Newton's First Law states: every body continues in its state of rest or uniform motion in a straight line unless compelled to change that state by an external unbalanced force (NCERT Class 11 Physics Chapter 4, page 50). This property of a body — its resistance to any change in velocity — is called inertia.
The trap NEET exploits: Students conflate Newton's First Law with Newton's Third Law. A book on a table is in equilibrium — net force is zero — so it stays at rest (First Law). But when asked "are the weight of the book and the normal force from the table an action-reaction pair?", many answer yes. They are not. Both forces act on the same body (the book). An action-reaction pair under the Third Law always acts on two different bodies. The weight's true reaction partner is the gravitational pull the book exerts on the Earth; the normal's true partner is the force the book pushes down on the table.
Inertia is not a force. A body at rest does not need a force to "keep it at rest." It stays at rest because no net external force acts on it. Similarly, an object moving at constant velocity on a frictionless surface needs no force to maintain that motion — First Law directly.
Connection to Newton's Second Law. When F_net = 0, the Second Law gives a = 0 — the body's velocity is constant (possibly zero). The First Law is sometimes called the "zero-force special case" of the Second Law, but it carries a deeper role: it defines what an inertial reference frame is. A frame where a free body (no net force) has zero acceleration is inertial. Non-inertial frames (accelerating bus, rotating platform) violate this and require pseudo-forces.
Watch out: NEET questions on the First Law are mostly recall or conceptual-application. They test whether you understand what inertia means, which reference frames are inertial, and whether you can correctly identify action-reaction pairs versus equilibrium force pairs.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Newton's First Law of motion is also known as the law of:
Answer: C. Newton's First Law describes the tendency of a body to resist changes in its state of motion. This property is called inertia. The law is also known as the law of inertia (NCERT Class 11 Physics Chapter 4, page 50).
Why A is wrong: A is wrong because energy is defined through work-energy concepts (Chapter 6), not through the First Law.
Why B is wrong: B is wrong because there is no ‘law of force’ — force is quantitatively defined by Newton’s Second Law (F = ma). The First Law is named for the property it describes, inertia.
Why D is wrong: D is wrong because momentum (p = mv) is defined as the product of mass and velocity, which is formalised through the Second Law's dp/dt formulation.
A passenger standing in a bus lurches forward when the bus suddenly brakes. This is a direct illustration of:
Answer: A. The passenger's body was in motion at the bus's speed. When the bus decelerates, no immediate force acts on the passenger's upper body to decelerate it — so it continues forward (inertia). This is Newton's First Law (NCERT Class 11 Physics Chapter 4, page 50).
Why B is wrong: B is wrong because the Third Law describes equal-and-opposite force pairs on two different bodies. The lurching forward is about the passenger's own inertia, not an action-reaction pair.
Why C is wrong: C is wrong because the Second Law quantifies F = ma; the scenario illustrates the tendency to maintain existing motion (inertia), not a force-acceleration calculation.
Why D is wrong: D is wrong because conservation of momentum applies to isolated systems with zero net external force. A braking bus has friction (external force) — this scenario illustrates inertia, not momentum conservation.
Which of the following is an inertial reference frame?
Answer: D. An inertial frame is one where a body not acted upon by any net force remains at rest or moves with constant velocity — Newton's First Law holds (NCERT Class 11 Physics Chapter 4, page 50). A train at constant velocity on a straight track satisfies this.
Why A is wrong: A is wrong because an accelerating car is a non-inertial frame. Objects inside experience pseudo-forces (pushed backward) because the First Law doesn't hold in this frame.
Why B is wrong: B is wrong because rotation at constant angular speed still involves centripetal acceleration. A rotating frame is non-inertial — objects inside experience centrifugal pseudo-force.
Why C is wrong: C is wrong because a satellite in orbit undergoes centripetal acceleration (free fall toward Earth). It is a non-inertial frame for Newtonian purposes — though for objects inside, it approximates a zero-gravity environment, the frame itself accelerates.
