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Laws of Motion

Newton's laws, friction and pulleys. Lighter than in the early papers, but its free-body habit runs through all of mechanics.

Questions in the bank
101
q/paper in 2025–26
0.52
Numeric answer
15%
Notes pages
5

Tier: Long tail

When you’ll see it

Forces on one or more bodies — strings, pulleys, friction, a lift or an accelerating frame — with an acceleration, a tension or a reaction to find.

How this chapter is tested

Almost every question is solved the same way: draw each body on its own, mark every force on it, and write F = ma along the direction it can move. Connected bodies share one acceleration; then cut the string at one body to get the tension.

Friction questions are mostly about the normal reaction, because a force at an angle or a tilted surface changes N and the friction with it. Check that a block moves at all before using kinetic friction.

The chapter is set less often than in the early papers. Marks are lost on a sign or a direction: a pseudo force put along the acceleration instead of against it, or a rebound's momentum change taken as mv instead of 2mv.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Second law, impulse and variable mass

    Net force is the rate of change of momentum: ma for fixed mass, an impulse for a brief force, v dm/dt for rockets, belts and jets.

  • Equilibrium of forces

    At rest or at constant velocity the forces along any two perpendicular directions each add to zero.

  • Connected bodies and pulleys

    Treat the bodies as one system for the acceleration, then one body for each tension; a movable pulley halves the load's acceleration.

  • Static and kinetic friction

    Find N first; static friction takes any value up to μs N, kinetic friction is μk N, and on an incline N = mg cos θ.

  • Lifts, pseudo forces and circular motion

    In an accelerating frame add −ma₀ to every body; in circular motion the real forces supply mv²/r towards the centre.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • The pseudo force along the acceleration

    A bob in a car speeding up forward swings back. The pseudo force points against the frame's acceleration, and it is used in that frame only.

  • A rebound taken as mv

    Bouncing back at the same speed, Δp = 2mv. Using mv halves the force, and that wrong answer is always an option.

  • Kinetic friction before checking

    If the driving force is below the largest static friction, a = 0. Static friction is only as large as it needs to be.

  • N taken as mg

    A push down at an angle adds F sin θ to N and a pull up takes it away; on an incline N = mg cos θ. Using mg gives one of the wrong options.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Laws of Motion notes

Drill every Laws of Motion question

101 questions from the bank, across 5 subtopics.

Drill the 101 questions

Drill one subtopic at a time

The 5 subtopics, in teaching order.

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