Newton's Laws of Motion Explained Simply (With Formulas for Exams)

Newton gave us three laws that together explain why things move, stop, or change direction. Think of them as three friends who always work as a team: one talks about staying still, one about speeding up, and one about pushing back.

The animated lesson calls them the Big Three. First law: inertia, meaning objects resist change. Second law: F = ma, meaning force causes acceleration. Third law: action equals reaction. Learn these three one-liners and half your revision is done.

These laws aren't just for exams. A cricket ball that keeps flying after you hit it, a rocket lifting off, a book resting on your desk — all of it follows the same three rules.

The first law says an object stays at rest, or keeps moving at a constant velocity, unless a net force acts on it. That's it. No force, no change.

This tendency to keep doing what you're already doing is called inertia. And here's the bit most students get wrong: inertia is not a force. It's a property of matter — a resistance to change. Nothing is pushing you forward in that braking auto; your body just carries on moving because no force has stopped it yet.

Inertia depends on mass. A loaded truck has far more inertia than a cycle, which is why stopping a truck takes so much longer. Heavier object, bigger resistance to change.

The second law is the most tested formula in physics: F = m × a. Force equals mass times acceleration.

The units matter as much as the formula. Force is measured in newtons (N), mass in kilograms (kg), and acceleration in metres per second squared (m/s²). One newton is the force needed to accelerate a 1 kg mass at 1 m/s².

This one equation solves most numericals in the chapter. If the question gives you any two of force, mass and acceleration, you can always find the third by rearranging.

• F = m × a (find force)
• m = F ÷ a (find mass)
• a = F ÷ m (find acceleration)
• Unit check: N = kg × m/s²

Every action has an equal and opposite reaction. The forces are equal in magnitude, opposite in direction, and they act on different objects.

The rocket example makes it clear. The rocket pushes gas downwards; the gas pushes the rocket upwards. Two forces, equal size, opposite ways, on two different things. That's how the rocket rises.

The same thing happens when you walk. You push the ground backwards, and the ground pushes you forwards. Without that reaction, you'd just slip in place.

Here's the hook from the lesson: 'F = ma, objects resist, and forces come in pairs.' Three laws, three ideas, one sentence. Say it out loud before you enter the exam hall.

Map it back: objects resist is the first law (inertia), F = ma is the second law, and forces come in pairs is the third law. Once the hook is in your head, the rest follows.

Keep the units close too — N, kg, m/s². Examiners love a missing unit, and it's the easiest mark to lose.

Use this as your last-minute checklist. Read it once the night before and once in the morning.

If you can state each law in one line, give its formula, and name its trap, you're ready for almost any question from this chapter.

• First law: an object stays at rest or in constant motion unless a net force acts — this is inertia
• Second law: F = ma — force causes acceleration, units N, kg, m/s²
• Third law: every action has an equal and opposite reaction on a different object
• Trap 1: inertia is not a force
• Trap 2: force and acceleration point the same way
• Trap 3: action and reaction act on different objects, so they never cancel