Navigating through the newly revised CBSE Class 9 Science curriculum (Exploration) requires a thorough physical and mathematical understanding of balanced and unbalanced forces, the concept of inertia and mass, Newton’s three fundamental laws of motion, momentum (p = m × v), impulse, the quantitative derivation of force (F = m × a), action-reaction pairs, frictional resistance, and the mechanics of momentum conservation in collisions. Chapter 6 of Class 9 Physics, “How Forces Affect Motion”, establishes the foundational principles of classical dynamics. It investigates why objects require no net force to maintain uniform straight-line velocity in frictionless space; explains how a net unbalanced force produces linear acceleration proportional to force and inversely proportional to mass; analyzes why a cricket fielder pulls their hands backward during a high-speed catch to reduce impact force; details the action-reaction dynamics of rowing boats, recoiling firearms, and magnetic compass needles; and derives mathematical expressions for composite systems. To help students master every aspect of this high-weightage chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE Class 9 evaluation standards.
Every question presented in the official NCERT textbook—ranging from all introductory “Think It Over” sections and in-text “Pause and Ponder” prompts (Pages 94, 95, and 97) to the complete end-of-chapter “Revise, Reflect, Refine” exercises (Questions 1 to 15 on Pages 112–115)—has been solved with exhaustive detail. Numerical problems follow a step-by-step box format with explicit variable legends, standard SI unit conversions, and algebraic substitutions using clean plain-text symbols. Key scoring terms, official CBSE exam tags, and dynamic summary tables have been highlighted to ensure students secure maximum marks in their examinations.
Master Concept & Comparative Summary Tables
1. Newton’s Three Laws of Motion Summary Sheet
| Law of Motion | Formal Physical Statement | Mathematical Formulation | Everyday Physical Applications |
|---|---|---|---|
| Newton’s First Law (Law of Inertia) | An object remains in its state of rest or of uniform motion in a straight line unless acted upon by an external net unbalanced force. | If Net Force = 0, then Acceleration = 0 (Velocity is constant or object is at rest). | • Passengers jerk backward when a bus starts suddenly. • Dust falls out when a carpet is beaten with a stick. • Luggage on bus roofs must be tied with a rope. |
| Newton’s Second Law (Law of Force & Acceleration) | The rate of change of momentum of an object is directly proportional to the applied unbalanced force and takes place in the direction of the force. | Force = Change in momentum / Time = m × (v – u) / t = m × a (Unit: 1 Newton = 1 kg × m/s²) | • A cricket fielder pulls hands backward to cushion a fast ball. • Athletes in high jump land on cushioned foam mattresses. • Seatbelts stretch slightly to increase stopping time. |
| Newton’s Third Law (Law of Action & Reaction) | To every action, there is always an equal and opposite reaction; forces always occur in matched pairs acting on two different bodies. | Force on B by A = – (Force on A by B) (Action and Reaction act on different bodies) | • A canoeist pushes water backward to move the boat forward. • Recoil of a heavy gun when a bullet is fired forward. • Rocket propulsion via high-velocity exhaust gas ejection. |
2. Comparison: Balanced Forces vs. Unbalanced Forces
| Feature / Parameter | Balanced Forces | Unbalanced Forces |
|---|---|---|
| Net Resultant Force | Net resultant force is strictly Zero (Net Force = 0). | Net resultant force is Non-Zero (Net Force > 0). |
| Effect on State of Rest | An object at rest remains at rest. | An object at rest begins to move (accelerates). |
| Effect on Moving Object | A moving object continues moving with constant speed in a straight line. | A moving object speeds up, slows down, stops, or changes its direction. |
| Acceleration Produced | Zero acceleration (a = 0). | Non-zero acceleration (a = Net Force / mass). |
| Effect on Object Shape | Can change the physical shape or size of an elastic body (e.g., squeezing a rubber ball). | Can change both the kinematic state of motion and the shape of the object. |
NCERT In-Text Questions: “Think It Over”
Page No. 94: Think It Over (Questions 1 to 3)
Question 1 Why does a canoe move forward when the canoeist pushes water backwards with their paddle, and why does it move faster when they push harder? [Exam Favorite]
Answer:
- Why the Canoe Moves Forward: This is a direct manifestation of Newton’s Third Law of Motion. When the canoeist pushes the water in the backward direction with their paddle (Action force), the water exerts an equal and opposite force pushing the paddle and the canoe in the forward direction (Reaction force). This forward reaction force accelerates the canoe.