A book lies at rest on a table. A student claims the weight of the book and the normal reaction from the table form a Newton's Third Law action-reaction pair. This claim is:
Answer: B. Action-reaction pairs under Newton's Third Law always act on two different bodies. Weight (Earth pulls book) and normal (table pushes book) both act on the book — they form an equilibrium pair, not a Third Law pair. The Third Law partner of the book's weight is the gravitational pull the book exerts on the Earth (NCERT Class 11 Physics Chapter 4, page 51).
Why A is wrong: A is wrong because being equal and opposite is necessary but not sufficient for a Third Law pair. The two forces must also act on DIFFERENT bodies. Here both act on the book (trap: confusing equilibrium with action-reaction).
Why C is wrong: C is wrong because causal dependence is not the criterion for a Third Law pair. The criterion is: same type of force, same magnitude, opposite direction, acting on TWO DIFFERENT bodies.
Why D is wrong: D is wrong because on a table at rest, the normal force equals the weight exactly (equilibrium). The claim fails not because of magnitude mismatch, but because both forces act on the same body.
A hockey puck slides on a perfectly frictionless ice surface at constant speed. The net force acting on the puck is:
Answer: A. On a frictionless surface, no horizontal force acts on the puck after the initial push. By Newton's First Law, it continues at constant velocity. The vertical forces (weight and normal) cancel. Net force = 0 (NCERT Class 11 Physics Chapter 4, page 50).
Why B is wrong: B is wrong because the initial push is a contact force that acts only during the push. After release on a frictionless surface, no horizontal force remains. Inertia maintains the motion, not a continued force.
Why C is wrong: C is wrong because friction is the force that would decelerate the puck — but the surface is frictionless. No decelerating force exists here.
Why D is wrong: D is wrong because the normal force acts vertically upward, and weight acts vertically downward — they cancel each other. The NET force (including vertical) is zero, not just the normal alone.
A body of mass 2 kg moves due east at 5 m/s. A net force acts on it for a brief interval, after which it moves due north at 5 m/s. The direction of the net force during that interval was:
Answer: C. F is along Δp = m(v_f − v_i). Take east as +x, north as +y. v_i = 5x̂, v_f = 5ŷ. Δv = 5ŷ − 5x̂ = (−5x̂ + 5ŷ). This vector points into the second quadrant: equal parts west and north, i.e. exactly north-west at 45° from both axes (NCERT Class 11 Physics Chapter 4, page 54; trap: confusing force direction with final velocity direction).
Why A is wrong: A is wrong because the force is NOT in the direction of final motion. Force is along Δp = p_f − p_i, not along p_f. The initial eastward momentum must be subtracted, giving a westward component as well (trap: picking direction of motion as force direction).
Why B is wrong: B is wrong because the force is not along the initial velocity. The body's direction changed — the force must have a component perpendicular to the original motion.
Why D is wrong: D is wrong because south-east is the direction of v_i − v_f, the reverse of the momentum change. Force is along Δp = p_f − p_i, so the subtraction must run from initial to final (trap: taking the momenta in the wrong order).
When a moving bus brakes suddenly, the standing passengers lurch forward. This is best explained by:
Answer: B. B is correct. The brakes act on the bus, and through the floor on the passengers' feet, but nothing pushes the upper body backward at the same moment. By the first law, the upper body keeps moving forward at the speed it had. The passengers do not accelerate forward; the bus and their feet slow down beneath them.
Why A is wrong: A is wrong because there is no forward force on the passengers. Relative to the ground, their upper bodies simply carry on at their earlier speed while the bus decelerates.
Why C is wrong: C is wrong because the third-law partner of the braking force acts on the road (or the brake components), not on the passengers, so it cannot explain their motion.
Why D is wrong: D is wrong because braking cannot increase anyone's momentum in the forward direction. The passengers' momentum is unchanged for an instant, which is exactly why they seem to lurch.
Newton's First Law is sometimes called the law of inertia. Which statement about inertia is correct?
