Beam balance and weight box
PhysicsMeasurement & UncertaintyAges 15–16Virtual lab
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A laboratory beam (physical) balance with a weight box from 100 g down to 10 mg and a 10 mg rider on the graduated beam. Check the empty balance and turn the balancing nut to remove the zero error, release the beam and find the rest point from three turning points, add weights and move the rider until it balances, and read the mass in g and kg. Move your eye to see parallax at the pointer; double weighing (swapping the pans) cancels unequal arms: m = √(m₁·m₂).
Lesson: Physical balance: zero error, parallax and double weighing
What it shows
A beam balance compares moments: when the beam rests level, the mass on one pan times its arm equals the mass on the other pan times its arm. With equal arms the object's mass equals the weights plus the rider reading. The pointer swings, so its rest point is found from three turning points. A zero error must be removed first with the balancing nut, and the eye must be in line with the pointer to avoid parallax. If the arms differ, weighing the object on each pan in turn gives m = √(m₁·m₂). The model uses a linear pointer response, slow damping and no air buoyancy.
How to use
Release the beam with the pans empty and turn the Balancing nut until the pointer swings evenly about 0. Press Arrest beam, choose the Object, add weights from the Weight box and move the Rider, then release the beam again. When the status says it balances, press Record weighing. For double weighing press Swap pans and balance again.
Parameters you can change
- Weighing method Single weighing, Double weighing (Gauss)
- Object on the pan No object, Object A, Object B
- Balance starts with a zero error
- Balance arms Equal, Unequal (faulty balance)
- Eye position relative to the pointer -3–3
Questions to explore
- Why do we find the rest point from three turning points instead of waiting for the pointer to stop?
- How does a zero error that is not corrected change every reading?
- Why does double weighing give the true mass even when the arms are unequal?