- Why It Moves Faster When Pushing Harder: According to Newton’s Second Law of Motion (Force = mass × acceleration), when the canoeist pushes the paddle harder, the magnitude of the applied backward action force increases, generating a correspondingly larger forward reaction force. A larger net force produces greater forward acceleration, making the canoe surge forward at a much higher speed.
Question 2 Suppose the same canoeist uses the same paddle force in two different canoes, one empty and one carrying another passenger. In which case will the canoe move faster? [Exam Favorite]
Answer: The empty canoe will move faster (achieve greater acceleration).
- Scientific Justification: According to Newton’s Second Law of Motion, acceleration is inversely proportional to the total mass of the system for a given constant force (Acceleration = Force / mass).
- The canoe carrying another passenger has a substantially larger total mass compared to the empty canoe. Because the applied paddle force is identical in both cases, the heavier canoe experiences a smaller acceleration, whereas the lighter empty canoe experiences greater acceleration and reaches higher velocity.
Question 3 Is there an underlying cause for a change in position and velocity of an object? What is the nature of this cause? Do all motions require a cause? [Exam Favorite]
Answer:
- Underlying Cause for Change in Velocity: Yes. A change in the velocity of an object (which means acceleration or deceleration) is fundamentally caused by a Net Unbalanced Force.
- Nature of the Cause: A force is an external push, pull, gravitational attraction, electromagnetic interaction, or frictional drag exerted by one body upon another.
- Do All Motions Require a Cause? No, uniform steady motion does not require a continuous force. As established by Galileo and Newton’s First Law, an object moving with constant velocity along a straight line in frictionless space will continue moving indefinitely without any force acting upon it. A net force is required only to change the state of motion (to start, stop, speed up, slow down, or change direction).
Page No. 95: Think It Over (Questions 4 & 5)
Question 4 How can we measure the magnitude of a force? Do you remember using a spring balance earlier to measure the weight of objects? Do you also remember the weight of an object is the gravitational force with which the Earth pulls the object? [Exam Favorite]
Answer:
- Measuring Force with a Spring Balance: The magnitude of a mechanical force can be measured using a calibrated Spring Balance.
- Operating Principle: A spring balance operates on the principle of linear elastic deformation (Hooke’s Law). When an external force acts on the hook, the internal helical spring stretches. The elongation of the spring is directly proportional to the magnitude of the applied pulling force.
- Gravitational Weight as Force: The weight of an object is the downward gravitational pull exerted on its mass by the Earth (Weight = mass × gravity). When an object is suspended from a spring balance, its weight stretches the spring, and the pointer directly indicates the magnitude of the gravitational force in Newtons (N).
Question 5 In such cases, what is the effect of forces when more than one force is acting on an object at rest or in motion? [Exam Favorite]
Answer: When multiple forces act simultaneously on an object, their combined physical effect is determined by their vector sum, known as the Net Force:
- If Net Force is Zero (Balanced Forces): The forces cancel each other out completely. An object at rest remains stationary, and an object in motion continues moving with constant velocity (Acceleration = 0).
- If Net Force is Non-Zero (Unbalanced Forces): The object undergoes linear acceleration in the exact direction of the resultant net force (Acceleration = Net Force / mass), altering its speed, trajectory, or both.
Page No. 97: Pause and Ponder (Questions 1 & 2)
Question 1 A weightlifter lifts a barbell. List two forces that are acting on the barbell. Are these forces balanced if the weightlifter keeps the barbell steady? [Exam Favorite]
Answer:
- Two Forces Acting on the Barbell:
- Downward Gravitational Force (Weight): Earth’s gravity pulling the barbell vertically downward.
- Upward Muscular Normal Force: The upward mechanical contact force exerted by the weightlifter’s hands against the barbell.
- Are the Forces Balanced? Yes, the forces are completely balanced when the barbell is held steady. Because the barbell is stationary in mid-air (Velocity = 0, Acceleration = 0), the upward muscular force exerted by the weightlifter is exactly equal in magnitude and opposite in direction to the downward gravitational weight (Upward Force = Downward Weight, so Net Force = 0).
Question 2 Two players, R and S, are participating in an arm-wrestling match. At the instant when the arms tilt to the front direction (out of the page towards you), are the forces exerted by the players balanced? If not, which player exerted the larger force? [Exam Favorite]
Answer:
- Are the Forces Balanced? No, the forces are unbalanced.
- Scientific Reasoning: If the gripped hands and arms tilt and accelerate in the front direction, there is a non-zero net resultant force acting in that direction.
