Newton’s First Law of Motion is one of the cornerstones of IB Physics. Often called the law of inertia, it explains why objects behave the way they do when no external forces act on them. Although the statement itself feels simple, it is fundamental to understanding dynamics, free-body diagrams, equilibrium, and motion analysis throughout the course. Students aiming for top marks quickly realise that mastering these basics strengthens their performance across every physics topic. This is why guides like The Quest for a 7 in IB Physics (https://www.revisiondojo.com/blog/the-quest-for-a-7-in-ib-physics) emphasise the importance of getting core principles right before moving to more complex problems.
What Newton’s First Law Actually States
Newton’s First Law says:
“An object will remain at rest or continue moving at constant velocity unless acted upon by a net external force.”
This statement has two equally important parts:
- If the object is at rest, it stays at rest.
- If the object is moving, it continues to move in a straight line at constant speed.
In other words, motion only changes when a net external force acts. This is why the law is often described as the law of inertia—objects naturally resist changes to their motion.
Understanding this concept early helps enormously when tackling later mechanics problems, especially in exams. The ability to identify when the net force is zero is essential, and students who train this skill tend to do well across Papers 1 and 2. The article How to Get a 7 in IB Physics (New Syllabus 2025 Onward) (https://www.revisiondojo.com/blog/how-to-get-a-7-in-ib-physics-new-syllabus-2025-onward) highlights this as a key habit of high-scoring students.
Why Newton’s First Law Matters in IB Physics
Newton’s First Law forms the basis for:
- Recognising equilibrium situations
