1.5.2

Turning effect of forces

Cambridge IGCSE Physics (0625)  · Unit 1: Motion, forces and energy  · 12 flashcards

Turning effect of forces is topic 1.5.2 in the Cambridge IGCSE Physics (0625) syllabus , positioned in Unit 1 — Motion, forces and energy , alongside Physical quantities and measurement techniques, Motion and Mass and weight.  In one line: The moment of a force is a measure of its turning effect around a pivot. It is calculated as the force multiplied by the perpendicular distance from the line of action of the force to the pivot.

This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical).

The deck below contains 12 flashcards — 2 definitions — covering the precise wording mark schemes reward.  Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.

Key definition

The moment of a force

The moment of a force is a measure of its turning effect around a pivot. It is calculated as the force multiplied by the perpendicular distance from the line of action of the force to the pivot.

What the Cambridge 0625 syllabus says

Official 2026-2028 spec

These are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.

  1. Describe Describe the moment of a force as a measure of its turning effect and give everyday examples
  2. Define Define the moment of a force as moment = force x perpendicular distance from the pivot; recall and use this equation
  3. Apply Apply the principle of moments to situations with one force each side of the pivot, including balancing of a beam
  4. State State that, when there is no resultant force and no resultant moment, an object is in equilibrium
  5. Apply Apply the principle of moments to other situations, including those with more than one force each side of the pivot Supplement
  6. Describe Describe an experiment to demonstrate that there is no resultant moment on an object in equilibrium Supplement
Definition Flip

Define the moment of a force.

Answer Flip

The moment of a force is a measure of its turning effect around a pivot. It is calculated as the force multiplied by the perpendicular distance from the line of action of the force to the pivot.

Key Concept Flip

A person pushes on a door handle. State one way to increase the moment the person applies to the door.

Answer Flip

Increasing the force applied to the door handle, or pushing further away from the hinges (pivot) would both increase the moment. The moment is force multiplied by distance from the pivot.

Key Concept Flip

Define the moment of a force. A mechanic uses a wrench to tighten a bolt. They apply a force of 20 N at a perpendicular distance of 0.25 m from the center of the bolt (the pivot). Calculate the moment of the force applied to the bolt.

Answer Flip

Definition: The moment of a force is the turning effect of the force about a pivot.

Calculation:
* Moment = Force x Perpendicular distance from pivot
* Moment = 20 N x 0.25 m
* Moment = 5.0 Nm

Therefore, the moment of the force applied to the bolt is 5.0 Nm.

Key Concept Flip

A door requires a moment of 12 Nm to open. You apply a force of 6 N to the door handle. State the perpendicular distance from the hinge (pivot) at which the handle must be located for you to successfully open the door.

Answer Flip

* Moment = Force x Perpendicular distance
* Rearrange: Perpendicular distance = Moment / Force
* Perpendicular distance = 12 Nm / 6 N
* Perpendicular distance = 2.0 m

Therefore, the door handle must be located 2.0 m from the hinge for you to successfully open the door.

Key Concept Flip

A 2.0 m long beam is pivoted at its center. A weight of 3.0 N is placed 0.4 m from the pivot on one side. Calculate the weight that must be placed 0.5 m from the pivot on the *other* side to balance the beam.

Answer Flip

Answer:
Principle of Moments: Sum of clockwise moments = Sum of anticlockwise moments
(Force 1 x Distance 1) = (Force 2 x Distance 2)
(3.0 N x 0.4 m) = (Force 2 x 0.5 m)
1.2 Nm = Force 2 x 0.5 m
Force 2 = 1.2 Nm / 0.5 m
Force 2 = 2.4 N

Explanation: To balance, the clockwise moment created by the 3N weight must equal the anticlockwise moment created by the unknown weight.

Key Concept Flip

A uniform beam is balanced on a pivot. A weight is placed on one side of the pivot. Explain why the beam is able to remain balanced using the principle of moments. Your explanation must include reference to both forces and distances.

Answer Flip

Answer:
For the beam to be balanced, the total clockwise moment around the pivot must equal the total anticlockwise moment. A moment is the product of a force and the perpendicular distance from the pivot. Therefore, the force exerted by the weight multiplied by its distance from the pivot on one side must equal the force of another weight (or combination of weights) multiplied by *their* distance(s) from the pivot on the other side. If these values are equal, the beam will not rotate in either direction and remains balanced.

Definition Flip

State the conditions necessary for an object to be in equilibrium.

Answer Flip

For an object to be in equilibrium:
1. The resultant force acting on the object must be zero.
2. The resultant moment about any point must be zero.

Equilibrium means no net force and no net turning effect.

Key Concept Flip

A 2N weight is placed 0.3m from a pivot. A force of 1.2N is applied on the other side of the pivot at a distance of 0.5m. Determine whether the object is in equilibrium, stating your reasoning.

