WJEC Physics for AS Level Student Book 2nd Edition

18 WJEC Physics for AS Level 1.1.6 Moments of forces (a) The turning effect of a force Sometimes forces cause things to accelerate. Sometimes they stretch or compress an object or make it rotate. The forces (red arrows) in Fig. 1.1.15 cause the wheelbarrow, and the spanner and nut to turn about the pivot. Anyone who has used a spanner knows that the longer the handle, the easier it is to undo a nut. In other words, the turning effect of the force is bigger if it is applied further away from the pivot. An easy experiment to show this difference in turning effect is for two people to push on either side of a door. It is easy for a child to hold a door shut against an adult – if the adult pushes close to the hinge! (See Fig. 1.1.16) (b) The principle of moments The turning effect of a force about a point depends upon its direction as well as the distance from a point – see Fig. 1.1.17. We take account of this when we define the moment of a force, which is the mathematical expression of its turning effect: Fig. 1.1.15 Forces causing rotation lift force to loosen the nut weight Fig. 1.1.16 Forces on a door large force small force pivot upwards force, from the ground Key term Moment: The moment of a force about a point is the product of the force and the perpendicular distance from the point to the line of action of the force. Fig. 1.1.17 Direction matters large turning effect smaller turning effect no turning effect