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Laws of Motion

NCERT Class 11 · Physics Based on NCERT Class 11 Physics textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter transitions from describing motion (kinematics) to understanding what causes motion—force. It debunks Aristotle's fallacy, establishes Galileo's law of inertia, and builds Newton's three laws of motion, which form the foundation of classical mechanics. The one big idea a student must walk away with is that a net external force is required to change the state of motion of a body (i.e., to produce acceleration), and that forces always occur in equal and opposite pairs, leading to the conservation of momentum.
Key Concepts (15)
Aristotle's fallacy
The incorrect view that an external force is required to keep a body in motion.
e.g. A child pulling a toy car thinks a constant force is needed to keep it moving, but the car actually stops due to friction, not because force is inherently needed for motion.
Law of inertia (Galileo's inference)
An object moving on a frictionless horizontal plane will continue to move with constant velocity; the state of rest and uniform linear motion are equivalent, with no net force acting on the body.
e.g. A ball released on a double inclined plane will climb to the same height on the other side; if the second plane is horizontal, the ball travels an infinite distance (ideally).
Inertia
The property of a body to resist any change in its state of rest or of uniform motion along a straight line.
e.g. When a bus suddenly starts, passengers are thrown backward because their body tends to remain at rest while the bus moves forward.
Newton's first law of motion
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.
e.g. A book on a table remains at rest because the upward normal force balances its weight, resulting in zero net force.
Momentum
The product of the mass and velocity of a body, denoted by p = m v. It is a vector quantity.
e.g. A heavy truck moving at the same speed as a small car has greater momentum and requires a larger force to stop in the same time.
Newton's second law of motion
The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts. Mathematically, F = dp/dt = m a.
e.g. A bullet of mass 0.04 kg moving at 90 m/s entering a wooden block experiences an average retarding force of 270 N to stop it in 60 cm.
Impulse
The product of force and the time duration for which it acts, equal to the change in momentum of the body. Impulse = Force × time = Δp.
e.g. A batsman hitting a ball of mass 0.15 kg at 12 m/s and reversing its direction imparts an impulse of 3.6 N s.
Newton's third law of motion
To every action, there is always an equal and opposite reaction. Forces always occur in pairs: force on A by B is equal and opposite to force on B by A.
e.g. When you press a coiled spring, the spring exerts an equal and opposite force on your hand.
Law of conservation of momentum
The total momentum of an isolated system of interacting particles is conserved.
e.g. When a bullet is fired from a gun, the momentum of the bullet forward equals the recoil momentum of the gun backward, so total momentum remains zero.
Equilibrium of a particle
A particle is in equilibrium when the net external force on it is zero, meaning it is either at rest or in uniform motion.
e.g. A mass of 6 kg suspended by a rope with a horizontal force of 50 N at its midpoint reaches equilibrium when the rope makes an angle tan⁻¹(5/6) with the vertical.
Static friction
The frictional force that opposes impending motion between two surfaces in contact, acting parallel to the surfaces. It satisfies f_s ≤ μ_s N.
e.g. A book on a table does not move when a small horizontal force is applied because static friction balances it.
Kinetic (sliding) friction
The frictional force that opposes relative motion between surfaces in contact once motion has started. It satisfies f_k = μ_k N, and μ_k is usually less than μ_s.
e.g. A box sliding on a floor experiences kinetic friction that opposes its motion, eventually bringing it to rest.
Normal reaction
The component of the contact force perpendicular to the surfaces in contact.
e.g. A book on a table experiences an upward normal force from the table that balances its weight.
Tension
The restoring force in an inextensible string of negligible mass, assumed constant throughout the string.
e.g. In the equilibrium problem of a 6 kg mass suspended by a rope, the tension in the rope supports the weight.
Spring force
The restoring force generated when a spring is compressed or extended, given by F = – k x, where k is the force constant and x is the displacement from the unstretched state.
e.g. Pulling a spring by 5 cm produces a restoring force proportional to 5 cm, opposing the pull.
Formulas & Equations (8)
p = m v
Reads as: Momentum equals mass times velocity
Variables: p = momentum (kg m/s), m = mass (kg), v = velocity (m/s)
Use when: Calculating the momentum of a body given its mass and velocity.
⚠ Forgetting that momentum is a vector; students often treat it as a scalar and ignore direction.
F = dp/dt = m a
Reads as: Force equals the rate of change of momentum, which equals mass times acceleration
Variables: F = net external force (N), dp/dt = rate of change of momentum (kg m/s²), m = mass (kg), a = acceleration (m/s²)
Use when: Relating net force to acceleration for a body of constant mass, or calculating force from change in momentum over time.
⚠ Using F = m a when mass is not constant, or forgetting that F is the net external force, not any individual force.
Impulse = Force × time = Δp
Reads as: Impulse equals force times time, which equals change in momentum
