Lesson overview
Moments and balance: How can a child balance an adult on a seesaw?
Balance a uniform metre rule on a knife-edge pivot: find where it balances on its own, then slide hooked masses along it until it rests level, work out the moments and the pivot’s upward push, and (Supplement) balance two masses on each side. Then move the pivot off the middle to weigh the rule with one 50 g mass, and find the centre of gravity of an uneven card by hanging it from a pin with a plumb line.
Suggested sequence
- Lesson 1 · Turning forces. The moment of a force as its turning effect, moment = force × perpendicular distance from the pivot; the principle of moments for a balanced beam; equilibrium as no resultant force and no resultant moment; Supplement: several forces each side and the experiment that shows it. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Centre of gravity. Centre of gravity: where the weight of an object acts; finding it for an irregular lamina with a plumb line; how its position affects the stability of simple objects. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Turning forces
- Describe the moment of a force as a measure of its turning effect and give everyday examples (1.5.2.1)
- Define the moment of a force as moment = force × perpendicular distance from the pivot; recall and use this equation (1.5.2.2)
- Apply the principle of moments to situations with one force each side of the pivot, including balancing of a beam (1.5.2.3)
- State that, when there is no resultant force and no resultant moment, an object is in equilibrium (1.5.2.4)
- Supplement: apply the principle of moments to other situations, including those with more than one force each side of the pivot (1.5.2.5)
- Supplement: describe an experiment to demonstrate that there is no resultant moment on an object in equilibrium (1.5.2.6)
Part 2: Centre of gravity
- State what is meant by centre of gravity (1.5.3.1)
- Describe an experiment to determine the position of the centre of gravity of an irregularly shaped plane lamina (1.5.3.2)
- Describe, qualitatively, the effect of the position of the centre of gravity on the stability of simple objects (1.5.3.3)
Misconceptions to listen for
- “Equilibrium means that no forces act on the object.” — A balanced beam has its weights pulling down and the pivot pushing up. In equilibrium the forces and their moments cancel; the forces are still there.
- “The perpendicular distance is the distance along the object to where the force acts.” — Only when the force is at right angles to the object. In general, measure from the pivot to the line of action, at right angles to it.
- “A wheelbarrow just lifts the load straight up.” — The barrow turns about its wheel as you lift the handles: the wheel is the pivot.
- “Pulling a sledge uses a moment.” — The sledge moves along in a straight line; nothing turns about a pivot.
- “R, because it acts at the handle too.” — R acts at the handle, but at a slant. Draw its line of action: the perpendicular distance from the hinge to that line is shorter than the width of the door.
- “They are all the same, because the pushes are the same size.” — A moment depends on the distance as well as the force. S, through the hinge, has no moment at all.
- “600 N m.” — You used 20 cm without changing it to metres; that gives 600 N cm. In metres the distance is 0.20 m, so the moment is 6.0 N m.
- “150 N m, from 30 ÷ 0.20.” — A moment is the force multiplied by the distance, not divided by it.
- “(3) newtons.” — A moment is a force times a distance, so its unit is the newton metre, N m.
- “(5) force.” — Both are needed: no resultant force and no resultant moment. The second condition is about moments.
Scope
The moment of a force as a measure of its turning effect, with everyday examples (1.5.2.1); moment = force × perpendicular distance from the pivot (1.5.2.2); the principle of moments with one force each side of the pivot, including balancing a beam (1.5.2.3); no resultant force and no resultant moment means equilibrium (1.5.2.4); Supplement: the principle of moments with more than one force each side (1.5.2.5) and an experiment to show there is no resultant moment on an object in equilibrium (1.5.2.6); centre of gravity (1.5.3.1), finding it for an irregular plane lamina (1.5.3.2) and its effect on stability (1.5.3.3). Couples, torque by name, forces at an angle and levers as machines are not used. Resultant forces are in Resultant force, momentum in Momentum and collisions, and pressure in Pressure.