Forces
Resultant force and Newton's laws, friction, springs and Hooke's law, circular motion, turning effect and equilibrium, and centre of gravity.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 1.5.1Describe the effects of forces on shape and motion, and determine the resultant of forces acting along the same line.
- 1.5.2Recall and use F = ma.
- 1.5.3Describe the extension of a spring and interpret load–extension graphs, including the limit of proportionality.
- 1.5.4Calculate the moment of a force about a pivot and apply the principle of moments to a balanced system.
- 1.5.5Describe motion in a circle as requiring a resultant force directed towards the centre.Supplement
8 minute read
Resultant force and Newton's laws
A force is a push or a pull. It can change an object's speed, its direction, or its shape.
Resultant force
When several forces act on an object, only the resultant matters. Along a straight line, choose one direction as positive and add them with signs.
Newton's laws in the language 0625 uses:
- If the resultant force is zero, a stationary object stays stationary and a moving object keeps moving at constant velocity.
- If the resultant force is not zero, the object accelerates in the direction of that force, with
F = ma. - Forces come in pairs: if A pushes B, B pushes A with an equal force in the opposite direction.
Friction and drag
Friction opposes motion between surfaces in contact. Air resistance (drag) is friction with a fluid and grows with speed. Both transfer energy to the internal (thermal) store of the surroundings.
Springs and Hooke's law
For a spring, extension is directly proportional to load — up to the limit of proportionality. On a load–extension graph this is the point where the straight line starts to curve. Beyond the elastic limit the spring will not return to its original length.
Be precise: the graph is a straight line through the origin while the spring obeys the law.
Turning effects
The moment of a force about a pivot is moment = force × perpendicular distance from the pivot, in N m.
For a body in equilibrium, two conditions hold at once:
- the resultant force is zero
- the sum of clockwise moments equals the sum of anticlockwise moments about any point
That second statement is the principle of moments, and it solves almost every beam question in the paper.
Think of it like this
A moment is why a long spanner loosens a bolt that a short one cannot: the same push, applied further from the pivot, produces a bigger turning effect.
Worked examples
Method, step by step
A 1200 kg car experiences a resultant forward force of 3000 N. Calculate its acceleration.
- 1F = ma, so a = F / m
- 2a = 3000 / 1200
a = 2.5 m/s²
A uniform beam is pivoted at its centre. A 20 N weight sits 0.30 m to the left of the pivot. How far to the right must a 12 N weight be placed to balance it?
- 1Principle of moments: clockwise moments = anticlockwise moments
- 220 × 0.30 = 12 × d
- 36.0 = 12d, so d = 0.50 m
d = 0.50 m from the pivot
Common misconceptions
- Thinking a moving object needs a constant force to keep moving. With zero resultant force it keeps moving at constant velocity.
- Using the distance along the beam rather than the perpendicular distance from the pivot when the force is at an angle.
- Confusing the limit of proportionality with the elastic limit — they are different points on the graph.
In the exam
- For balanced-beam questions, always write "clockwise moments = anticlockwise moments" as your first line. It is often a mark on its own.
- Moments are in N m — do not convert distances to centimetres and forget to change back.
- When a question says "explain in terms of forces", name each force and say whether it is bigger, smaller or equal to the others.