Variables: Impulse (N s or kg m/s), Force (N), time (s), Δp = change in momentum (kg m/s)
Use when: Calculating the effect of a large force acting over a short time, e.g., in collisions or impacts.
⚠ Confusing impulse with force; impulse is the product, not the force itself.
F_AB = – F_BA
Reads as: Force on A by B equals the negative of force on B by A
Variables: F_AB = force on body A due to body B (N), F_BA = force on body B due to body A (N)
Use when: Applying Newton's third law to identify action-reaction pairs.
⚠ Adding the two forces and claiming net force is zero on a single body; they act on different bodies.
p_A + p_B = p'_A + p'_B
Reads as: Total initial momentum equals total final momentum
Variables: p_A, p_B = initial momenta of bodies A and B (kg m/s), p'_A, p'_B = final momenta of bodies A and B (kg m/s)
Use when: Solving collision problems where no external force acts on the system.
⚠ Applying conservation of momentum even when external forces (like friction) are present and significant.
f_s ≤ μ_s N
Reads as: Static friction is less than or equal to the coefficient of static friction times the normal force
Variables: f_s = static friction force (N), μ_s = coefficient of static friction (dimensionless), N = normal force (N)
Use when: Determining the maximum static friction that can act before motion begins.
⚠ Writing f_s = μ_s N always; it is an inequality, and equality holds only at the limiting value.
f_k = μ_k N
Reads as: Kinetic friction equals the coefficient of kinetic friction times the normal force
Variables: f_k = kinetic friction force (N), μ_k = coefficient of kinetic friction (dimensionless), N = normal force (N)
Use when: Calculating the friction force when surfaces are sliding relative to each other.
⚠ Assuming μ_k = μ_s; in reality, μ_k is usually less than μ_s.
F = – k x
Reads as: Spring force equals negative of the force constant times displacement
Variables: F = restoring force (N), k = force constant (N/m), x = displacement from equilibrium (m)
Use when: Calculating the force exerted by a spring when compressed or stretched.
⚠ Forgetting the negative sign, which indicates the force opposes the displacement.
How It Works
Galileo's double inclined plane experiment (thought experiment)
  1. Step 1: Release a ball from rest on one inclined plane; it rolls down and climbs up the opposite plane.
  2. Step 2: If the planes are smooth (frictionless), the ball reaches nearly the same height as its starting height.
  3. Step 3: Decrease the slope of the second plane; the ball travels a longer distance to reach the same height.
  4. Step 4: In the limiting case where the second plane is horizontal (zero slope), the ball travels an infinite distance, never stopping.
  5. Step 5: Conclude that on a frictionless horizontal surface, a body in motion continues with constant velocity—no force needed to keep it moving.
💡 This is an idealised thought experiment; in reality, friction always exists, so the ball stops. The key insight is that force is needed to overcome friction, not to sustain motion itself.
Applying Newton's first law to infer forces from motion
  1. Step 1: Observe the state of motion of a body (at rest or uniform linear motion).
  2. Step 2: Conclude that its acceleration is zero.
  3. Step 3: From the first law, infer that the net external force on the body must be zero.
  4. Step 4: Use this to deduce unknown forces (e.g., normal force equals weight for a book at rest on a table).
💡 Do not say 'forces cancel so the body is at rest.' The correct logic is: 'The body is at rest, so net force must be zero, implying forces are equal and opposite.'
Deriving Newton's second law from momentum change
  1. Step 1: Define momentum p = m v.
  2. Step 2: State that force is proportional to rate of change of momentum: F ∝ Δp/Δt.
  3. Step 3: Take limit Δt → 0 to get F = k dp/dt.
  4. Step 4: For constant mass, dp/dt = m dv/dt = m a.
  5. Step 5: Choose k = 1 to define the unit of force (1 N = 1 kg m/s²), giving F = m a.
💡 The second law is a vector law; it applies component-wise: F_x = m a_x, F_y = m a_y, F_z = m a_z.
Conservation of momentum during collision
  1. Step 1: Consider two bodies A and B in an isolated system (no external force).
  2. Step 2: During collision, forces F_AB and F_BA act for time Δt.
  3. Step 3: By second law: F_AB Δt = p'_A – p_A and F_BA Δt = p'_B – p_B.
  4. Step 4: By third law: F_AB = – F_BA, so p'_A – p_A = – (p'_B – p_B).
  5. Step 5: Rearrange to get p_A + p_B = p'_A + p'_B, i.e., total momentum conserved.
💡 This holds for both elastic and inelastic collisions; only momentum is conserved in all cases, not kinetic energy.
Equilibrium of a particle under concurrent forces
  1. Step 1: Identify all forces acting on the particle.
  2. Step 2: For equilibrium, vector sum of forces must be zero: ΣF = 0.
  3. Step 3: Resolve forces into components along x, y, z axes.
  4. Step 4: Set each component sum to zero: ΣF_x = 0, ΣF_y = 0, ΣF_z = 0.
  5. Step 5: Solve the equations to find unknown forces or angles.
💡 The three forces in equilibrium can be represented by the sides of a closed triangle taken in the same sense.
Static friction opposing impending motion
  1. Step 1: Apply a horizontal force F to a body at rest on a surface.
  2. Step 2: Static friction f_s arises, equal and opposite to F, keeping the body at rest.
  3. Step 3: As F increases, f_s increases up to a maximum value (f_s)_max = μ_s N.
  4. Step 4: If F exceeds (f_s)_max, the body begins to slide.
  5. Step 5: Once sliding, kinetic friction f_k = μ_k N acts, which is usually less than (f_s)_max.
