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Laws of Motion — NEET & JEE MCQs with answers
Newton's three laws of motion form the foundation of classical mechanics: an object remains at rest or in uniform motion unless acted upon by a net force (1st law / inertia); the net force equals the rate of change of momentum, $\mathbf{F} = d\mathbf{p}/dt = m\mathbf{a}$ (2nd law); forces always occur in equal and opposite action-reaction pairs (3rd law). The chapter applies these laws to friction, circular motion, and equilibrium.
Topics in this chapter
- 4.1 Introduction
- 4.2 Aristotle's Fallacy
- 4.3 The Law of Inertia
- 4.4 Newton's First Law of Motion
- 4.5 Newton's Second Law of Motion
- 4.6 Newton's Third Law of Motion
- 4.7 Conservation of Momentum
- 4.8 Equilibrium of a Particle
- 4.9 Common Forces in Mechanics
- 4.10 Circular Motion
- 4.11 Solving Problems in Mechanics
Free practice questions
In the block–trolley system, a $3\,\text{kg}$ block hangs and pulls a $20\,\text{kg}$ trolley on a horizontal surface via a string over a smooth pulley. The coefficient of kinetic friction between trolley and surface is $\mu_k = 0.04$. Taking $g = 10\,\text{m s}^{-2}$, the kinetic friction on the trolley is:
- A. $1.2\,\text{N}$
- B. $8\,\text{N}$
- C. $80\,\text{N}$
- D. $0.8\,\text{N}$
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Answer: B — Normal force on trolley $N = 20 \times 10 = 200\,\text{N}$; $f_k = \mu_k N = 0.04 \times 200 = 8\,\text{N}$.
NCERT Class 11 Physics · Ch 4 · 4.9.1 Friction (Example 4.9)
According to Newtonian mechanics, force:
- A. occurs in pairs only for contact forces
- B. can occur singly on an isolated body
- C. never occurs singly; it always occurs in pairs
- D. occurs only when bodies are in contact
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Answer: C — NCERT: 'force never occurs singly in nature. Force is the mutual interaction between two bodies. Forces always occur in pairs.'
NCERT Class 11 Physics · Ch 4 · 4.6 Newton's Third Law of Motion
Express $1\,\text{newton}$ in base SI units.
- A. $1\,\text{kg m s}^{-1}$
- B. $1\,\text{kg m}^{-1}\,\text{s}^{-2}$
- C. $1\,\text{kg m}^{2}\,\text{s}^{-2}$
- D. $1\,\text{kg m s}^{-2}$
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Answer: D — By definition $1\,\text{N}=1\,\text{kg m s}^{-2}$, the force giving $1\,\text{m s}^{-2}$ to $1\,\text{kg}$.
NCERT Class 11 Physics · Ch 4 · 4.5 Newton's Second Law of Motion
Banking of a road is done in order to
- A. reduce the dependence on friction for providing the centripetal force
- B. make the road perfectly frictionless
- C. increase the weight of the car
- D. eliminate the need for a normal reaction
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Answer: A — Banking the road reduces the contribution of friction to the circular motion of the car; the horizontal component of the normal reaction helps provide the centripetal force (NCERT 4.10).
NCERT Class 11 Physics · Ch 4 · 4.10 Circular Motion
A particle is in equilibrium under three concurrent forces $\vec{F_1}=2\hat{i}+3\hat{j}-\hat{k}\,\text{N}$, $\vec{F_2}=-\hat{i}+2\hat{j}+4\hat{k}\,\text{N}$ and $\vec{F_3}$. The magnitude of $\vec{F_3}$ is:
- A. $\approx 3.74\,\text{N}$
- B. $35\,\text{N}$
- C. $\approx 9\,\text{N}$
- D. $\approx 5.92\,\text{N}$
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Answer: D — $\vec{F_3} = -(\vec{F_1}+\vec{F_2}) = -((2-1)\hat{i}+(3+2)\hat{j}+(-1+4)\hat{k}) = -(1\hat{i}+5\hat{j}+3\hat{k})$. Magnitude $= \sqrt{1^2+5^2+3^2} = \sqrt{1+25+9} = \sqrt{35} \approx 5.92\,\text{N}$.
NCERT Class 11 Physics · Ch 4 · 4.8 Equilibrium of a Particle
Suppose, in Example 4.12, the block ($2\ \text{kg}$) + cylinder ($25\ \text{kg}$) instead remained at rest on a rigid floor ($g = 10\ \text{m s}^{-2}$). The normal force of the floor on the system would then be:
- A. 270 N
- B. 250 N
- C. 267.3 N
- D. 272.7 N
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Answer: A — If the system is at rest ($a = 0$), the second law gives $R' = mg = 27 \times 10 = 270\ \text{N}$, exactly equal to the weight — unlike the yielding-floor case where $R' = 267.3\ \text{N}$ due to the $0.1\ \text{m s}^{-2}$ downward acceleration.
NCERT Class 11 Physics · Ch 4 · 4.11 Solving Problems in Mechanics (Example 4.12)
According to NCERT 4.3, which of the following statements is CORRECT?
- A. A body in uniform motion always accelerates
- B. No net force is needed to keep a body in uniform motion
- C. Rest requires a force but uniform motion does not
- D. A net force is needed to keep a body in uniform motion
Show answer
Answer: B — NCERT 4.3 explicitly says it is incorrect to assume that a net force is needed to keep a body in uniform motion; rest and uniform linear motion are equivalent (zero net force). Therefore the correct statement is that no net force is needed to keep a body in uniform motion.
NCERT Class 11 Physics · Ch 4 · 4.3 The Law of Inertia
In the preceding chapter (kinematics), uniform motion was described using which single concept?
- A. Acceleration alone
- B. Force alone
- C. Velocity alone
- D. Momentum alone
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Answer: C — The introduction recalls that uniform motion needs the concept of velocity alone, whereas non-uniform motion additionally requires acceleration.
NCERT Class 11 Physics · Ch 4 · 4.1 Introduction
The first law of motion is sometimes called the law of:
- A. action and reaction
- B. inertia
- C. gravitation
- D. momentum
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Answer: B — The text identifies Galileo's law of inertia as the basis Newton formulated as the first law; the property described (a body resisting change in its state of rest or uniform motion) is inertia. The first law is thus also known as the law of inertia.
NCERT Class 11 Physics · Ch 4 · 4.4 Newton's First Law of Motion
A machine gun of mass $10\,\text{kg}$ fires bullets of mass $0.02\,\text{kg}$ at $400\,\text{m s}^{-1}$, $5$ bullets per second. The average recoil force needed to hold the gun steady is
- A. $200\,\text{N}$
- B. $40\,\text{N}$
- C. $20\,\text{N}$
- D. $8\,\text{N}$
Show answer
Answer: B — Momentum per bullet $= 0.02 \times 400 = 8\,\text{kg m s}^{-1}$. Rate $= 8 \times 5 = 40\,\text{kg m s}^{-1}$ per second, so the force $= 40\,\text{N}$.
NCERT Class 11 Physics · Ch 4 · 4.7 Conservation of Momentum
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