Investigate turning effects by building beams, pivots, hanging masses and connected mobiles. Compare clockwise and anticlockwise moments, examine the effect of beam weight, and distinguish the forces in an apparatus at rest from those in one that is moving.
What you can investigate
- How mass and perpendicular distance determine the moment of a force
- How moving a pivot changes the balance of a uniform beam
- How every beam in a hanging mobile must balance independently
- The effect of beam weight, string tension and centre of mass
- How pivots and additional angled anchor strings share support forces
- The difference between a balanced construction and an apparatus that is moving
How can a smaller mass balance a larger one?
- Begin with a horizontal uniform beam supported at its centre. Hang masses on opposite sides and measure their perpendicular distances from the pivot.
- Place a smaller mass twice as far from the pivot as a mass twice its size on the other side. Compare the two turning effects after the apparatus settles.
- Move one mass or shift the pivot. Predict the direction of turning, then adjust the construction to restore balance and account for the beam’s own weight.
Moment = force × perpendicular distance from the pivot to the force’s line of action. A horizontal beam can balance when the total clockwise and anticlockwise moments are equal. Static equilibrium also requires the resultant force to be zero.
Interpreting the model
Include every relevant force. The beam’s weight acts through its centre of mass; it only has zero moment about a pivot on that line of action. For angled strings, use perpendicular distance rather than distance along the beam.
Revision and practice