IP Physics Notes (Upper Secondary, Year 3-4): 4) Turning Forces

Study guide30 Sep 2025, 00:00 ZUpdated 30 Nov 2025

Apply the moment formula, balance clockwise and anticlockwise torques, locate the centre of gravity, and judge stability in real levers.

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Q: What does IP Physics Notes (Upper Secondary, Year 3-4): 4) Turning Forces cover?
A: Apply the moment formula, balance clockwise and anticlockwise torques, locate the centre of gravity, and judge stability in real levers.

Quick recap -- A turning effect appears when a force acts with an offset from a pivot. Balance comes from matching clockwise and anticlockwise moments while keeping the net force zero.

The core idea is simple: Moment equals force times perpendicular distance from the pivot.

Use it as a working check: Balance turning effects by matching clockwise and anticlockwise moments. Stability depends on where the centre of gravity falls relative to the base.

Then go one layer deeper: Use the beam and stability examples to practise marking the pivot, measuring perpendicular distances, writing moment equations, and checking force balance.

Keep your practice loop tight via our IP Physics tuition hub-it links each topic here to quizzes, diagnostics, and WA-style problem sets.

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These notes align with SEAB GCE O-Level Physics (6091) content used in IP programmes (exams from 2026).

Status: SEAB O-Level Physics 6091 syllabus (exams from 2026) checked 2025-11-30 - scope unchanged; remains the reference for these notes.

Moment of a Force

The moment (torque) of a force measures its turning effect about a pivot: $\tau = F d_\perp$

Reviewed by

Chee Wei Jie·Academic Advisor (Physics)

Situation in the diagram	Best pivot choice	Why it helps	Common trap
A support reaction at a hinge is unknown	Take moments about the hinge.	The reaction acts through the pivot, so its perpendicular distance is zero.	Taking moments about the load and then carrying an extra unknown reaction.
A beam has two support reactions	Take moments about one support first.	One reaction disappears, leaving the other support reaction easier to find.	Forgetting to use force balance after the first reaction is found.
A force is angled to the beam	Use the stated pivot, then resolve the force perpendicular to the arm.	Only the perpendicular component creates the turning effect.	Multiplying the full angled force by the beam length.
The object is uniform and has weight	Mark the centre of gravity before choosing moments.	The weight acts at the centre of gravity and may create its own moment.	Treating the beam as light when the question gives its weight or mass.

Step	What to mark	Decision to make	Common trap
1	Mark the base of support, including the left and right edges touching the ground.	The object can only resist tipping while the weight acts through this base.	Treating the whole object width as the base when only one small contact patch touches the ground.
2	Draw a vertical line down from the COG.	If the line falls inside the base, the weight creates a restoring moment.	Looking only at COG height and ignoring the vertical line.
3	Check whether the vertical line passes exactly through a base edge.	This is the limiting position: a tiny extra tilt can make the object topple.	Calling it stable just because it has not fallen yet.
4	Check whether the vertical line falls outside the base.	The weight now creates an overturning moment about the base edge.	Saying a low COG prevents toppling in every tilted position.

IP Physics Notes (Upper Secondary, Year 3-4): 4) Turning Forces

Moment of a Force

Principle of Moments

Pivot-choice checkpoint

Common Moment Scenarios

Centre of Gravity

Stability and Equilibrium Types

Stability line-of-action checkpoint

Worked Example: Uniform Beam with Hanging Masses

Practice Quiz

Key Takeaways

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