Grades 3–5 · Everyday Life
How does a level balance compare total masses?
A balance has a beam that can turn around a pivot. The pans pull downward on opposite sides, so they tend to turn the beam in opposite directions. In an equal-arm balance, both pans hang the same distance from the pivot. Equal downward pulls can then balance each other.
In the diagramA level equal-arm balance compares massIdeal model · known masses labeled in gramsUnknown2 g3 gEqual arm lengths; the beam is level.Unknown mass = 2 + 3 = 5 g.
The ideal balance in the picture is already level. An unknown object sits on one pan, while known masses of 2 grams and 3 grams sit on the other. Their combined mass is 5 grams. With matching arms and the same local gravity, the level beam tells us that the unknown object's mass is also 5 grams.
The comparison uses the total on each side, not just one known piece. A level balance with 3 grams plus 4 grams would represent a different unknown object of 7 grams. Real balances need calibration and cannot measure with unlimited precision. This drawing leaves those complications out so the equality of the two total masses remains clear.
About this explanation
Starting knowledge: Add small whole numbers, recognize grams as a mass unit and understand equal distances on two sides of a pivot.
Sources
- National Institute of Standards and Technology — Automating Data Acquisition on a Mechanical Equal Arm Balance Used in Large Mass Calibrations