Grade 10 general science Natural Physical Science – Turning effect of force Notes

• Determine the moment of a force about a point.
• Calculate the moment produced by anti-parallel (equal and opposite) forces — a couple.
• Apply the turning effect of force to explain common Kenyan everyday situations.
• Appreciate the importance of turning effects in daily life and safety.

The turning effect of a force about a point is called the moment (or torque). Moment = Force × Perpendicular distance from the point to the line of action of the force.

Formula: moment (M) = F × d

Units: Force in newtons (N), distance in metres (m), moment in newton-metres (N·m).

Direction: If the force makes something turn clockwise (CW) we may call that clockwise moment; if it makes it turn anticlockwise (ACW) we call that anticlockwise moment. When solving, compare clockwise and anticlockwise moments.

Example above: moments about pivot = 10×0.8 (clockwise) and 6×0.4 (anticlockwise).

1. Identify the point (pivot) about which you want the moment.
2. Find the perpendicular distance d from the pivot to the line of action of the force (convert cm → m when needed: 1 cm = 0.01 m).
3. Multiply the force (in N) by the perpendicular distance (in m): M = F × d.
4. Decide direction: clockwise (CW) or anticlockwise (ACW). Compare total CW and total ACW moments.

Anti-parallel forces are two equal and opposite forces whose lines of action are parallel but separated by a distance. They form a couple and produce pure rotation (no net force).

Moment (of a couple) = one force × distance between the forces.

If F and −F are separated by perpendicular distance d, the couple moment M = F × d (direction given by right-hand rule or by which way it turns — CW or ACW).

1. Seesaw activity (outdoor): Two learners sit at different distances from the pivot. Measure weights and distances; calculate moments and observe balance. Relate to swapping positions until seesaw balances (sum CW = sum ACW).
2. Ruler-balance experiment: Place a pencil as pivot under a metre rule. Add small weights (coins, washers) at known distances, calculate moments, and find the balance point (centre of mass). Record and compare calculations to observation.
3. Spanner and nut demonstration: Using a spanner and a weight hanging at the end, measure force and distance. Show that longer spanner gives larger turning effect — explain safe methods for loosening tight nuts (e.g., on bicycle or boda boda wheel) with a longer spanner.
4. Group problem-solving: Given everyday Kenyan contexts (opening a heavy water tank lid, using a wheelbarrow, opening a gate), ask groups to identify pivot, forces and distances, compute required force or distance to reduce effort.
5. Field observation / homework: Ask learners to list 5 tools used by people in their community that depend on turning effects (e.g., lever, crowbar, door, steering wheel, spanner) and explain why length/position matters.

• Seesaws at playgrounds — balancing by changing distance from pivot.
• Using a spanner to loosen/tighten nuts on bicycles or boda bodas — longer spanner = greater turning effect.
• Opening heavy tank lids or gates — pushing farther from hinge reduces required force.
• Wheelbarrow — handles act as levers; load position changes effort needed.
• Turning a tap or a steering wheel — small torque produces rotation useful for control.

1. A door handle is 0.9 m from the hinge. A person pushes with a force of 15 N perpendicular to the door. Find the moment about the hinge.
   M = F × d = 15 × 0.9 = 13.5 N·m (turning effect is 13.5 N·m).
2. Two equal anti-parallel forces of 12 N act on a rod; their lines are 0.25 m apart. What is the moment of the couple?
   M = F × d = 12 × 0.25 = 3.0 N·m.
3. A student sits 1.2 m from the pivot on a seesaw and weighs 420 N. What opposing weight located 0.84 m on the other side will balance the seesaw? (Assume only these two weights.)
   Take moments: 420 × 1.2 (CW) = W × 0.84 (ACW). So W = (420 × 1.2) / 0.84 = (504) / 0.84 = 600 N.

1. Calculate the moment of a 25 N force acting 0.4 m from a pivot.
2. Two forces 8 N and 5 N act on opposite sides of a pivot at distances 0.35 m and 0.5 m respectively. Which way will the object turn? Show working.
3. Explain why using a long spanner makes it easier to loosen a tight bolt. Use the moment formula in your answer.
4. (Challenge) Two anti-parallel forces 10 N are separated by 0.6 m. A third force of 15 N is applied at a point 0.2 m from the pivot on one side. Find the net moment (give direction) about the pivot.

• Safety: When using levers or spanners, ensure stable footing and keep hands clear of moving parts. Use suitable tools — never stand over heavy loads that could rotate unexpectedly.
• Misconception: More force always means more turning — actually turning effect depends on both force and perpendicular distance. Pushing closer to pivot may need much larger force.