Ask students if one could weigh still smaller things on this balance: a ring, a pin, a hair? Try hanging a hair alone on one end of the balance. Do this demonstration very quickly and lightly to suggest the need for a sensitive 'microbalance'.
4 Students given a 'design and build' task for homework can come up with some very ingenious designs, perhaps made with Lego. Top-pan balances of varying mechanical advantage can be built and some very small masses measured.
5 Students should have some practice at guessing the masses of various objects. They should then check their guesses by using a balance to find the object's mass more accurately. This can be treated as a game around the class.
This experiment was safety-checked in July 2007