Ch 9 · 1.5.5 Turning Effect of Forces · O Level Physics 5054

Topic 9 of 17 in Motion, Forces and Energy — O Level Physics 5054 — 48 lessons.

Introduction

Why is a door handle on the side furthest from the hinges? Why does a long spanner loosen a tight nut more easily? Forces can turn things as well as push them along. The turning effect of a force is called its moment. (Syllabus section 1.5.5.)

Key ideas — The moment of a force

The moment of a force is a measure of its turning effect about a pivot. Everyday examples: opening a door, turning a spanner or a steering wheel, using a crowbar or a wheelbarrow, balancing a see-saw.

🧑‍🏫 Teacher's explanation

Imagine you are trying to open a heavy door. If you push near the hinges, it is incredibly difficult. If you push at the handle, it is easy. Even though you are applying the same force, the 'turning effect' changes because of where you apply it. This turning effect is what we call the 'moment'.

📖 Definitions

🔍 Worked example — Easy

A force of 20 N is applied perpendicularly to a door at a distance of 0.8 m from the hinges. Calculate the moment of the force.

🔍 Worked example — Standard

A uniform meter ruler is balanced at its center (the 50 cm mark). A weight of 4 N is placed at the 10 cm mark. Where must a 2 N weight be placed to keep the ruler in equilibrium?

🔍 Worked example — Exam-style

A lever is 2.0 m long and pivots at its center. A force of 50 N is applied at one end. A second force is applied at the other end to maintain equilibrium. Calculate the magnitude of the second force.

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — Calculating a moment

moment = force × perpendicular distance from the pivot

🧑‍🏫 Teacher's explanation

Imagine you are trying to open a heavy door. If you push near the hinges, it feels incredibly difficult. If you push at the handle, far from the hinges, it opens easily. This is because a 'moment' is just a scientific way of measuring how much a force wants to rotate an object around a fixed point, called a pivot.

📖 Definitions

🔍 Worked example — Easy

A force of 20 N is applied perpendicularly to a door at a distance of 0.8 m from the hinges. Calculate the moment of the force.

🔍 Worked example — Standard

A uniform meter ruler is balanced at its center (the 50 cm mark). A weight of 4 N is placed at the 10 cm mark. Where must a 2 N weight be placed to keep the ruler in equilibrium? Give your answer as a distance from the center.

🔍 Worked example — Exam-style

A lever is used to lift a heavy rock. A force of 500 N is applied at the end of a 2.5 m lever. The rock requires a moment of 1250 N m to be lifted. Calculate the perpendicular distance from the pivot to the point where the rock's weight acts, assuming the force is applied perpendicularly.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Clockwise and anticlockwise

A moment can turn something clockwise or anticlockwise about the pivot. Work out the direction for each force separately.

🧑‍🏫 Teacher's explanation

Imagine you are trying to open a heavy door. If you push near the hinges, it is very hard; if you push far from the hinges, it is easy. This 'turning effect' is what we call a moment. The further you are from the pivot (the hinge), the more 'turning power' you have for the same amount of force.

📖 Definitions

🔍 Worked example — Easy

A force of 15 N is applied perpendicular to a door at a distance of 0.8 m from the hinge. Calculate the moment of the force.

🔍 Worked example — Standard

A uniform meter ruler is balanced at its center (the 50 cm mark). A weight of 4.0 N is placed at the 10 cm mark. Where must a 2.0 N weight be placed to maintain equilibrium?

🔍 Worked example — Exam-style

A lever is pivoted at point P. A 50 N force is applied downwards at a distance of 2.0 m to the left of P. A 125 N force is applied downwards at a distance of x to the right of P. Find x for the lever to be in equilibrium.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — The principle of moments

*For an object in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point.*

🧑‍🏫 Teacher's explanation

Imagine you are trying to open a heavy door. If you push near the hinges, it is incredibly hard to move. If you push far from the hinges, it is very easy. This is because the 'turning effect' depends not just on how hard you push, but where you push. The further you are from the pivot (the hinge), the more leverage you have.

📖 Definitions

🔍 Worked example — Easy

A force of 15 N is applied perpendicularly to a door at a distance of 0.8 m from the hinges. Calculate the moment of the force.

🔍 Worked example — Standard

A uniform meter ruler is pivoted at the 50 cm mark. A 2.0 N weight is placed at the 10 cm mark. Calculate the magnitude of a single force required at the 90 cm mark to balance the ruler.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Practical notes

Result: in every balanced arrangement, the total clockwise moment equals the total anticlockwise moment (within experimental error), which verifies the principle. Hang the weights from loops of thread so the distances can be measured precisely, and read the rule at eye level.

Worked example

Example 1: a spanner. A force of 50 N is applied at right angles to a spanner, 0.30 m from the nut.

🔍 Worked example — Easy

A spanner is used to tighten a nut. A force of 50 N is applied at right angles to the handle, 0.30 m from the nut. Calculate the moment of the force.

🔍 Worked example — Standard

A see-saw is balanced. Ali, weighing 300 N, sits 2.0 m to the left of the pivot. Sara, weighing 400 N, sits on the right side. How far from the pivot must Sara sit to maintain equilibrium?

⚠️ Common errors

Common mistakes

Practice

Answers: (1) 25 × 0.80 = 20\ N m. (2) At the handle the perpendicular distance from the pivot (hinge) is larger, so a smaller force produces the moment needed. (3) For an object in equilibrium, the sum of the clockwise moments about a point equals the sum of the anticlockwise moments about the same point. (4) 600 × 1.0 = 400 × d, so d = 1.5\ m. (5) Balance the rule at its centre; hang weights on each side and adjust until it balances; measure the distances; show that the total clockwise moment equals the total anticlockwise moment; repeat for other arrangements.

Ch 9 · 1.5.5 Turning Effect of Forces · O Level Physics 5054

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