Newton's Laws of Motion: quick revision, 15 bites

Three Laws That Govern All Motion

Everything that moves obeys these 3 rules.

Newton's three laws form a complete framework for understanding force and motion in everyday life.

Takeaway. Master these three laws and you can predict how any object responds to forces.

Newton's First Law: The Law of Inertia

Objects are lazy — they resist change.

Takeaway. No net force means no change in motion — inertia is why you lurch forward when a bus brakes.

Newton's Second Law: F = ma

The equation that links force, mass, and acceleration.

Takeaway. Force and acceleration are directly proportional; mass is the resistance to acceleration.

Newton's Third Law: Action and Reaction

Push something — it pushes back equally.

The forces do **not** cancel because they act on **different objects**.

Takeaway. Forces always come in pairs — but each force acts on a different object, so they never cancel out.

Action–Reaction Forces Don't Cancel

They're equal — so why does anything move?

Takeaway. Cancellation requires forces on the same body — action–reaction pairs act on two different bodies.

Why You Lurch Forward When a Bus Brakes

Your body keeps moving even when the bus stops.

Takeaway. Seatbelts exist because of inertia — they provide the force needed to stop your body with the car.

Rockets Work Because of the Third Law

Rockets push gas down to go up.

A rocket expels hot gas downward at high speed. By Newton's Third Law, the gas pushes the rocket upward with an equal and opposite force.

Takeaway. Rockets don't push against air — they push against their own exhaust, and the exhaust pushes back.

Calculating Acceleration with F = ma

Plug in force and mass — get acceleration.

Takeaway. Rearrange F = ma to a = F/m whenever you need acceleration from force and mass.

Mass vs. Weight: Not the Same Thing

Mass is matter; weight is force.

Takeaway. Mass is intrinsic and constant; weight depends on the local gravitational field.

Weight Is a Force: W = mg

Your weight is just gravity pulling on your mass.

A 60 kg student weighs about $60 \times 9.8 = 588$ N on Earth — but only about 98 N on the Moon.

Takeaway. Weight is the force of gravity on a mass — it changes with g, but mass does not.

Remember the Three Laws with 'I-F-R'

Three letters, three laws.

Takeaway. I-F-R: Inertia, Force, Reaction — in that order.

Always Draw a Free-Body Diagram First

One diagram saves ten mistakes.

Takeaway. Free-body diagrams turn messy force problems into clean equations — examiners award method marks for them.

Newton's Laws Only Hold in Inertial Frames

These laws need a stable viewpoint.

Newton's laws are valid only in **inertial (non-accelerating) reference frames**. In an accelerating frame, you must add pseudo-forces (like centrifugal force) to make the laws work.

Takeaway. Newton's laws assume an inertial frame — accelerating frames require fictitious forces.

Newton Published the Laws in 1687

Over 300 years old — still used daily.

Isaac Newton published his three laws in *Philosophiæ Naturalis Principia Mathematica* in **1687**. They remain the backbone of classical mechanics and are still used to send spacecraft to other planets.

Takeaway. Newton's 1687 Principia introduced the three laws that still power modern engineering.

Check Units to Catch Mistakes

Wrong units = wrong answer.

Takeaway. Dimensional analysis catches most algebra slips before they cost you marks.