CDS Physics · Free notes

Laws of Motion: CDS Physics

Laws of Motion and Forces has 9 past-year questions in CDS General Knowledge, from 2017 (II) to 2026 (II): 4 EASY and 5 MODERATE. They ask what Newton's laws say far more often than they ask you to calculate. Watch for two traps: a word like 'always' in a statement about momentum, and a stem that credits a result to the wrong law. So learn each law in its exact words, learn the one condition for momentum to be conserved, and practise the short sum that turns a force into a change of speed.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

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Subtopic notes

Newton's Laws and Momentum
9 PYQs
What happens when the net force is zero, how a net force changes momentum, and when the total momentum of a system stays the same.

Formula & revision sheet

2 formulas · 1 reference tables · 8 gotchas across all subtopics — the exam-eve cheat-sheet

Newton's Laws and Momentum

Formulas (2)

Second law: force changes momentum · Second law of motion $p = mv \qquad F = \dfrac{\Delta p}{\Delta t} = ma \qquad \Delta v = \dfrac{Ft}{m} \qquad K = \tfrac{1}{2}mv^{2} = \dfrac{p^{2}}{2m}$
Conservation of momentum · Conservation of momentum $m_1u_1 + m_2u_2 = m_1v_1 + m_2v_2 \qquad \text{recoil from rest: } m_1v_1 = -m_2v_2$

Reference tables (1)

Zero net force: inertia and equilibrium5 rows

Situation	What happens
Object sliding on a frictionless horizontal surface	Moves with constant velocity (first law)Q CDS 2017 (II): with the forces balanced and no friction, the object moves with constant velocity.
Rod balanced on one end	Unstable equilibriumQ CDS 2026 (II): a rod balanced vertically on one end is in unstable equilibrium.
Cone resting on its flat base	Stable equilibrium
Ball on a level floor	Neutral equilibrium
Rope with a weight at its middle, pulled straight	Needs an infinitely large tensionQ CDS 2018 (II): the tension needed to make the rope perfectly straight is infinitely large.

Watch out for (8)

Weight and normal reaction are not an action-reaction pair
→ Zero net force: inertia and equilibrium
Zero net force does not mean at rest
→ Zero net force: inertia and equilibrium
Tilt test for equilibrium
→ Zero net force: inertia and equilibrium
Force changes momentum, not displacement or acceleration
→ Second law: force changes momentum
Change the speed into m/s before using F = ma
→ Second law: force changes momentum
Watch the word 'always'
→ Conservation of momentum
Running and rolling need friction
→ Conservation of momentum
Torque is not the condition for linear momentum
→ Conservation of momentum

PYQ weightage by concept

3 concepts · 9 PYQs — where the marks actually sit, so you know what to drill first

Newton's Laws and Momentum9 PYQs · 100%

Concept	PYQs	Share
Zero net force: inertia and equilibrium	3	33%
Second law: force changes momentum	3	33%
Conservation of momentum	3	33%

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