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2. Force and Rotation Forces Causing Rotation Levers and Simple Machines Moments – Turning Effect of a Force Principle of Moments (Single Pivot)
Year 8 • Physics
Force and Rotation
This lesson explains how forces can cause rotation, how levers and simple machines work, and how the turning effect of a force (moment) is calculated using the principle of moments.
Lesson Objectives
- Understand how forces cause rotation.
- Identify levers and simple machines.
- Explain moments as the turning effect of a force.
- Calculate moments using force and distance.
- Apply the principle of moments for a single pivot.
1. Forces Causing Rotation
A force can cause an object to rotate if it acts at a distance from a fixed point.
This fixed point is called a pivot.
Examples:
• Opening a door about its hinges
• Turning a spanner to loosen a nut
• A seesaw rotating about the centre
• Opening a door about its hinges
• Turning a spanner to loosen a nut
• A seesaw rotating about the centre
2. Levers and Simple Machines
A lever is a simple machine that helps us apply a force more easily.
Levers rotate around a pivot.
Examples of levers:
• Seesaw
• Crowbar
• Wheelbarrow
• Scissors
• Seesaw
• Crowbar
• Wheelbarrow
• Scissors
3. Moments – Turning Effect of a Force
The moment of a force is the turning effect produced by a force.
Moment Formula:
Moment = Force × Distance from pivot
Moment = Force × Distance from pivot
• Force is measured in newtons (N)
• Distance is measured in metres (m)
• Moment is measured in newton metres (Nm)
4. Increasing or Decreasing Moments
A moment can be increased by:
• Increasing the force
• Increasing the distance from the pivot
• Increasing the force
• Increasing the distance from the pivot
This is why doors open more easily when pushed at the handle rather than near the hinge.
5. Principle of Moments (Single Pivot)
When an object is balanced and not rotating, the total clockwise moment equals the total anticlockwise moment.
Principle of Moments:
Clockwise moments = Anticlockwise moments
Clockwise moments = Anticlockwise moments
6. Everyday Examples
• Seesaws balancing children of different weights
• Tightening bolts using a long spanner
• Balancing scales
• Opening heavy doors
• Tightening bolts using a long spanner
• Balancing scales
• Opening heavy doors
Practice Questions
A. Fill in the Blanks
- A fixed point of rotation is called a __________.
- The turning effect of a force is called a __________.
- Moment = force × __________.
- Moment is measured in __________.
- A seesaw is an example of a __________.
B. True or False
- A force can only cause movement, not rotation.
- Pushing further from a pivot increases the moment.
- A lever is a simple machine.
- Balanced objects have unequal moments.
- Moments depend on force and distance.
C. Short Answer
- What is a pivot?
- Define moment.
- Name two examples of levers.
- Why is a long spanner useful?
- What happens when clockwise moments equal anticlockwise moments?
D. Numerical Problems
- A force of 10 N acts 0.5 m from a pivot. Calculate the moment.
- A force of 5 N acts 2 m from a pivot. Find the moment.
- A 20 N force acts 0.25 m from a pivot. Calculate the moment.
- If the moment is 12 Nm and distance is 3 m, find the force.
- If a force of 4 N creates a moment of 8 Nm, find the distance.
✅ Show Answer Key
A. Fill in the Blanks
- pivot
- moment
- distance
- newton metres (Nm)
- lever
B. True or False
- False
- True
- True
- False
- True
C. Short Answer
- A fixed point about which an object rotates.
- The turning effect of a force.
- Seesaw, spanner (any two).
- It increases the distance from the pivot, increasing the moment.
- The object is balanced and does not rotate.
D. Numerical Problems
- 10 × 0.5 = 5 Nm
- 5 × 2 = 10 Nm
- 20 × 0.25 = 5 Nm
- 12 ÷ 3 = 4 N
- 8 ÷ 4 = 2 m
© Aviate Learning – Force and Rotation (Year 8)
⚖️ Interactive Seesaw – Principle of Moments
Adjust the forces and distances on both sides of the pivot.
The simulation will automatically calculate moments and show whether the seesaw is balanced.
⬅️ Left Side
➡️ Right Side
Left Moment: 20 Nm
Right Moment: 20 Nm
✅ Balanced