- Which Player Exerted the Larger Force? The player who pushes or pulls in the forward tilt direction (Player S) exerted a larger muscular force than Player R, overcoming the opposing force and generating net forward acceleration.
NCERT Chapter-End Exercises: “Revise, Reflect, Refine” (Pages 112–115)
Question 1 Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table? [Exam Favorite]
Answer:
================================================================================ DEDUCTION USING NEWTON'S FIRST LAW: -------------------------------------------------------------------------------- 1. State of Motion: The table is moving with a CONSTANT VELOCITY. 2. Acceleration: Since velocity is constant, acceleration a = 0. 3. Net Force Condition: According to Newton's Second Law: Net Force = mass × acceleration = 0 4. Force Equilibrium: Net Force = Applied Force (F) - Frictional Force = 0 Frictional Force = F FINAL ANSWER: The frictional force exerted by the floor on the table is EXACTLY EQUAL IN MAGNITUDE to the applied force F, and acts in the OPPOSITE DIRECTION to the motion. ================================================================================
Question 2 For a ball moving on a smooth frictionless surface, choose the appropriate option that will make the following statements physically correct: (i) If no net force is applied on the ball, the velocity of the ball will (remain the same / increase / decrease). (ii) If a net force is applied on the ball in the direction of its motion, the magnitude of the velocity of the ball will (remain the same / increase / decrease). (iii) If a net force is applied on the ball in a direction opposite to the direction of its motion, the magnitude of the velocity of the ball will (remain the same / increase / decrease). [Exam Favorite]
Answer:
- (i) If no net force is applied on the ball, the velocity of the ball will remain the same. (Newton’s First Law: an object in motion maintains constant velocity in the absence of net force).
- (ii) If a net force is applied on the ball in the direction of its motion, the magnitude of the velocity of the ball will increase. (Force produces positive acceleration in the direction of motion).
- (iii) If a net force is applied on the ball in a direction opposite to the direction of its motion, the magnitude of the velocity of the ball will decrease. (Opposing force produces negative acceleration or deceleration).
Question 3 What is inertia? How does the inertia of an object depend upon its mass? Give two everyday examples of inertia of rest and inertia of motion. [Exam Favorite]
Answer:
- Definition of Inertia: Inertia is the inherent natural property of all material bodies to resist any change in their existing state of rest or of uniform straight-line motion.
- Relationship with Mass: Mass is the quantitative measure of inertia. An object with larger mass offers greater resistance to changing its motion and possesses greater inertia than a lighter object.
- Everyday Examples:
- Inertia of Rest: When a stationary bus suddenly accelerates forward, passengers jerk backward because their lower bodies move forward with the bus while their upper bodies tend to remain at rest due to inertia.
- Inertia of Motion: When a speeding bus applies emergency brakes, passengers lurch forward because their bodies tend to continue moving forward at the original speed due to inertia.
Question 4 While practising for the snake boat race (Vallam Kali), a team has 100 oarsmen. 95 oarsmen row in the forward direction while 5 oarsmen accidentally row in the backward direction. If each oarsman applies a horizontal force of 200 N, what is the net forward force on the snake boat? (Ignore water drag forces). [Exam Favorite]
Answer:
================================================================================ NUMERICAL SOLUTION (RESULTANT FORCE ON SNAKE BOAT): -------------------------------------------------------------------------------- GIVEN DATA: • Number of forward oarsmen = 95 • Number of backward oarsmen = 5 • Force applied by each man = 200 N STEP 1: Calculating Total Forward Force Forward Force = 95 × 200 N = 19,000 N STEP 2: Calculating Total Opposing Backward Force Backward Force = 5 × 200 N = 1,000 N STEP 3: Net Resultant Horizontal Force Net Force = Forward Force - Backward Force Net Force = 19,000 N - 1,000 N = 18,000 N (or 18 kN) Alternative Quick Method: Effective number of forward oarsmen = 95 - 5 = 90 Net Force = 90 × 200 N = 18,000 N FINAL ANSWER: The net forward force acting on the snake boat is 18,000 N (18 kN). ================================================================================
Question 5 When a carpet is beaten with a stick, dust particles fall out. Explain the physical reason using Newton’s first law of motion. [Exam Favorite]
Answer: This happens due to the Inertia of Rest:
- Initially, both the carpet fibers and the dust particles resting inside them are in a state of rest.
- When the carpet is struck vigorously with a stick, the flexible carpet fibers move forward suddenly.
- However, the loosely held dust particles tend to remain in their original state of rest due to inertia of rest.
- As the carpet moves away from beneath them, the dust particles become detached in air and fall downward under the action of gravity.
