Class 10 · Physics · FBISE Islamabad

Chapter 4: Turning Effect of Forces

ReadyExercise 4.1

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Concept 2

Numerical 3

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Concept

1 3 mkConcept

Define torque (moment of a force). On what factors does the turning effect of a force depend?

DEFINE Torque is the turning effect produced by a force about a pivot or axis of rotation.

WRITE τ = F × L, where L is the perpendicular distance from the pivot to the line of action of the force.

FACTOR 1 The magnitude of the applied force — a larger force gives a larger turning effect.

FACTOR 2 The perpendicular distance of the force from the pivot — the longer the arm, the larger the torque.

Answer Torque τ = F × L; it depends on the force and on its perpendicular distance from the pivot.

Common mistake

Marks are commonly lost by writing "distance" instead of "perpendicular distance". The word perpendicular carries a mark.

4 4 mkConcept

Why is it easier to open a door by pushing at its outer edge rather than near the hinge?

REASON 1 Torque depends on the perpendicular distance from the pivot, here the hinge.

REASON 2 Pushing at the outer edge makes that distance as large as the door allows.

REASON 3 With a larger arm, the same force produces a larger torque.

REASON 4 Near the hinge the arm is very small, so a much larger force would be needed for the same turning effect.

Answer The outer edge gives the largest moment arm, so the same push produces the greatest torque.

Numerical

2 4 mkNumerical

A force of 200 N is applied at right angles to a spanner of length 25 cm. Find the torque produced.

F = 200 NL = 25 cmFind τ

STEP 1 τ = F × L

STEP 2 L = 25 cm = 0.25 m

STEP 3 τ = 200 N × 0.25 m

STEP 4 τ = 50 N m

Answer τ = 50 N m

Common mistake

Convert centimetres to metres before substituting, otherwise the answer comes out 5000 and the unit is wrong.

3 5 mkNumerical

Two children sit on a see-saw. A 30 kg child sits 1.6 m from the pivot. How far from the pivot must a 40 kg child sit so that the see-saw balances?

m₁ = 30 kgL₁ = 1.6 mm₂ = 40 kgFind L₂

PRINCIPLE For balance, the anticlockwise torque equals the clockwise torque.

STEP 1 w₁ × L₁ = w₂ × L₂

STEP 2 Weight is proportional to mass, so m₁ L₁ = m₂ L₂

STEP 3 30 × 1.6 = 40 × L₂

STEP 4 L₂ = 48 ÷ 40

STEP 5 L₂ = 1.2 m

Answer L₂ = 1.2 m from the pivot

Common mistake

g cancels from both sides, so masses may be used directly. Write that step down — the board scheme awards a mark for it.

5 5 mkNumerical

A couple is formed by two equal and opposite forces of 50 N each, acting on a steering wheel of diameter 40 cm. Find the torque of the couple.

F = 50 Nd = 40 cmFind torque of couple

STEP 1 Torque of a couple = F × d, where d is the perpendicular distance between the two forces.

STEP 2 Here d is the diameter of the wheel: d = 40 cm = 0.4 m

STEP 3 τ = 50 N × 0.4 m

STEP 4 τ = 20 N m

Answer Torque of the couple = 20 N m

Common mistake

For a couple use the full separation between the forces — the diameter, not the radius.

This sheet was checked against the FBISE Islamabad 2024 marking scheme.