💡 Static friction is self-adjusting up to a limit; it does not have a fixed value. The inequality f_s ≤ μ_s N is crucial.
Exam Traps — Don't Make These Mistakes
⚠ Confusing action-reaction pairs with balanced forces on the same body
✗ The weight of the book and the normal force from the table are action-reaction pairs because they are equal and opposite.
→ Action-reaction forces act on different bodies. Weight acts on the book (by earth), normal acts on the book (by table). They act on the same body (the book), so they are not an action-reaction pair; they are balanced forces.
✓ The action-reaction pair is: Earth pulls book down (force on book by earth) and book pulls earth up (force on earth by book). The normal force on the book by the table and the force on the table by the book form another action-reaction pair.
⚠ Misapplying F = ma to find net force from a single force
✗ A car moving with constant velocity has a net force F = ma, so since a = 0, F = 0. But the engine is applying a force, so the net force cannot be zero.
→ The engine force is cancelled by friction and air resistance, so the net external force is indeed zero. Constant velocity implies zero acceleration, hence zero net force.
✓ When a car moves with constant velocity, the net external force on it is zero because the driving force is exactly balanced by opposing forces (friction, drag).
⚠ Forgetting that impulse is a vector
✗ A ball hits a wall at 30° and rebounds at the same speed; the impulse is 2 m u.
→ Impulse is a vector; only the component of velocity perpendicular to the wall changes sign. For a ball hitting at 30°, the change in momentum is 2 m u cos 30°, not 2 m u.
✓ Impulse = change in momentum vector. For a ball hitting a wall at angle θ, the component parallel to the wall remains unchanged, so impulse magnitude = 2 m u cos θ (normal to wall).
⚠ Assuming kinetic friction equals static friction
✗ The friction force on a sliding block is μ_s N.
→ Once the block is sliding, kinetic friction applies, which uses μ_k, not μ_s. μ_k is generally less than μ_s.
✓ For a sliding block, the friction force is kinetic friction: f_k = μ_k N, where μ_k < μ_s.
⚠ Adding action and reaction forces to get zero net force on a system
✗ The net force on a gun-bullet system is zero because the force on the bullet and the force on the gun are equal and opposite.
→ While it is true that internal forces cancel when considering the system as a whole, the statement is often misinterpreted to mean the bullet does not accelerate. The bullet accelerates because only the force on the bullet (by the gun) acts on it.
✓ For the gun-bullet system, the internal forces cancel, so the net external force determines the acceleration of the centre of mass. Individually, the bullet experiences a forward force and accelerates, while the gun experiences an equal backward force and recoils.
Compare & Contrast
Nature of opposition
Static friction
Kinetic friction
Static friction opposes impending relative motion (before sliding starts) and is self-adjusting up to μ_s N. Kinetic friction opposes actual relative motion (during sliding) and has a constant value μ_k N, with μ_k < μ_s.
Condition of applicability
Newton's first law
Newton's second law
First law applies when net external force is zero, describing bodies at rest or in uniform motion. Second law applies when net external force is non-zero, relating force to acceleration (F = m a). The first law is a special case of the second law (F = 0 implies a = 0).
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 SA-I (2 marks), 1 SA-II (3 marks), 1 LA (5 marks) — total approximately 12 marks.
Most asked: Newton's second law and its applications (F = ma, impulse-momentum) · Conservation of momentum in collisions · Free-body diagrams and equilibrium of a particle · Friction (static and kinetic) problems on inclined planes and horizontal surfaces · Newton's third law and identification of action-reaction pairs
Don't skip: Numerical problems combining F = ma, friction, and free-body diagrams (5 marks); Conservation of momentum in collision or recoil problems (3 marks); Impulse-momentum theorem application (3 marks)
One-Line Revision (read aloud before the exam)
  1. Aristotle's fallacy: an external force is needed to keep a body in motion — this is wrong because friction is the real opposing force.
  2. Galileo's law of inertia: on a frictionless surface, a body in motion stays in motion with constant velocity.
  3. Inertia is the resistance of a body to any change in its state of rest or uniform motion.
  4. Newton's first law: every body continues in its state of rest or uniform motion unless compelled by an external force.
  5. Momentum p = m v is a vector; its rate of change equals the net external force.
  6. Newton's second law: F = dp/dt = m a, where F is net external force.
  7. Impulse = Force × time = change in momentum (Δp).
  8. Newton's third law: forces always occur in equal and opposite pairs acting on different bodies.
  9. Law of conservation of momentum: total momentum of an isolated system remains constant.
  10. Equilibrium of a particle: net external force = 0, so the particle is at rest or in uniform motion.
  11. Static friction f_s ≤ μ_s N opposes impending motion; kinetic friction f_k = μ_k N opposes actual sliding.
  12. Spring force F = – k x, where the negative sign indicates the force opposes displacement.
  13. In a free-body diagram, only forces acting on the body are shown; action-reaction pairs are on different bodies.
  14. For a body moving with constant velocity, net force is zero because all forces balance.
  15. The coefficient of kinetic friction μ_k is always less than the coefficient of static friction μ_s.

