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Laws of Motion and Forces: SPI Physics

Laws of Motion and Forces has 5 questions in the SPI bank, and two of them are from the 2022 paper. Two are EASY and three MODERATE. Three are about momentum and need one short calculation each, with the units and directions kept straight; the other two ask for a named fact about forces. So this chapter rewards care more than cleverness: bullets per minute turned into per second, grams into kilograms, and momenta in opposite directions subtracted, not added.

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.

SPI home All formulas on one page

Subtopic notes

Formula & revision sheet

3 formulas · 2 reference tables · 9 gotchas across all subtopics — the exam-eve cheat-sheet

Momentum and Force

Formulas (3)

Newton's three laws of motion · Newton's second law $F = ma$
Momentum and force as its rate of change · Momentum and force $p = mv \qquad F = \frac{\Delta p}{\Delta t} \qquad F = n \times m v \text{ for } n \text{ bullets each second}$
Conservation of momentum in collisions and recoil · Conservation of momentum $m_1 u_1 + m_2 u_2 = m_1 v_1 + m_2 v_2$

Watch out for (6)

Cancelling action against reaction
→ Newton's three laws of motion
Using bullets per minute
→ Momentum and force as its rate of change
Bringing the gun's mass into the force
→ Momentum and force as its rate of change
Leaving grams in the momentum
→ Momentum and force as its rate of change
Adding momenta that point opposite ways
→ Conservation of momentum in collisions and recoil
Forgetting the direction of the answer
→ Conservation of momentum in collisions and recoil

Fundamental Forces and Levers

Reference tables (2)

The four fundamental forces4 rows

Force	Strength (strong nuclear = 1)	Range	Acts between
Strong nuclear	1 (strongest)	About 10⁻¹⁵ m, inside a nucleus	Protons and neutrons
Electromagnetic	About 10⁻²	Unlimited	Charged particles
Weak nuclear	About 10⁻¹³	Less than 10⁻¹⁶ m	Particles in radioactive (beta) decay
Gravitational	About 10⁻³⁹ (weakest)	Unlimited	All bodies with mass

Weakest to strongest: gravitational, weak nuclear, electromagnetic, strong nuclear.

The three classes of lever3 rows

Class	In the middle	Examples
First	Fulcrum	See-saw, scissors, pliers, crowbar, beam balance
Second	Load	Nutcracker, wheelbarrow, bottle opener, door on hinges
Third	Effort	Fire tongs, forceps, fishing rod, the human forearm

Memory key, in class order 1, 2, 3, the middle point is F, L, E.

Watch out for (3)

Thinking gravity is strong because planets feel it
→ The four fundamental forces
Classing by where the fulcrum is
→ The three classes of lever
Calling scissors a second class lever
→ The three classes of lever

Question weightage by concept

5 concepts · 5 questions — where the marks actually sit, so you know what to drill first

Momentum and Force3 questions · 60%

Concept	Qs	Share
Momentum and force as its rate of change	2	40%
Conservation of momentum in collisions and recoil	1	20%
Newton's three laws of motionfoundation	—	—

Fundamental Forces and Levers2 questions · 40%

Concept	Qs	Share
The four fundamental forces	1	20%
The three classes of lever	1	20%

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