Revision summary

Moment of a force

Moment = F x d. F in N; d is the perpendicular distance in m from the pivot to the force's line of action. Give N m and the clockwise or anticlockwise turning sense.

Principle of moments

For equilibrium, total clockwise moment = total anticlockwise moment about the same point. A stationary body also needs zero resultant force.

Before calculating a moment

• Name the pivot and extend the force's line of action.
• Choose the perpendicular distance to that line, not automatically the handle length.
• Convert distances to metres for a result in N m. Convert mass to force using W = mg when needed.
• A force whose line of action passes through the pivot has zero moment about it; the force may still be non-zero.

Build a balance equation

Include every force with a turning effect, including the object's own weight. Measure all lever arms from one common pivot and classify their turning senses. For a uniform metre rule, its weight acts at the 50 cm mark; a support elsewhere does not automatically remove that moment.

Centre of gravity and suspension

Represent the body's combined weight by one downward force at G. A uniform symmetrical object's G is at its geometric centre, but an irregular object needs evidence.

1. Suspend the lamina freely and let it settle.
2. Mark the vertical plumb-line position from that same suspension point.
3. Repeat from another hole. The intersection of the two traces estimates G.
4. Use a third suspension to check; thin, well-separated trace directions give a more useful intersection.

For a moments investigation, include hanger weights, keep forces vertical and measure from the actual pivot. For suspension, avoid clamping, parallax and a moving plumb line. Explain a specific limitation and its effect.

Pure: stability

About a possible toppling edge, the weight line determines whether weight gives a restoring or toppling moment. At the boundary it passes through the edge. With no sliding and otherwise comparable conditions, a lower G or a wider base in the tilt direction allows more tilt before toppling.