MCQs — 10 Questions with Answers

Q1. A book rests on a table. According to Newton's First Law, why does it remain at rest?

  • A. No net external force acts on the book ✓
  • B. The book has very large mass
  • C. Gravity is balanced by an equal force from the table
  • D. The book does not experience any forces

Answer: A — Newton's First Law states that if net force is zero, a body at rest remains at rest; option C is too specific—normal force balances weight, but the principle is that net force = 0.

Q2. Which statement best explains Aristotle's fallacy regarding motion?

  • A. He believed all moving objects must eventually stop
  • B. He failed to recognize that friction opposes motion and must be overcome to maintain uniform motion ✓
  • C. He thought objects accelerate when pushed
  • D. He did not understand the concept of velocity

Answer: B — Aristotle observed that bodies stop due to friction but incorrectly concluded that force is always needed to keep motion, not realizing friction was the hidden cause.

Q3. In Galileo's double incline experiment, as the slope of the second plane approaches zero (becomes horizontal), what happens to the ball?

  • A. The ball decelerates and stops quickly
  • B. The ball accelerates downward
  • C. The ball continues to move indefinitely at constant velocity ✓
  • D. The ball oscillates back and forth

Answer: C — On a frictionless horizontal surface, there is no component of gravity along the plane and no opposing force, so the ball moves with constant velocity forever.

Q4. A skater moves on smooth ice at constant velocity with no external forces applied. According to Newton's First Law, this is possible because:

  • A. The skater has large momentum
  • B. The net external force on the skater is zero ✓
  • C. Ice is frictionless, so the skater experiences no forces
  • D. The skater's inertia increases with velocity

Answer: B — Smooth ice provides negligible friction, making net force approximately zero; constant velocity motion occurs because acceleration = 0.