Answer Flip

Taking clockwise moments as positive, the clockwise moment = 2N * 0.3m = 0.6 Nm.
The anticlockwise moment = 1.2N * 0.5m = 0.6 Nm.

Since the clockwise and anticlockwise moments are equal (0.6Nm), the resultant moment is zero. Assuming that the resultant force in the system is zero, then the object is in equilibrium.

In reality, if there's a non-zero vertical force, there would be no equilibrium.

Key Concept Flip

A 2.0 m long plank is pivoted at its center. A 30 N weight is placed 0.5 m from one end, and a 20 N weight is placed 0.3m from the other end. Calculate the additional force needed, applied at the opposite end to the 30N weight, to balance the plank.

Answer Flip

The principle of moments states that for equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments.

Taking moments about the pivot:
Clockwise moment = (20 N * 0.7 m) = 14 Nm
Anticlockwise moment = (30 N * 0.5 m) = 15 Nm
Net anticlockwise moment = 15 Nm - 14 Nm = 1 Nm

To balance, additional clockwise moment needed = 1 Nm
Let the additional force be F.
F * 0.5 m = 1 Nm
F = 1 Nm / 0.5 m = 2 N
Answer: 2 N

Key Concept Flip

A seesaw is balanced with two children on either side of the pivot. One child exerts a force of 300N at a distance of 1.2m from the pivot. The other child is sitting 1.0m from the pivot. Explain why the seesaw is balanced even though the forces exerted by the children are different.

Answer Flip

The seesaw is balanced because the sum of the clockwise moment is equal to the sum of the anticlockwise moment. The moment is the turning effect of the force, which is calculated by multiplying the force by the perpendicular distance from the pivot. Therefore, even though the forces are different, if the product of force and distance are the same on both sides, the seesaw will be balanced. In this instance, the anticlockwise moment is 300N * 1.2m = 360 Nm. The force on the other side must be 360N to create a balancing clockwise moment of 360Nm (360N * 1.0m)

Key Concept Flip

Describe an experiment, including necessary apparatus and method, to demonstrate that there is no resultant moment on a pivoted metre rule when it is in equilibrium. Two weights are to be suspended from the rule. Include details on how you would ensure accuracy.

Answer Flip

Apparatus: Metre rule, pivot stand, two known weights (

Example: 1.0 N and 2.0 N), string, ruler.<br><br><strong>Method:</strong><br>1. Balance the metre rule on the pivot stand *without* any weights attached. Adjust the pivot until the rule is balanced horizontally and the centre of mass of the rule lies directly above the pivot.<br>2. Hang the 1.0 N weight at a known distance (. 20 cm) from the pivot on one side.<br>3. Hang the 2.0 N weight on the *other* side of the pivot. Adjust its position until the metre rule is again balanced horizontally.<br>4. Measure the distance of the 2.0 N weight from the pivot.<br>5. Calculate the moments: Moment 1 = Force 1 × distance 1 = 1.0 N × 0.20 m = 0.20 Nm. Moment 2 = Force 2 × distance 2 = 2.0 N × distance 2. If the rule is in equilibrium, moment 2 should be equal to moment 1.<br>6. Repeat with different weights and distances.<br><br><strong>Accuracy:</strong> Ensure the string is vertical, read measurements at eye level to avoid parallax error, and use a spirit level to ensure the table is horizontal.
Key Concept Flip

State what measurements would be taken in an experiment to show that there is no resultant moment acting on an object in equilibrium when weights are hung on either side of a pivot.

Answer Flip

The measurements that would be taken are:
1. The weight (force) of each hanging object, typically measured in Newtons (N).
2. The perpendicular distance from the pivot to the line of action of each weight, typically measured in meters (m).

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1.5.1 Effects of forces 1.5.3 Centre of gravity

Key Questions: Turning effect of forces

Define the moment of a force.

The moment of a force is a measure of its turning effect around a pivot. It is calculated as the force multiplied by the perpendicular distance from the line of action of the force to the pivot.

State the conditions necessary for an object to be in equilibrium.

For an object to be in equilibrium:
1. The resultant force acting on the object must be zero.
2. The resultant moment about any point must be zero.

Equilibrium means no net force and no net turning effect.

More topics in Unit 1 — Motion, forces and energy

Turning effect of forces sits alongside these Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.

1.1 Physical quantities and measurement techniques

14 flashcards
1.2 Motion

26 flashcards
1.3 Mass and weight

10 flashcards
1.4 Density

8 flashcards
1.5.1 Effects of forces

24 flashcards
1.5.3 Centre of gravity

6 flashcards
1.6 Momentum

8 flashcards
1.7.1 Energy

12 flashcards
1.7.2 Work

4 flashcards
1.7.3 Energy resources

14 flashcards
1.7.4 Power

2 flashcards
1.8 Pressure

8 flashcards

Key terms covered in this Turning effect of forces deck

Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.

The moment of a force
Conditions necessary for an object to be in equilibrium

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