Imagine you're sitting in a bus that suddenly lurches forward. Your body jerks backward, pressed into the seat. Then the bus brakes hard — you fly forward. That feeling in your body, the way things seem to "want" to keep doing what they were doing, is the raw experience Newton captured in his first law.
The Core Intuition
Motion is not natural or unnatural. Objects don't "prefer" to be at rest. They don't "prefer" to move either. They simply keep doing whatever they're already doing — unless something forces them to change.
A ball on a table stays put. A hockey puck sliding on ice keeps sliding. The difference? Friction. On ice, there's almost no force to stop it, so it keeps going. On a rough table, friction acts as a force that slows it down. Newton realised: if you could remove all forces, an object would keep moving forever in a straight line at constant speed.
This was revolutionary. Before Newton, people thought you needed a continuous push to keep something moving. Newton said: no, you only need a push to change its motion — to start it, stop it, speed it up, slow it down, or turn it.
The Three Laws — Precisely
First Law (Law of Inertia):
An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction, unless acted upon by an unbalanced external force.
The key word is unbalanced. If multiple forces act on an object but cancel each other out (like a book on a table — gravity pulls down, the table pushes up equally), the net force is zero. The object behaves as if no force acts on it.
Watch out
The first law is not about objects that are obviously at rest. It's about what happens when the net force is zero. A car moving at 100 km/h on a straight highway with no acceleration has zero net force — it's obeying the first law, not the second.
Second Law:
The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass.
Fnet=ma
This is the workhorse. It tells you exactly how much motion changes when you apply a force.
Force (F) is what causes acceleration — measured in newtons (N).
Mass (m) is the measure of inertia — how much an object resists acceleration. A heavier object needs more force to achieve the same acceleration.
Acceleration (a) is the rate of change of velocity — how quickly speed or direction changes.
Tip
The second law is a vector equation. Force and acceleration always point in the same direction. If you push a box east, it accelerates east — not north.
Example: A 2 kg block is pushed with a net force of 10 N.
a=mF=210=5 m/s2
Third Law:
For every action, there is an equal and opposite reaction.
F12=−F21
If object A exerts a force on object B, then object B exerts a force of the same magnitude but opposite direction on object A. These forces always come in pairs, act on different objects, and are simultaneous.
Common confusion: If action and reaction are equal and opposite, why does anything move? Because they act on different bodies. When you push a wall, the wall pushes you back with equal force. Your hand doesn't move through the wall — but you can move backward (your feet push the floor, the floor pushes you forward). The pair of forces never cancel because they're on different objects.
Note
The third law is why rockets work in space. The rocket pushes exhaust gases backward (action), and the gases push the rocket forward (reaction). No air needed.
Putting It All Together — A Simple Problem
A 5 kg box is pulled horizontally with a force of 20 N. The friction opposing motion is 5 N.
Step 1: Find the net force.
Fnet=20−5=15 N
Step 2: Apply the second law.
a=mFnet=515=3 m/s2
The box accelerates at 3 m/s² in the direction of the pull. …
Newton's first law defines inertia and equilibrium, and Newton's second law quantifies how force changes a body's momentum; together they lead to the definition of the newton as the SI unit of force. …
Newton's first law defines inertia (no change without an unbalanced force); Newton's second law gives F = ma; one newton is defined as the force producing 1 m/s^2 acceleration in a 1 kg mass.
Newton's first law of motion: Every body continues to remain in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by an external unbalanced force acting on it. This law defines inertia -- the natural tendency of a body to resist any change in its state of rest or of uniform motion -- and also implicitly defines force as that which changes (or tends to change) this state.
Newton's second law of motion: The rate of change of linear momentum of a body is directly proportional to the applied external force, and this change takes place in the direction of the applied force.
Mathematically, if p = mv is the momentum,
F proportional to dp/dt
For a body of constant mass m, dp/dt = m (dv/dt) = m a, so:
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2025Set ANNUAL1 markMCQ
Q.Launching of a rocket is based upon which principle?
(A) Nuclear fusion
(B) Nuclear fission
(C) Newton's law of motion
(D) Work-energy theorem
›Reveal solutionSolution
Rocket propulsion is based on Newton's third law: ejected exhaust gas (action) pushes the rocket forward (reaction).
A rocket burns fuel and ejects hot gas at very high velocity through its nozzle. By Newton's third law, for every action there is an equal and opposite reaction — the rocket exerts a backward force on the expelled gas, and the gas exerts an equal, forward force (thrust) on the rocket. Equivalently, this is conservation of momentum: the total momentum of (rocket + ejected gas) system stays constant, so as the gas gains backward momentum, …
Q.Assertion: A cricketer moves her hands forward to catch a ball so as to catch it easily without hurting. Reason: She tries to decrease the distance travelled by the ball so that it hurts less.
(a) (A) Assertion is correct and reason is correct; reason is correct explanation of assertion
(b) (B) Assertion is correct and reason is correct; reason is not a correct explanation of assertion
(c) (C) Assertion is correct and reason is incorrect
(d) (D) Assertion is incorrect and reason is correct
›Reveal solutionSolution
[!TLDR]
(B)
Why
Catching technique that reduces impact force is a real effect (assertion true in spirit), but the actual physics reason is the impulse-momentum theorem: moving the hands increases the TIME of contact (Ft=Δp), which reduces the force for the same change in moment …
(d) Rocket Propulsion
List-II: 1. Digitial logic electronic circuits 2. Newton's II and III Laws of Motion 3. Bernoulli's principle 4. Propagation of Electromagnetic waves.
(a) a-3, b-2, c-4, d-1
(b) a-3, b-1, c-4, d-2
(c) a-1, b-2, c-3, d-4
(d) a-2, b-3, c-4, d-1
›Reveal solutionSolution
Match each technology to the physics principle behind it: Aeroplane -> Bernoulli's principle, Computers -> digital logic circuits, Radio/TV -> EM wave propagation, Rocket -> Newton's II & III laws.
Each item in List-I is a piece of technology; List-II gives the physics principle behind it.
(a) Aeroplane: the shape of an aircraft wing (aerofoil) makes air move faster over the top surface than the bottom, so by Bernoulli's principle the pressure below the wing exceeds the pressure above, producing the lift force. -> matches 3.
(b) Computers: at the hardware level, a computer's circuits are built from AND/OR/NOT gates and other digital logic elements that manipulate binary (0/1) signals. -> matches 1. …