Q5. A child drags a toy car across a floor at constant speed by pulling the string horizontally. Which statement is true about the forces on the car?

  • A. Tension in string is greater than friction force
  • B. Tension in string equals friction force; net horizontal force is zero ✓
  • C. Friction force is zero because the car moves at constant speed
  • D. The net force equals the tension minus the car's weight

Answer: B — At constant speed, acceleration is zero, so by Newton's First Law, net force = 0; tension must equal friction for horizontal equilibrium.

Q6. Which pair of forces is an example of action-reaction pair according to Newton's Third Law? [ASSERTION-STYLE: Choose the most accurate pair]

  • A. Weight of a book and normal force from table on the book
  • B. Force exerted by Earth on a falling stone and force exerted by stone on Earth ✓
  • C. Tension in a rope and friction on an object pulled by rope
  • D. Applied force on a car and air resistance opposing the car

Answer: B — Action-reaction pairs act on different objects; the stone and Earth exert equal and opposite forces on each other, whereas weight and normal force are both on the book.

Q7. A book of mass 0.5 kg lies on a horizontal table. The normal force exerted by the table on the book is: (Take g = 10 m/s²)

  • A. 0.5 N
  • B. 5 N ✓
  • C. 50 N
  • D. 0.05 N

Answer: B — Normal force balances weight: N = mg = 0.5 × 10 = 5 N; this is the contact force the table exerts upward.

Q8. Which of the following is NOT an example of inertia? [NEGATIVE MCQ]

  • A. A passenger lurches forward when a bus brakes suddenly
  • B. A moving ball continues rolling when no external force is applied
  • C. A book accelerates downward when dropped from a height ✓
  • D. A skater glides across frictionless ice without slowing down

Answer: C — Inertia is resistance to change in motion state; a dropped book accelerates (changes state) due to gravity, not inertia—inertia would resist this acceleration.

Q9. A magnet attracts an iron nail from a distance of 5 cm. This demonstrates that forces can: [ASSERTION-STYLE]

  • A. Only act through direct physical contact
  • B. Act on a body even without physical contact (action-at-distance forces exist) ✓
  • C. Only act on objects at rest
  • D. Only change an object's motion, not direction

Answer: B — Magnetic and gravitational forces act without contact, illustrating that forces need not require direct physical contact between objects.

Q10. Two blocks, A (mass 2 kg) and B (mass 1 kg), are on a frictionless table and connected by a light string. A horizontal force of 6 N is applied to block A. What is the acceleration of the system? [NUMERICAL/HOTS]

  • A. 1 m/s²
  • B. 2 m/s² ✓
  • C. 3 m/s²
  • D. 6 m/s²

Answer: B — Total mass = 2 + 1 = 3 kg; by Newton's Second Law (F = ma), acceleration = 6 / 3 = 2 m/s²; the string ensures both blocks accelerate together.

Flashcards

What is Aristotle's fallacy about motion?

Aristotle incorrectly believed external force is needed to keep a body moving, but he ignored friction which actually causes bodies to slow down.

Define inertia in one sentence.

Inertia is the property of a body to resist change in its state of rest or uniform motion.

State Newton's First Law of Motion.

A body at rest remains at rest and a body in uniform motion continues in uniform motion unless an external net force acts on it.

Why does a toy car need constant force to move on a floor?

Friction force from the floor opposes motion, so applied force must equal friction force to maintain uniform motion with zero net force.

What is the significance of Galileo's double incline experiment?

It proved that on a frictionless horizontal surface, a ball would move forever at constant velocity, showing uniform motion needs no force.

Distinguish between rest and uniform motion according to Newton's First Law.

Both rest and uniform motion are equivalent states where net external force is zero and the body does not accelerate.

Why is friction force crucial in understanding Aristotle's error?

Aristotle observed real-world motion where friction always opposes movement, but failed to imagine a frictionless world where uniform motion needs no force.

What does 'net external force' mean in the context of the First Law?

Net external force is the vector sum of all forces acting on a body; when it equals zero, the body's motion state does not change.

Give one example where force acts on a body without physical contact.

Gravitational force: a stone released from a building accelerates downward due to Earth's gravity acting at a distance.

How does the concept of inertia explain why passengers lean forward when a bus stops suddenly?

Passengers have inertia and tend to continue their forward motion; the bus stops due to friction but passengers' bodies resist this change.

