Year 9 Physics
At the end of their mechanics topic on moments of force, Year 9 were given the challenge of making a microbalance. They applied their knowledge of the principle of moments to a pivoted straw with a small weight in one end and pivoted with a needle positioned so that the other end rocked up and down over a mm paper scale on a lolly stick. The first had to work out the mass of a 1mm squared piece if paper by finding the mass of an A4 sheet and dividing this mass by the number of 1mm squares on the paper. Then they took the tiny square of paper and placed it using tweezers into the end of the straw and made delicate adjustments to the pivot and/ or mass of the counterweight ( using tiny balls of Blu Tack)so that the straw moved across a good range of the lolly stick. The scale was then created making it possible to weigh objects as small as a human hair or grain of sand. An extremely steady hand, much patience and a high level of dexterity were needed (and breathing was undesirable throughout the experiment due to the sensitivity of the device). Given that the smallest mass we can measure on a laboratory balance is 0.01g it was satisfying to make a junk model balance that could weigh objects weighing as little as 10 micrograms 0.000010g.