A level Physics · Edexcel 9PH0 · Topic 2B
Forces and Newton's laws
Key points
- Free-body force diagrams show every force acting on one object, drawn from the object as labelled arrows.
- Newton's first law: an object stays at rest or moves at constant velocity unless a resultant force acts.
- Newton's second law: (for constant mass). Resultant force and acceleration are in the same direction.
- Newton's third law: if A exerts a force on B, B exerts an equal and opposite force of the same type on A. These forces never act on the same body.
- Weight acts from the centre of gravity; an object topples when its centre of gravity moves outside its base.
- Moment = force × perpendicular distance from the pivot. Principle of moments: for equilibrium, total clockwise moment = total anticlockwise moment about any point.
- In equilibrium the resultant force and resultant moment are both zero; a closed vector triangle shows three forces in equilibrium.
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Newton's laws explain why motion changes: a resultant force produces an acceleration. This sub-topic also covers weight, centre of gravity, moments and the conditions for an object to be in equilibrium.
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