Important Board Questions

State Newton's First Law of Motion and give one real-life example that illustrates inertia. [2 marks]

Define the law (net force = 0 → no change in motion state), then provide a clear example of rest inertia (book on table) or motion inertia (skater on ice).

Explain why Aristotle's view that 'force is required to keep a body in motion' is incorrect. Use the concept of friction to justify your answer and show how Galileo's experiment with the double inclined plane disproved this view. [5 marks]

Explain that Aristotle ignored friction; friction opposes motion and bodies slow down because of it, not because force is absent. Show that on a frictionless horizontal plane (Galileo's limit), a ball would move forever at constant velocity, disproving Aristotle. Conclude: uniform motion needs no force if friction is absent.

A block of mass 3 kg is placed on a horizontal surface. A student applies a horizontal force of 15 N to the right. The block moves at constant velocity. (a) What is the magnitude of friction force acting on the block? (b) If the applied force is increased to 21 N and the block now accelerates, what is the new acceleration? (c) Explain why the block moved at constant velocity initially but accelerates when the force is increased. [6 marks]

For (a), use Newton's First Law: at constant velocity, net force = 0, so friction = 15 N. For (b), apply Newton's Second Law: F_net = F_applied − friction = 21 − 15 = 6 N; find a = F_net / m = 6 / 3 = 2 m/s². For (c), explain that initially applied force equaled friction (net force zero), but increased force exceeds friction (net force non-zero), causing acceleration per F = ma.

Assertion-Reason Questions for Laws of Motion

Q1. Assertion (A):

A body moving with constant velocity on a horizontal frictionless surface requires a net external force to maintain its motion.

Reason (R):

According to Newton's first law, a body in motion continues to move with constant velocity if no net external force acts upon it.

Answer: D — A is false but R is true — The assertion is false; no net force is required for constant velocity motion on a frictionless surface. The reason is true and states the correct principle. The assertion contradicts Newton's first law, making option D correct.

Q2. Assertion (A):

Inertia is the property of a body that causes it to remain in its current state of motion unless disturbed by an external force.

Reason (R):

Inertia is directly proportional to the mass of the body and represents resistance to change in motion.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The reason correctly explains why the assertion is true—inertia (mass-dependent resistance) keeps a body in its state unless an external force acts. The reason is the correct explanation of the assertion.

Q3. Assertion (A):

Aristotle's law of motion is incorrect because it fails to account for friction and other opposing forces in real-world scenarios.

Reason (R):

In the absence of friction, a body in uniform motion would continue indefinitely without external force, which contradicts Aristotle's view.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both the assertion and reason are true—friction does invalidate Aristotle's law, and frictionless motion confirms Galileo's insight. However, the reason does not directly explain the assertion; it provides a counterexample rather than an explanation of why Aristotle was wrong. The assertion is about the flaw itself; the reason supports it but is not the direct causal explanation.

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Case-Based Questions (CBQ) for Laws of Motion

Case Study:

A cyclist rides on a straight road at a constant speed of 10 m/s. The cyclist pedals continuously to overcome friction and air resistance, which together produce a retarding force of 50 N. The cyclist's mass (including the bicycle) is 80 kg. After riding for 20 seconds at constant speed, the cyclist stops pedalling. Due to the retarding forces, the cyclist decelerates uniformly and comes to rest after 10 seconds.

Q1. While riding at constant 10 m/s, what is the net force on the cyclist?

  • A. 0 N ✓
  • B. 50 N forward
  • C. 50 N backward
  • D. 800 N forward

Answer: A

Q2. What pedalling force must the cyclist apply to maintain constant speed against the 50 N retarding force?

  • A. 0 N
  • B. 25 N
  • C. 50 N ✓
  • D. 80 N

Answer: C

Q3. After stopping pedalling, what is the deceleration of the cyclist?

  • A. 0.625 m/s² ✓
  • B. 1.25 m/s²
  • C. 5 m/s²
  • D. 10 m/s²

Answer: A

Q4. Which concept from the chapter best explains why the cyclist continues at 10 m/s without pedalling briefly, but eventually slows down?

  • A. Aristotle's law—external force is always needed
  • B. Inertia—the cyclist's velocity would remain constant without the retarding forces acting ✓
  • C. Newton's third law—the road pushes back on the cyclist
  • D. Acceleration is zero when velocity is constant

Answer: B

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