Newton's Second Law: Force, Mass & Acceleration
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Lesson overview
Computing acceleration from a resultant force and mass using F = ma.
Go to lesson quiz →Exam guidance · WAEC WASSCE Paper 1 + Paper 2 Lens
Core idea (Kinematics,SUVAT & Newton's Laws of Motion): Pick a positive direction first, list knowns/unknowns (u, v, a, s, t), draw the free-body diagram before writing ΣF = ma, and check vector signs.
Pacing
Objective speed on Paper 1 + structured step-by-step method marks on Paper 2 Theory/Practical.
How WAEC marks this
Examiners award method marks (M1) for stating the formula/definition and accuracy marks (A1) ONLY when SI units are written.
Watch out for
Writing bare numbers without working or omitting SI units loses up to 50% of Paper 2 marks even when the final value is right.
Lesson content
Overview — Newton's Second Law: Force, Mass & Acceleration
Computing acceleration from a resultant force and mass using F = ma. This lesson is at the beginner level — by the end, you'll be able to recognise this question type, apply a reliable method, and avoid the traps that cost marks.
Key Concept
Newton's Second Law states that the acceleration of an object is directly proportional to the resultant force acting on it, and inversely proportional to its mass: F = ma, where F is in newtons (N), m in kilograms (kg), and a in metres per second squared (m/s²).
Why It's Tested This Way
WAEC and JAMB test this almost every year because it's the single equation linking force, mass and motion — the foundation for later topics like momentum, work and power. Expect it standalone, or combined with a diagram showing several forces acting on one object.
Method
To find acceleration, rearrange the formula: a = F ÷ m. Always check your units are in newtons and kilograms before dividing.
Worked Example
Worked example: A resultant force of 20 N acts on a 4 kg trolley. Acceleration = 20 ÷ 4 = 5 m/s².
Second Worked Example
Second worked example: A resultant force of 15 N acts on a 3 kg object. Acceleration = 15 ÷ 3 = 5 m/s² — notice a different force and mass can still give the same acceleration, since it's the RATIO that matters.
Common Traps
Common trap: students often forget that F must be the RESULTANT (net) force — if multiple forces act on an object, add or subtract them first before applying F = ma.
Lesson Summary
Remember: (1) identify what is being asked, (2) apply the WAEC strategy, (3) check against the traps. Complete the 10-question quiz below to unlock the next lesson — you need at least 60% to pass.
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