O-Level Physics Moments Practical: Balancing a Beam and Measuring Arms

A small load far from a pivot can balance a larger load nearby. To explain the balance, compare turning effects rather than masses alone. Each moment is the force multiplied by its perpendicular distance from the pivot.

This guide uses a low, supported beam with trainer-specified hanging loads. Read the general lab safety guide before handling the weights. Keep fingers and feet out of the area below them.

Set up a useful balance

The 2026 O-Level Physics syllabus, 6091, includes moments investigations on pages 31-32 of its practical scope. The Institute of Physics beam activity illustrates balancing loads at different distances.

Use the assigned beam, pivot, supports, hangers and masses. The trainer checks support stability and permitted loads. Locate the beam's own centre of gravity and record the pivot position. If the pivot is at that centre, the beam's weight contributes no moment about it. If the pivot is elsewhere, include the beam's weight and its arm in the calculation.

For a horizontal beam with vertical hanging forces, the horizontal distance from pivot to each load's line of action is the perpendicular arm. Measuring from the end of the ruler, or using an arbitrary sloping distance, answers a different question.

Record balanced conditions