Moments and equilibrium
Learn how a force turns a door, spanner or seesaw; calculate the turning effect; and check when an object is truly balanced. Core ideas come first, with Supplement / Extended ideas clearly marked. This is Cambridge International IGCSE Physics 0625, separate from Egypt’s Egyptian Baccalaureate / Thanaweya Amma and not a Year 3 course.
A force can make something turn
A moment is the turning effect of a force about a pivot. A door turns about its hinges; a spanner turns a nut; a seesaw turns about its support. The pivot is the fixed point or line that the object turns around.
Open the worked example
Push a door near its handle. It turns easily because the push acts far from the hinge. Push just beside the hinge and the same force has a much smaller turning effect.
Quick check: Name the pivot of a door.
Think, then reveal: The hinge is the pivot: the door turns about the hinge line.
Change the forces; inspect the turning
Interactive lever · balance the moments
Model: a light horizontal beam with one vertical downward force on each side of a central pivot. Left gives an anticlockwise moment; right gives a clockwise moment. This model checks turning balance only; the support’s upward force balances the total weight.
Anticlockwise moment is larger by 3.00 N m.
Keep the distances fixed and try a force that makes the two moments equal. To balance the current left force at this right distance, the right force would need to be 16.00 N.
Practise, then read the explanation
Questions 1–9 practise Core ideas; questions 10–12 are Supplement / Extended only. Choose an answer, open a hint if useful, then submit to see the reasoning.
Optional hint
Think about what a force on a door or spanner can make it do.
Original lesson content aligns to Cambridge IGCSE Physics 0625 syllabus objectives 1.5.2.1–1.5.2.6. Core: define moment, use moment = force × perpendicular distance, apply the principle of moments for one force on each side, and state both equilibrium conditions. Supplement / Extended only: balance several moments on each side and describe a metre-rule experiment. Source reading: Physics-0625 Volume A, PDF pp. 77–83 (printed pp. 69–75). The slider activity is an illustrative model, not experimental data.