Q.If external force on a body is zero, its acceleration is :
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Newton's First Law: The Law of Inertia
Imagine you're sitting in a bus that's stopped at a signal. The bus suddenly lurches forward. What happens to you? You jerk backwards against the seat. Now imagine the bus is moving at a steady speed and the driver slams the brakes. You lurch forwards toward the front.
Why? Your body was trying to keep doing what it was already doing — staying still when the bus was still, and moving forward when the bus was moving. That instinct is the heart of Newton's First Law.
The Intuition: Objects Are Lazy
Things don't change their motion on their own. A ball sitting on the ground will stay sitting forever unless something pushes or pulls it. A rolling ball will keep rolling forever in a straight line — unless friction, air resistance, or a wall stops it.
This "laziness" of objects is called inertia. The more massive an object, the more inertia it has — a truck is much harder to start moving or stop than a bicycle.
The Precise Statement
Newton's 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.
Let's break that down:
- "At rest stays at rest" — A book on a table won't slide off unless you push it or the table shakes.
- "In motion stays in motion" — A hockey puck on ice keeps gliding because friction is very low. On rough ground, it stops quickly — but that's because friction (a force) is acting on it.
- "Unbalanced external force" — If you push a box and someone else pushes it equally from the opposite side, the forces cancel (balanced). The box doesn't move. Only when the net force is non-zero does motion change.
A common mistake: students think a moving object needs a force to keep moving. That's false. A moving object needs a force only to change its motion — speed up, slow down, or turn. In the absence of forces, it keeps moving forever.
Why This Law Matters
Newton's First Law defines what a force is: anything that changes an object's state of motion. It also introduces the idea of inertial reference frames — if you're in a smoothly moving train with no windows, you can't tell you're moving at all. All physics works the same as if you were at rest.
The First Law is really a special case of the Second Law (F=ma). If net force F=0, then acceleration a=0, meaning velocity is constant — either zero (rest) or some steady value (uniform motion). But Newton listed it first because it's the foundation: it tells us what happens when no forces act.
Real-Life Examples
| Situation | What happens | Why |
|-----------|--------------|-----| …
Newton's second law directly connects net force and acceleration. …
Newton's second law gives a = F/m; with no net external force, acceleration must be zero.
Newton's second law of motion states that the acceleration of a body is directly proportional to the net external force acting on it:
F=ma⇒a=mF
If the net external force F = 0, then since mass m is never zero for a real body:
a=m0=0
…
Showing the 12 most recent of 15 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.It is common to give a sudden jerk to a wet cloth to remove water from it. In doing so, we take advantage of which rule?(a) Newton's first law of motion(b) Newton's second law of motion(c) Newton's third law of motion(d) None of these
›Reveal solutionSolution
Giving a wet cloth a sudden jerk relies on inertia (Newton's first law): the water droplets keep moving due to their own inertia even when the cloth's motion is abruptly changed, so they separate from the cloth.
Newton's first law states that a body continues in its state of rest or uniform motion unless acted upon by an external force -- this property is called inertia.
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- CBSE 2026Set ANNUAL1 markMCQQ.When moving bus suddenly stops, the passenger fall forward, it is due to :(a) Inertia of Motion.(b) Inertia of Direction.(c) Inertia of Rest.(d) Gravitational pull of Earth.
›Reveal solutionSolution
A passenger falls forward when a moving bus stops suddenly because their body, which was moving with the bus, tends to keep moving forward — this is inertia of motion.
Newton's first law states that a body continues in its state of rest or of uniform motion unless acted upon by an external unbalanced force. This tendency to resist a change in the state of motion is called inertia.
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- CBSE 2025Set ANNUAL1 markMCQQ.Net force acting on an object is found to be zero. It can be inferred that the object:(a) May be at rest(b) May be in uniform motion(c) May be in uniformly accelerated motion(d) Both (A) and (B)
›Reveal solutionSolution
Zero net force means zero acceleration (Newton's first/second law), not zero velocity — so the object could be at rest or moving at constant velocity; either is consistent.
By Newton's second law, Fnet=ma. If Fnet=0, then a=0 (assuming m=0).
Zero acceleration means the velocity is not changing — but it says nothing about what that (constant) velocity actually is. Two cases are consistent with a=0:
- The object could be permanently at rest (v=0, unchanging).
- The object could be moving with a constant, non-zero velocity in a straight line (uniform motion). …
- CBSE 2025Set ANNUAL1 markMCQQ.When the object is moving at constant velocity on the rough surface:(a) only external force acts on the object(b) net force on the object is zero(c) only kinetic friction acts on the object(d) no force acts on the object
›Reveal solutionSolution
Constant velocity means zero acceleration, and by Newton's Second Law (F_net = ma), zero acceleration means the net force on the object is zero -- even on a rough surface where kinetic friction is actively acting.
