By Thread Academy · 15 September 2026 · Physics
Newton's laws of motion are three simple rules that describe how objects move when forces act on them. Written over 300 years ago, they still explain everything from a falling apple to a rocket launch. Each law is easy to state, but the exam rewards students who understand what they really mean — so let us take them one at a time.
Newton's first law: inertia
First law: An object stays at rest or keeps moving at a constant velocity unless a resultant force acts on it.
This is the law of inertia — the tendency of objects to resist changes in motion. A book on a table stays put. A hockey puck sliding on ice keeps gliding with almost no change in speed because friction is tiny.
The first law also explains why passengers lurch forward when a bus brakes suddenly. Their bodies tend to keep moving at the original speed while the bus slows beneath them. Seatbelts provide the force that changes the passenger's motion safely.
Newton's second law: F = ma
Second law: The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass.
This gives the equation:
F = ma
where F is the resultant force in newtons, m is the mass in kilograms and a is the acceleration in metres per second squared.
Worked example 1: Force from mass and acceleration
Question: A 900 kg car has a resultant force of 2,700 N acting on it. Calculate its acceleration.
Step 1: Rearrange F = ma to make a the subject: a = F / m.
Step 2: Substitute: a = 2,700 / 900 = 3.
Answer: The acceleration is 3 m/s^2 in the direction of the resultant force.
Worked example 2: Force with friction
Question: A 60 kg crate is pushed with a force of 240 N. Friction opposes the motion with 120 N. Calculate the acceleration of the crate.
Step 1: Find the resultant force first. The push and friction oppose each other: 240 - 120 = 120 N forward.
Step 2: Use a = F / m = 120 / 60 = 2.
Answer: The crate accelerates at 2 m/s^2.
Note the trap: using 240 N instead of the resultant 120 N would give the wrong answer. Always subtract opposing forces first.
Newton's third law: action and reaction
Third law: For every action there is an equal and opposite reaction.
Whenever one object pushes on another, the second object pushes back just as hard in the opposite direction. These are called action-reaction pairs.
Everyday examples are everywhere:
- You push down on the ground as you walk; the ground pushes you forward, which is what actually moves you.
- A rocket expels hot gases downward; the gases push the rocket upward.
- A swimmer pushes water backward; the water pushes the swimmer forward.
The crucial point is that the two forces act on different objects. The force on the rocket pushes it up; the force on the gases pushes them down. Because they act on different objects, they never cancel each other out.
Worked example 3: Using all three laws
Question: A 2 kg book rests on a table. Explain the forces acting on it using Newton's laws.
Step 1 (first law): The book is at rest, so the resultant force on it must be zero. The forces are balanced.
Step 2 (identify forces): Gravity pulls the book down with a weight of 2 times 10 = 20 N. The table pushes up on the book with a normal contact force of 20 N.
Step 3 (third law): The book pushes down on the table with 20 N, and the table pushes back up on the book with 20 N — an action-reaction pair acting on different objects.
Answer: Weight (20 N down) and normal force (20 N up) balance, so the book stays at rest. The book-table push pair shows Newton's third law in action.
Common exam misconceptions
Misconception 1: "Objects need a force to keep moving." Not true — the first law says a moving object keeps moving unless a force stops it. You only need a force to change the motion.
Misconception 2: "Action and reaction cancel out." They cannot cancel, because each acts on a different object. Cancellation only happens when two opposite forces act on the same object.
Misconception 3: "F = ma uses the biggest force." It uses the resultant force — the sum of all forces after subtracting opposites. Forgetting friction is the most common arithmetic error.
Misconception 4: "Heavier objects fall faster." In a vacuum, everything falls with the same acceleration because although gravity pulls harder on a heavier object, that object also has more inertia to overcome. Air resistance, not mass, makes feathers fall slowly.
Key takeaways
- First law: no resultant force means no change in motion — an object at rest stays at rest, and a moving object keeps its velocity.
- Second law: F = ma links resultant force, mass and acceleration; always use the resultant force, not just the push.
- Third law: forces come in equal, opposite pairs acting on different objects — this is why rockets, walking and swimming work.
- Action and reaction never cancel because they act on different objects.
- A force is not needed to keep an object moving, only to change its motion.