Chapter 4: Laws of Motion Short Questions | physics Class 11 CBSE

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Chapter 4: Laws of Motion Short Questions | physics Class 11 CBSE | ByteNotes.in

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Question 1

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State Aristotle's law of motion and identify the fundamental flaw in his reasoning.

Aristotle stated that an external force is required to keep a body in motion; without a continuous force, a moving body will come to rest.
The flaw is that Aristotle ignored the role of friction: bodies in everyday experience stop not because of the absence of a driving force, but because the opposing force of friction decelerates them.
If friction were absent, no external force would be needed to maintain uniform motion, as demonstrated by Galileo's idealised experiments on frictionless surfaces.

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Question 2

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How did Galileo's double inclined plane experiment lead to the law of inertia?

A ball released from rest on one inclined plane rolls down and climbs the other to nearly the same height; in an ideal frictionless case it reaches exactly the same height.
When the slope of the second plane is progressively decreased, the ball still reaches the same height but travels a greater horizontal distance to do so.
In the limiting case when the second plane is horizontal, the ball would travel an infinite distance—meaning it moves forever with constant velocity, establishing the law of inertia.

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Question 3

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State Newton's First Law of motion and express it in terms of acceleration.

Newton's First Law states: every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise.
In terms of acceleration, the law is expressed as: if the net external force on a body is zero, its acceleration is zero.
Conversely, acceleration can be non-zero only if there is a net external force acting on the body; the law thus defines both force and inertia simultaneously.

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Question 4

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Define momentum and explain why it is considered a more fundamental quantity than speed for analysing the effect of force on motion.

Momentum is defined as the product of the mass and velocity of a body: p = mv; it is a vector quantity with SI unit kg m s⁻¹ or N s.
For a given mass, a greater speed requires a greater opposing force to stop the body in a given time; for a given speed, a more massive body requires a greater force, showing that both mass and velocity together determine the difficulty of changing motion.
The same force applied for the same time produces the same change in momentum for bodies of different masses, making momentum the key variable linking force and motion in Newton's Second Law.

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Question 5

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Write Newton's Second Law in its mathematical form and state any two important features of this law.

Newton's Second Law is mathematically expressed as F = dp/dt = ma, where F is the net external force, p is momentum, m is mass, and a is acceleration; the SI unit of force is the newton (1 N = 1 kg m s⁻²).
It is a vector law, equivalent to three independent scalar equations for each component (Fx = max, Fy = may, Fz = maz), meaning force changes only the component of velocity along its own direction.
It is a local law: the acceleration of a body at any instant depends only on the net force at that instant and not on the history of the body's motion.

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Question 6

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Define impulse and explain why it is a useful concept in mechanics.

Impulse is defined as the product of force and the time duration for which it acts; it is equal to the change in momentum of the body: Impulse = F × Δt = Δp.
It is useful in situations where a large force acts for a very short time (impulsive force), making it difficult to measure the force and time duration separately.
In such cases (e.g., a bat hitting a ball), the impulse—the change in momentum—remains a measurable and meaningful quantity even when individual values of force and time are not known.

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Question 7

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State Newton's Third Law of Motion and list two important clarifications about the terms 'action' and 'reaction'.

Newton's Third Law states: to every action there is always an equal and opposite reaction; or equivalently, the force on body A by body B is equal and opposite to the force on body B by A.
Action and reaction forces act simultaneously—there is no cause-effect relationship between them, and either of the two mutual forces may be called the action.
Action and reaction forces always act on different bodies; because they act on different bodies, they cannot cancel each other and should never be added to give zero net force on a single body.

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Question 8

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Why do internal forces in a system of particles cancel in pairs, and what is the significance of this fact?

By Newton's Third Law, for every internal force that body A exerts on body B within the system, body B exerts an equal and opposite force on body A, so each pair of internal forces sums to zero.
When all internal force pairs cancel, only the external forces contribute to changing the total momentum of the system, making the net internal force on the system as a whole equal to zero.
This fact enables Newton's Second Law to be applied to a system of particles or a rigid body, where F refers to total external force and a refers to the acceleration of the centre of mass of the system.

