Overview
This lecture introduces the turning effect of forces (moment), explains how to calculate it, and discusses conditions for balance in systems like see-saws.
Turning Effect of Forces (Moment)
- A force is a push or pull that changes the motion, shape, or size of an object.
- The moment of a force (turning effect) is its tendency to rotate an object about a fixed point or pivot.
- The direction of the moment is either clockwise or anti-clockwise, depending on the direction of rotation caused.
Calculating Moments
- Moment (Nm) = Force (N) × Perpendicular distance from pivot to the line of action of force (m).
- The SI unit for moment is the newton-metre (Nm).
- Perpendicular distance is measured from the pivot to the force’s line of action, at a 90° angle.
- It's easier to close a heavy door by pushing further from the pivot, increasing the perpendicular distance.
Examples and Applications
- Henry applies 70 N at a perpendicular distance of 0.4 m: moment = 70 N × 0.4 m = 28 Nm.
- On a see-saw:
- Clockwise moment: 250 N × 1.2 m = 300 Nm.
- Anti-clockwise moment: 200 N × 1.5 m = 300 Nm.
- When clockwise and anti-clockwise moments are equal, the system is balanced and does not turn.
Balancing Systems
- Balance is achieved when the clockwise moment equals the anti-clockwise moment.
- To balance a see-saw or beam balance, adjust force or distance on one side to match the moment on the opposite side.
- If one moment is greater, the object rotates in that direction.
Key Terms & Definitions
- Force — A push or pull that changes or tends to change the motion or shape of an object.
- Moment (of force) — The turning effect produced by a force about a pivot.
- Pivot/Fulcrum — The fixed point about which an object rotates.
- Line of action of force — The straight line through the point of application in the direction of the force.
Action Items / Next Steps
- Compare balancing methods for a see-saw and beam balance and identify what factor changes in each case.
- Practice calculating moments using different forces and distances.