In IB Physics, Newton’s First Law feels like one of those sentences you already “know,” right up until an exam question quietly asks for a free-body diagram, or a justification of why acceleration is zero, and suddenly the simplicity disappears.
That’s the strange power of the First Law. It’s not flashy. It doesn’t promise a clever shortcut. But it sits underneath almost every mechanics mark you’ll ever earn: equilibrium, constant velocity, friction, drag, and the moment you decide whether (\sum F = 0) or not.
If you can make Newton’s First Law automatic in IB Physics, the rest of dynamics becomes calmer.
Newton’s First Law (IB Physics definition, unpacked)
Newton’s First Law states:
An object remains at rest or continues in uniform motion in a straight line unless acted upon by a net external force.
In IB Physics, two phrases do most of the work:
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“Net external force” means the vector sum of all forces. Not “one force.” Not “the biggest force.” The resultant.
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“Uniform motion” means constant velocity (so constant speed and constant direction).
So the exam-friendly translation is:
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If (\sum F = 0), then (a = 0)
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If (a = 0), velocity is constant (including possibly (v = 0))
This is why the First Law is often called the law of inertia: matter resists changes to its motion.
For a tight syllabus-aligned summary, revise alongside IB Physics A.2.1 Newton’s Laws of Motion Notes.
A quick IB Physics checklist for First Law questions
When a problem smells like Newton’s First Law in IB Physics, run this checklist before you calculate anything:
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Are they hinting constant speed or constant velocity?
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Are they hinting “at rest”, “moves with steady speed”, or “terminal/constant speed”?
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Can you state (in words) whether (\sum F = 0)?
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Have you drawn a free-body diagram with all forces that actually act?
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Did you label forces clearly ((mg), (N), friction (f), drag (D), tension (T))?
For practice that matches examiner habits, the IB Physics A.2 Forces and Momentum Questionbank is ideal for drilling “net force or not?” decisions.
Why Newton’s First Law matters so much in IB Physics
Most students don’t lose marks because they “don’t know” Newton’s First Law. They lose marks because they don’t recognize when it’s the hidden premise.
In IB Physics, the First Law powers three high-frequency skills:
Free-body diagrams that actually earn marks
A correct free-body diagram is basically Newton’s First Law made visible: if arrows balance, acceleration is zero.
If your diagrams feel messy, rebuild the foundations with Notes for 2.2: Forces and then reinforce definitions with the IB Physics Glossary.
Equilibrium (including “moving equilibrium”)
Equilibrium is not a vibe. It’s a condition:
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Static equilibrium: (v = 0) and (\sum F = 0)
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Dynamic equilibrium: (v) constant and (\sum F = 0)
That second one is where IB Physics questions get sneaky. You see motion, so you assume there must be a net force. But constant velocity means no net force.
Setting up Newton’s Second Law correctly
Newton’s Second Law is the calculator. Newton’s First Law is the decision about whether you even need the calculator.
If (a = 0), then (\sum F = ma = 0). That single line saves time, reduces algebra, and prevents the classic “solve for acceleration that should be zero” mistake.
Everyday examples (and what IB Physics wants you to say)
Examples help, but in IB Physics the mark is usually in the explanation: naming forces, stating the resultant, and linking it to acceleration.
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A book on a table: weight (mg) down, normal (N) up, (\sum F = 0), so (a = 0). It stays at rest.
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A hockey puck sliding: if friction is negligible, resultant force is ~0, so velocity stays (approximately) constant. If friction acts, net force is opposite motion, so it slows.
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A passenger lurching forward when a car brakes: the person’s body tends to maintain its velocity (inertia). The seatbelt provides the net external force to change the motion.
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A spacecraft drifting: with thrusters off, net external force is ~0, so it continues at constant velocity.
If you want a related application with constant-speed reasoning, What Is Terminal Velocity? is a great extension because it’s Newton’s First Law in disguise: constant speed implies (\sum F = 0).
The misconceptions that cost marks in IB Physics
Newton’s First Law misconceptions are predictable, which is good news: you can train them out.
“If it’s moving, there must be a force in the direction of motion”
In IB Physics, constant velocity requires no resultant force, not a “maintaining force.” Motion continues because there is no net force to change it. Forces cause changes in motion, not motion itself.
“Net force equals zero means no forces act”
This is the biggest wording trap. You can have multiple forces that cancel (like (N) and (mg)), giving (\sum F = 0). Examiners love this distinction.
“Objects naturally slow down”
They slow down because of forces you forgot to include: friction, drag, resistive forces. If you include them in the diagram, Newton’s First Law stays intact.
To strengthen the “forces come from interactions” mindset, read How Forces Arise From Interactions Between Objects.
How to practise Newton’s First Law the RevisionDojo way
A useful pattern for IB Physics revision is: concept, diagram, question, feedback.
On RevisionDojo, that becomes:
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Start with concise theory in IB Physics A.2 Forces and Momentum Notes
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Drill exam-style items in the Mechanics Questionbank and 2.2 Forces Questionbank
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Lock in definitions with A.2 Flashcards
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Use AI Chat and Grading tools to explain where your diagram or net-force statement went wrong, then rebuild the habit
When you practise like this, Newton’s First Law stops being a quote you memorise and becomes a reflex you use.
Closing: make IB Physics feel simpler by mastering the first rule
Newton’s First Law is the quiet sentence that holds mechanics together. In IB Physics, it tells you when acceleration is zero, how to balance forces, and why the best free-body diagrams feel almost boring.
If you want that calm on exam day, build it now: revise the notes, drill targeted questions in RevisionDojo’s Questionbank, check your understanding with Flashcards and AI Chat, and use the Grading tools and Mock Exams to turn “I think it balances” into “(\sum F = 0), therefore (a = 0).” That’s how fundamentals become marks.
