Sir Isaac Newton gave three fundamental laws that explain how objects move. These laws form the foundation of classical mechanics. If you understand these laws properly, Physics becomes almost predictable—like knowing your friend will still come late even after saying “I’m on my way.” 😭
Let’s break them down nicely.
Statement:
A body will continue in its state of rest or uniform motion in a straight line unless acted upon by an external force.
Meaning:
Objects don’t just start moving or stop moving on their own.
Something must push or pull them.
This “resistance to change in motion” is called inertia.
Key Points:
Inertia depends on mass — the heavier the object, the more inertia it has.
Bigger mass = harder to start or stop.
Examples:
When a car stops suddenly, your body lunges forward.
A football stays where it is until someone kicks it.
A moving bicycle continues rolling until friction slows it down.
Real Life Trick:
If JAMB asks: “Which law explains why passengers fall forward when a bus stops?”
→ First Law (Inertia).
Statement:
The rate of change of momentum is directly proportional to the applied force and occurs in the direction of the force.
This can be simplified to the famous formula:
Where:
F = Force (N)
m = Mass (kg)
a = Acceleration (m/s²)
Meaning:
The more force you apply, the more acceleration you get.
But if the object is heavy (large mass), you need more force to get the same acceleration.
Key Points:
Momentum (p) = m × v
Force is the reason objects speed up, slow down, or change direction.
If no force → no acceleration → motion stays constant.
Examples:
Pushing a small boy makes him move faster than pushing a fat uncle (mass difference 😭).
A car accelerates when you press the pedal — engine provides force.
JAMB Tip:
If they ask: “What is the acceleration of a 10 kg box pushed by a 50 N force?”
Use a = F/m → 50/10 = 5 m/s².
Statement:
For every action, there is an equal and opposite reaction.
Meaning:
Forces come in pairs.
When one object pushes another, the other pushes back with equal force.
Key Points:
Action and reaction occur simultaneously, not one after the other.
They act on different bodies, never on the same object.
Examples:
When you jump, your legs push the ground → ground pushes you upward.
A gun recoils backward when fired.
A rocket moves upward because hot gases push downward.
JAMB Trick:
If they ask:
“Why does a swimmer move forward when pushing water backward?”
→ Third Law.
| Law | Key Idea | What It Explains |
|---|---|---|
| 1st | Inertia | Why objects stay still or move at constant speed |
| 2nd | Force causes change | How force affects acceleration |
| 3rd | Action–Reaction | Interactions between two bodies |
State Newton’s first law of motion.
Explain why a rocket moves in space.
A 5 kg mass is accelerated at 4 m/s². Calculate the force.
→ F = 5 × 4 = 20 N
Which law explains recoil of a gun?
→ Third Law