Explain the concept of inertia. Write two examples each for inertia of motion, inertia of rest, and inertia of

II. Short Answer Questions · Q1

Q. Explain the concept of inertia. Write two examples each for inertia of motion, inertia of rest, and inertia of direction.

Tamil Nadu DgeTextbookSubjectiveImportance★★★★★

2% · 2/89 Questions

✓ Free question

Concept understanding — Newton's First Law

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.

Watch out

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.

Important

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
A passenger not wearing a seatbelt in a car crash	Flies forward through the windshield	Body keeps moving forward; car stops suddenly (force from collision)
Dust shaken from a rug	Dust flies off	You move the rug (force on rug), but dust particles keep their state of rest
A coin on a card on a glass — flick the card	Coin drops straight into the glass	Card moves away (force from flick), coin stays at rest due to inertia, then gravity pulls it down

The Bottom Line

Newton's First Law says: No net force → no change in motion. Objects are stubborn — they keep doing exactly what they're already doing until something forces them to change. That stubbornness is inertia, and it's why you lurch in a bus, why a ball stops rolling on grass, and why seatbelts save lives.

Many students search for "Newton's First Law class 11 physics" or "Newton's First Law: Definition, Formula & Real-World Examples" while revising for boards, and Newton's First Law is drawn directly from the Laws of Motion coverage of the NCERT/CBSE Class 11 Physics syllabus and recurs often in JEE Main and NEET papers. Working through the worked examples above alongside the official NCERT Physics textbook is the most reliable way to turn this understanding into exam-ready recall.

Step 1. Newton's first law states that every object continues in its state of rest or of uniform straight-line motion unless acted upon by an external force; inertia is precisely this inability of a body to change its own state of motion (or rest, or direction) without an external force.

Step 2. Inertia of rest — resistance to starting to move. Example (i): when a stationary bus suddenly starts moving, a standing passenger is jerked backward because their body, at rest, tends to stay at rest while the bus floor moves forward beneath them. Example (ii): a coin placed on a stiff card balanced on a glass stays behind (and drops into the glass) when the card is flicked sharply away.

Step 3. Inertia of motion — resistance to stopping or slowing. Example (i): when a moving bus suddenly brakes, a standing passenger lurches forward because their body, already moving, tends to keep moving even though the bus has stopped. Example (ii): a running athlete continues moving for a few steps even after crossing the finish line.

Step 4. Inertia of direction — resistance to changing the direction of motion. Example (i): a stone whirled on a string flies off along the tangent to the circle, not along the curve, the instant the string is released or snaps. Example (ii): mud or sparks thrown off a rapidly spinning wheel fly off tangentially, not radially.

✓Final answer

Inertia of rest, motion, and direction, each illustrated with the standard textbook examples above.

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 15 on this concept.

CBSE 2026Set ANNUAL1 markMCQ
$Q. It is common to give a sudden jerk to a wet cloth to remove water from it. In doing so, we take advantage of which rule?$
(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.

When the wet cloth is jerked, the cloth fibres are suddenly forced to change their state of motion. The water droplets clinging to the cloth are not rigidly attached; due to inertia, they tend to continue moving with whatever velocity they had at that instant (their own inertia of motion resists the sudden change), so they separate from the cloth and fly off, effectively removing the water. This is a classic and commonly cited illustration of the law of inertia (Newton's first law), not the second law (which concerns force = rate of change of momentum) or the third law (action-reaction pairs).

✓Final answer
$(a) Newton's first law of motion.$
CBSE 2026Set ANNUAL1 markMCQ
$Q. When moving bus suddenly stops, the passenger fall forward, it is due to :$
(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.

While the bus is moving, the passenger's whole body is moving forward with the bus's velocity. When the brakes are applied suddenly, the bus (and the passenger's feet, in contact with the floor) decelerates quickly. The upper part of the passenger's body is not directly acted on by this stopping force, so it continues moving forward at the original speed due to inertia of motion, throwing the passenger forward.

(This is distinct from 'inertia of rest', which is why a standing passenger falls backward when a stationary bus suddenly starts moving.)

✓Final answer
$The correct option is (a) Inertia of Motion.$
CBSE 2025Set ANNUAL1 markMCQ
$Q. Net force acting on an object is found to be zero. It can be inferred that the object:$
(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, $F_{net} = ma$ . If $F_{net} = 0$ , then $a = 0$ (assuming $m \neq 0$ ).

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).

What is ruled out is any change in velocity — so uniformly accelerated motion (option c) is inconsistent with zero net force, since that requires a non-zero, constant acceleration.