Answer: C. Inertia is the tendency of a body to resist any change in its velocity (whether from rest or from uniform motion). It depends only on mass — a more massive body has greater inertia (NCERT Class 11 Physics Chapter 4, page 50).
Why A is wrong: A is wrong because inertia is NOT a force. It is a property. No force is needed to keep a body at rest — it stays at rest because no net external force acts on it (First Law directly).
Why B is wrong: B is wrong because inertia depends only on mass, not on velocity. A 10 kg block has the same inertia whether it is at rest or moving at 50 m/s. The quantity that depends on both mass and velocity is momentum.
Why D is wrong: D is wrong because momentum p = mv depends on both mass and velocity and is a vector quantity. Inertia depends only on mass and is a scalar property — they are distinct concepts.
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Given
A 0.50 kg ball moves due south at 4.0 m/s. After hitting a wall, it moves due west at 4.0 m/s. The collision lasts 0.010 s.
Required
Direction and magnitude of the average net force on the ball during the collision.
Concept
Newton's Second Law: F = Δp/Δt. The force is in the direction of the change in momentum, not the direction of final motion.
Formula
F = m(v_f − v_i) / Δt
Substitution
Choose south as +y, west as +x.
v_i = 4.0 ŷ (due south)
v_f = 4.0 x̂ (due west)
Δv = v_f − v_i = 4.0 x̂ − 4.0 ŷ = (4.0, −4.0) m/s
|Δv| = √(4.0² + 4.0²) = √32 = 4√2 m/s
Calculation
|F| = m|Δv|/Δt = 0.50 × 4√2 / 0.010 = 0.50 × 5.657 / 0.010 = 2.828 / 0.010 = 283 N
Direction of Δv: components (+4.0 west, −4.0 north-to-south reversed = toward north from south). In our coordinate system, Δv = (4.0 x̂ − 4.0 ŷ) points toward north-west — specifically 45° north of west, i.e. toward the direction that is simultaneously west (gaining) and away from south (losing southward momentum).
More precisely: with south = +y and west = +x, Δv = (+4.0, −4.0). This is in the fourth quadrant of our coordinate system (positive x, negative y) = west and anti-south = west and north. The angle from the +x (west) axis is arctan(4.0/4.0) = 45° toward the −y (north) direction. So the force points north-west, bisecting west and north.
Note: The mass 0.50 kg and time 0.010 s are exact problem-defined values; they do not limit significant figures. The speeds 4.0 m/s have 2 significant figures, so the final answer is reported to 2 significant figures.
Final answer
Average force ≈ 2.8 × 10² N, directed north-west (45° from both north and west).
Common trap
Students pick "due west" (direction of final motion) as the force direction. The force is along Δp = p_f − p_i, which has components in BOTH the west and north directions. Drawing the vector subtraction triangle is essential.
Similar NEET-style question
A 0.20 kg ball moving east at 3.0 m/s is struck and moves north at 3.0 m/s. Find the magnitude of the impulse on the ball.
Approach: J = mΔv. Δv = 3.0 ŷ − 3.0 x̂, |Δv| = 3√2. J = 0.20 × 3√2 = 0.60√2 ≈ 0.85 kg·m/s, directed north-west.
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Aristotle held that an external force is required to keep a body in motion. This view is intuitively appealing but flawed. Galileo's frictionless-plane argument and Newton's first law overturn it: a body with NO net external force continues in its state of rest or uniform motion in a straight line.
-- NCERT Class 11 Physics, Ch. 4, p. 50If no external force acts on a body, the body in motion continues to move uniformly in a straight line, and the body at rest continues to be at rest. Friction had been masking this principle in everyday observation.
-- NCERT Class 11 Physics, Ch. 4, p. 51Every body continues in its state of rest, or of uniform motion in a straight line, unless compelled by an external force to change that state. This identifies inertia as the property by which a body resists change in its state of motion.
-- NCERT Class 11 Physics, Ch. 4, p. 51More in Laws of Motion: 16 exam traps and mistakes · 9 formulas · 10 question patterns from its other lessons.
No question in our NEET 2020–2025 set targets this topic directly.
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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