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Question 9

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State the law of conservation of momentum and derive it from Newton's Second and Third Laws for a two-body collision.

The law of conservation of momentum states: the total momentum of an isolated system of interacting particles is conserved, provided no net external force acts on the system.
For two bodies A and B in contact for time Δt, the Second Law gives FAB Δt = p'A − pA and FBA Δt = p'B − pB; by the Third Law FAB = −FBA.
Substituting gives p'A − pA = −(p'B − pB), which rearranges to p'A + p'B = pA + pB, showing total final momentum equals total initial momentum.

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Question 10

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What is meant by equilibrium of a particle? State the condition for equilibrium under three concurrent forces.

Equilibrium of a particle refers to the condition when the net external force on the particle is zero; according to the First Law, the particle is either at rest or in uniform linear motion.
For three concurrent forces F₁, F₂, and F₃ to maintain equilibrium, their vector sum must be zero: F₁ + F₂ + F₃ = 0, meaning the resultant of any two forces must be equal and opposite to the third.
In component form, this requires the sum of x-components, the sum of y-components, and the sum of z-components of all three forces to each independently equal zero.

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Question 11

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Distinguish between static friction and kinetic friction, giving the mathematical law for each.

Static friction (fs) opposes impending relative motion between two surfaces in contact; it is self-adjusting and exists only when an applied force is present, up to a maximum value (fs)max = μsN.
Kinetic friction (fk) opposes actual relative motion between surfaces already sliding against each other and is given by fk = μkN; unlike static friction, it has a fixed value for given surfaces and normal force.
Experimentally, the coefficient of kinetic friction μk is always less than the coefficient of static friction μs; both are independent of the area of contact and depend only on the nature of the surfaces in contact.

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Question 12

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Explain why a block on a horizontal surface remains stationary even when a small horizontal force is applied to it.

When a small horizontal force F is applied, static friction fs immediately comes into play in the horizontal direction, opposing the impending motion of the block.
Static friction is self-adjusting; it increases to exactly equal F in magnitude, resulting in zero net horizontal force on the block, keeping it stationary.
This continues as long as F does not exceed the maximum static friction (fs)max = μsN; once F exceeds this limit, the block begins to slide and kinetic friction takes over.

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Question 13

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Define centripetal force and identify what provides it in each of these cases: (i) a stone whirled on a string, (ii) a planet orbiting the sun, (iii) a car taking a circular turn on a level road.

Centripetal force is the net inward force directed towards the centre of the circular path required to maintain circular motion; it equals mv²/R for a body of mass m moving with speed v in a circle of radius R.
For a stone whirled on a string, the tension in the string provides the centripetal force; for a planet orbiting the sun, the gravitational force of the sun on the planet provides it.
For a car taking a circular turn on a level road, the static friction between the tyres and the road surface provides the centripetal force; the maximum speed without slipping is vmax = √(μsRg).

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Chapter 4: Laws of Motion | Short Questions

State Aristotle's law of motion and identify the fundamental flaw in his reasoning.

How did Galileo's double inclined plane experiment lead to the law of inertia?

State Newton's First Law of motion and express it in terms of acceleration.

Define momentum and explain why it is considered a more fundamental quantity than speed for analysing the effect of force on motion.

Write Newton's Second Law in its mathematical form and state any two important features of this law.

Define impulse and explain why it is a useful concept in mechanics.

State Newton's Third Law of Motion and list two important clarifications about the terms 'action' and 'reaction'.

Why do internal forces in a system of particles cancel in pairs, and what is the significance of this fact?

State the law of conservation of momentum and derive it from Newton's Second and Third Laws for a two-body collision.

What is meant by equilibrium of a particle? State the condition for equilibrium under three concurrent forces.

Distinguish between static friction and kinetic friction, giving the mathematical law for each.

Explain why a block on a horizontal surface remains stationary even when a small horizontal force is applied to it.

Define centripetal force and identify what provides it in each of these cases: (i) a stone whirled on a string, (ii) a planet orbiting the sun, (iii) a car taking a circular turn on a level road.

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