State the three laws of motion

In one line

The rule

Newton's First Law: An object remains at rest or moves in a straight line at constant speed unless acted upon by an external force. Newton's Second Law: The force acting on an object is equal to the mass of the object multiplied by its acceleration. Newton's Third Law: For every action, there is an equal and opposite reaction.

Formulas:

• First Law: No formula (it is about what happens when no force acts)
• Second Law: F = m × a (F = force in newtons, m = mass in kg, a = acceleration in m/s²)
• Third Law: F₁ = −F₂ (F₁ = force by object 1 on 2, F₂ = force by object 2 on 1)

Let's understand it simply

Imagine you are sitting on a cycle. If you don't pedal or brake, the cycle keeps rolling at the same speed (First Law: things keep doing what they are doing unless something makes them change). Now, if you push a cricket ball and a football with the same force, the cricket ball moves faster (Second Law: lighter things speed up more for the same push). Finally, when you jump off a small boat, the boat moves backward (Third Law: your jump pushes the boat back as much as you go forward).

How it works

1. First Law: If no force acts, objects stay still or keep moving straight at the same speed.
2. Second Law: If a force acts, the object speeds up or slows down depending on its mass and the force size.
3. Third Law: When you push or pull something, it pushes or pulls you back equally but in the opposite direction.
4. Forces can be pushes or pulls—by hands, wind, gravity, or even friction.
5. These laws work together: every time something moves or stops, all three are at play.

The story behind it

Sir Isaac Newton, an English scientist, wrote these laws in 1687 in his book "Principia Mathematica." Before Newton, people did not clearly understand why things moved or stopped. Newton wanted to explain the motion of planets, falling apples, and even carts on the road. He showed that the same rules work for both the sky and the ground. Newton was inspired by simple things like falling fruit and rolling balls, and he used maths to write his ideas.

Where you see it in real life

• Cricket ball hit: The ball stays still until hit (First Law), speeds up based on force (Second Law), and the bat feels a push back (Third Law).
• Bus braking suddenly: You lurch forward (First Law: your body wants to keep moving).
• Rowing a boat: You push water backward, the boat moves forward (Third Law).
• Pushing a cart in the market: Heavier carts need more force to move (Second Law).
• Diwali rocket: The rocket pushes gases down, gases push the rocket up (Third Law).
• Rolling roti: The dough moves only when you push it (First Law).

Why it matters

• All machines: From cycles to cars, these laws explain how they move and stop.
• Engineering: Building bridges, lifts, and even amusement park rides uses these laws.
• Space travel: Rockets use the third law to lift off.
• Sports: Every kick, throw, or jump is explained by these laws.
• Future chapters: You will need these laws for chapters on friction, circular motion, work and energy, and more.

Worked example

Let's solve a problem using Newton's Second Law:

Given:  m = 5 kg,  F = 15 N
Step 1: Write the rule        F = m × a
Step 2: Rearrange for a       a = F / m
Step 3: Put in the numbers    a = 15 / 5
Step 4: Answer                a = 3 m/s²

Common mistakes

• Thinking motion needs force: Objects keep moving unless a force stops them (First Law).
• Mixing up action and reaction: The two forces act on different objects, not the same one (Third Law).
• Forgetting direction: Forces and acceleration have direction; always show which way they act.
• Ignoring mass: Heavier objects need more force for the same acceleration (Second Law).

Remember it

First: Things stay as they are unless pushed. Second: Force = mass × acceleration. Third: Every push gets a push back, equal and opposite.