On a rough surface, several forces act on a moving object simultaneously: the applied external force (if any), gravity, the normal reaction from the surface, and kinetic friction opposing the relative sliding motion. None of these individual forces is zero.
However, 'constant velocity' means the velocity is not changing -- neither in magnitude nor direction -- so the acceleration a = dv/dt = 0.
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- CBSE 2024Set ANNUAL1 markMCQQ.Which of the following is known as the law of inertia?(a) Newton's first law of motion(b) Newton's second law of motion(c) Newton's third law of motion(d) Law of conservation of mass
›Reveal solutionSolution
Newton's first law is called the law of inertia because it defines inertia: a body's natural tendency to resist any change in its state of rest or uniform motion.
Newton's first law: 'A body remains in a state of rest, or of uniform motion in a straight line, unless it is acted upon by an external unbalanced force.' This resistance to a change of state is precisely what we call inertia — hence the first law is also known as the law of inertia. New …
- CBSE 2024Set SET-AP55001 markMCQQ.Newton's laws of motion apply in:(a) A rotating frame(b) An accelerated frame(c) A non-inertial reference frame(d) An inertial reference frame
›Reveal solutionSolution
Newton's laws of motion are valid, in their standard form, only in an inertial (non-accelerating) frame of reference.
An inertial frame is one that is either at rest or moving with constant velocity (zero acceleration). Newton's first law itself defines this: a body continues in its state of rest or uniform motion unless acted upon by a net external force — this statement is only true if the observer's own frame is not accelerating.
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- CBSE 2024Set SET-AP55001 markQ.Newton's first law of motion is also called the law of ________.
›Reveal solutionSolution
Newton's first law states that a body remains in its state of rest or uniform motion unless compelled by an external force to change that state — this property is called inertia, so the law is known as the law of inertia.
Inertia is the natural tendency of any object to resist a change in its state of rest or of uniform motion in a straight line. Newton's first law is essentially a statement that every body possesses this property — it will not accelerate (change velocity …
- CBSE 2024Set ANNUAL1 markMCQQ.If external force on a body is zero, its acceleration is :(a) F/m(b) m/F(c) F(d) Zero
›Reveal solutionSolution
Newton's second law gives a = F/m; with no net external force, acceleration must be zero.
Newton's second law of motion states that the acceleration of a body is directly proportional to the net external force acting on it:
F=ma⇒a=mF
If the net external force F = 0, then since mass m is never zero for a real body:
a=m0=0
…
- CBSE 2023Set ANN1 markQ.________ is the measure of inertia.
›Reveal solutionSolution
Inertia is a body's inherent tendency to resist a change in its state of rest or uniform motion, and this tendency is quantified by its mass.
Inertia is the property of a body due to which it opposes any change in its state of rest or of uniform motion in a straight line. Newton's first law states that a body continues in its state of rest or uniform motion unless acted upon by an external force — this resistance to change is inertia.
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- CBSE 2023Set ANNUAL1 markQ.Fill in the blank: ____________ is the measure of the inertia of a body.
›Reveal solutionSolution
Mass is the quantitative measure of a body's inertia: the more mass a body has, the more it resists a change in its state of motion.
From Newton's second law, F = m*a, so for a given force F, the acceleration a = F/m. A larger mass m produces a smaller acceleration for the same force -- i.e., the body 'resists' the change in velocity more strongly. This resistance to a change in state of rest or uniform motion is called inertia, and mass is exactly the prop …
- CBSE 2022Set ANNUAL1 markMCQQ.When a car takes a sudden left turn in the curved road, passengers are pushed towards the right due to :(a) inertia of rest(b) inertia of direction(c) absence of inertia(d) inertia of motion
›Reveal solutionSolution
Inertia has three aspects: inertia of rest (resists starting to move), inertia of motion (resists stopping), and inertia of direction (resists a change in the direction of motion). A car swerving left is a change of direction, so the relevant property is inertia of direction.
By Newton's first law, a moving body continues in a straight line at constant velocity unless acted on by a net external force. When the car suddenly turns left, the car (and the seat, held by friction with the road) changes direction, but the passenger's body has no direct force turning it — it tends to keep moving along its original straight-line path.
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- CBSE 2021Set TERM11 markMCQQ.To shake off water from wet cloth, it is common to give it a sudden jerk. In doing so, we are taking advantage of:(a) Newton's first law of Motion(b) Newton's second law of Motion(c) Newton's third law of Motion(d) Impulse
›Reveal solutionSolution
Water drops on the cloth are not rigidly attached, so when the cloth is jerked into sudden motion, the water -- by its inertia (Newton's first law) -- tends to remain at rest and gets left behind, i.e. shaken off.
Newton's first law states that a body continues in its state of rest (or uniform motion) unless acted upon by an external unbalanced force -- this tendency to resist a change in the state of motion is called inertia.
…
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