Newton's Three Laws Explained Simply
May 9, 2026 · 5 min · newtons laws · physics help · mechanics
Newton's three laws cover almost every mechanics question on a physics paper. Here they are in plain English, with a question type for each.
First law (inertia)
An object stays at rest, or keeps moving at constant velocity, unless a force acts on it.
What the question looks like: "A 5 kg block sits on a frictionless surface. What happens when you stop pushing?" Answer: it keeps moving at constant velocity.
Second law
F = ma. Force equals mass times acceleration.
What the question looks like: "A 2 kg ball accelerates at 3 m/s². What force is acting?" Answer: 6 N.
This is the workhorse. Most numerical mechanics questions are F = ma in disguise.
Third law
Every action has an equal and opposite reaction.
What the question looks like: "A book sits on a table. Identify the action-reaction pair." Answer: book pushes table down with weight W, table pushes book up with W.
The trap: gravity (book pulled down by Earth) and the table's normal force (table pushes book up) are NOT a Newton's third law pair. They're two forces on the same object. A third law pair always involves two different objects.
Common pitfalls
- Forgetting that constant velocity (including zero velocity) means net force is zero
- Treating mass like weight. Mass is in kg. Weight is in N (mass × g).
- Mixing up Newton's third law pair vs balanced forces
Where the three laws are not enough
- Rotational motion (need angular momentum)
- Anything at the speed of light (need special relativity)
- Subatomic scales (need quantum mechanics)
For all of school physics through A Level, the three laws plus energy conservation cover everything.