✓Final answer
$The object may be at rest OR in uniform motion — option (d), both (A) and (B).$
CBSE 2025Set ANNUAL1 markMCQ
$Q. When the object is moving at constant velocity on the rough surface:$
(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.

By Newton's Second Law, F_net = ma. If a = 0, then F_net = 0 necessarily, regardless of how many individual (nonzero) forces are acting. This is the defining condition of what is called 'dynamic equilibrium' -- the applied force is exactly balanced by kinetic friction (and gravity is balanced by the normal force).

This rules out options (a), (c), and (d), which incorrectly claim only one type of force acts or that no force acts at all -- multiple forces DO act, they just sum to zero.

✓Final answer
$The correct option is (b) net force on the object is zero.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. Which of the following is known as the law of inertia?$
(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. Newton's second law relates force to the rate of change of momentum, and the third law is about action-reaction pairs; neither directly defines inertia.

✓Final answer
$(a) Newton's first law of motion.$
CBSE 2024Set SET-AP55001 markMCQ
$Q. Newton's laws of motion apply in:$
(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.

In a rotating frame, an accelerated frame, or any non-inertial frame, an object can appear to accelerate even with zero real net force acting on it (e.g., the centrifugal effect felt in a rotating frame). To make Newton's second law (F = ma) appear to work in such frames, one has to introduce fictitious 'pseudo-forces' — which shows the laws don't hold there in their pure form.

So among the options, only the inertial reference frame is where Newton's laws directly apply without correction.

✓Final answer
$The correct option is (d) An inertial reference frame.$
CBSE 2024Set SET-AP55001 mark
$Q. Newton's first law of motion is also called the law of ________.$
›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) on its own; only an external unbalanced force can do that. Since the entire content of the law is about this resistance-to-change property, it is called the law of inertia.

✓Final answer
$The law of inertia.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. If external force on a body is zero, its acceleration is :$
(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:

$F = ma \quad \Rightarrow \quad a = \frac{F}{m}$

If the net external force F = 0, then since mass m is never zero for a real body:

$a = \frac{0}{m} = 0$

This is consistent with Newton's first law (law of inertia): a body with zero net force continues in its state of rest or uniform motion, i.e., its acceleration is zero.

✓Final answer
$With zero net external force, the acceleration is zero. Option (d) Zero.$
CBSE 2023Set ANN1 mark
$Q.________ is the measure of inertia.$
›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.

Experience shows that a heavier body is harder to set into motion or stop than a lighter one — for example, it takes a much larger force to push a loaded truck than an empty cart. This shows that the amount of inertia a body has depends on its mass: the greater the mass, the greater the inertia.

Hence, mass is taken as the quantitative measure of inertia.

✓Final answer
$Mass is the measure of inertia.$
CBSE 2023Set ANNUAL1 mark
$Q. Fill in the blank: ____________ is the measure of the inertia of a body.$
›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 property that quantifies it. This is why heavier objects are harder to start moving, stop, or turn than lighter ones, even under identical forces.

✓Final answer
$Mass is the measure of the inertia of a body.$
CBSE 2022Set ANNUAL1 markMCQ
$Q. When a car takes a sudden left turn in the curved road, passengers are pushed towards the right due to :$
(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.

Relative to the car (which has now turned left), the passenger who is still moving 'straight ahead' appears to be pushed toward the right side of the car. This apparent push is not a real outward force; it is simply the passenger's inertia resisting the change in the direction of motion — hence 'inertia of direction'.

(This is distinct from 'inertia of rest', which is why a standing passenger falls backward when a bus suddenly starts, and 'inertia of motion', which is why a standing passenger falls forward when a moving bus suddenly brakes.)

✓Final answer
$The correct option is (b) inertia of direction.$
CBSE 2021Set TERM11 markMCQ
$Q. To shake off water from wet cloth, it is common to give it a sudden jerk. In doing so, we are taking advantage of:$
(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.

When a wet cloth is given a sudden jerk, the cloth fibres are forced to move quickly. The water droplets clinging to the cloth are only weakly held (by surface tension/adhesion) and are not rigidly part of the cloth. Because of their inertia, the water droplets tend to continue in their original state (at rest, or moving slower than the suddenly-jerked cloth) while the cloth itself moves away rapidly -- so the relative motion separates the water from the cloth.

✓Final answer
$The correct option is (a) Newton's first law of Motion.$

←Previous question · 3.9Q15 — If a person moves from the pole to the equator, the centrifugal force …→Next question · 3.9Q2 — State Newton's second law.

Q.Explain the concept of inertia. Write two examples each for inertia of motion, inertia of rest, and inertia of direction.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days