Question 6 Why is it advised to tie any luggage kept on the roof of a bus with a rope? [Exam Favorite]
Answer: Luggage on a bus roof is subject to inertia during sudden changes in velocity and direction:
- When the Bus Accelerates Suddenly: The luggage tends to remain at rest due to inertia of rest, causing it to slip backward and fall off the roof.
- When the Bus Brakes Suddenly: The luggage tends to continue moving forward due to inertia of motion, sliding forward over the roof.
- When the Bus Takes a Sharp Turn: The luggage tends to maintain its straight-line path due to directional inertia, sliding off sideways.
Tying the luggage firmly with a rope provides the necessary constraining contact forces to overcome these inertial slips.
Question 7 A constant force acts on an object of mass 5 kg for a duration of 2 s. It increases the object’s velocity from 3 m/s to 7 m/s. Find the magnitude of the applied force. Now, if the force was applied for a duration of 5 s, what would be the final velocity of the object? [Exam Favorite]
Answer:
================================================================================
NUMERICAL SOLUTION (FORCE AND FINAL VELOCITY):
--------------------------------------------------------------------------------
GIVEN DATA (PART 1):
• Mass (m) = 5 kg
• Initial Velocity (u) = 3 m/s
• Final Velocity (v) = 7 m/s
• Time Duration (t) = 2 s
STEP 1: Finding Acceleration (a) and Applied Force (F)
Formula: Acceleration a = (v - u) / t
a = (7 - 3) / 2 = 4 / 2 = 2 m/s²
Formula: Force F = m × a
F = 5 kg × 2 m/s² = 10 N
STEP 2: Finding Final Velocity if Force is Applied for 5 s
• Initial Velocity (u) = 3 m/s
• Acceleration (a) = 2 m/s² (Constant force F = 10 N)
• Time (t') = 5 s
Using Formula: Final Velocity v' = u + (a × t')
v' = 3 + (2 × 5)
v' = 3 + 10 = 13 m/s
FINAL ANSWER:
• Magnitude of Applied Force = 10 N
• Final Velocity after 5 s = 13 m/s
================================================================================
Question 8 Which would require a greater force: accelerating a 2 kg mass at 5 m/s² or a 4 kg mass at 2 m/s²? [Exam Favorite]
Answer:
================================================================================
COMPARISON OF FORCES:
--------------------------------------------------------------------------------
Case 1: Mass = 2 kg, Acceleration = 5 m/s²
Formula: Force F₁ = mass × acceleration
F₁ = 2 kg × 5 m/s² = 10 N
Case 2: Mass = 4 kg, Acceleration = 2 m/s²
Formula: Force F₂ = mass × acceleration
F₂ = 4 kg × 2 m/s² = 8 N
Comparison:
F₁ (10 N) is greater than F₂ (8 N)
FINAL ANSWER:
Accelerating a 2 kg mass at 5 m/s² requires a GREATER force (10 N vs 8 N).
================================================================================
Question 9 A hand exerts a horizontal force on a box of mass 10 kg resting on a rough floor. The maximum force of static friction between the box and the floor is 30 N. If the applied force is 20 N, what is the acceleration of the box? If the applied force is increased to 50 N, and the kinetic friction is 25 N, find the acceleration. [Exam Favorite]
Answer:
================================================================================ NUMERICAL SOLUTION (FRICTION AND ACCELERATION): -------------------------------------------------------------------------------- GIVEN DATA: • Mass of box (m) = 10 kg • Maximum Static Friction = 30 N • Kinetic Friction = 25 N CASE 1: Applied Force = 20 N • Since Applied Force (20 N) is LESS than Maximum Static Friction (30 N), the static friction self-adjusts to 20 N to balance the applied force. • Net Force = 20 N - 20 N = 0 N • The box remains AT REST. • Acceleration = 0 m/s² CASE 2: Applied Force = 50 N • Since Applied Force (50 N) EXCEEDS Maximum Static Friction (30 N), the box breaks into motion, and kinetic friction (25 N) opposes motion. • Net Force = Applied Force - Kinetic Friction Net Force = 50 N - 25 N = 25 N • Acceleration = Net Force / mass = 25 N / 10 kg = 2.5 m/s² FINAL ANSWER: • In Case 1 (20 N force) : Acceleration = 0 m/s² (Box remains at rest). • In Case 2 (50 N force) : Acceleration = 2.5 m/s². ================================================================================
Question 10 A bullet of mass 10 g travelling horizontally with a velocity of 150 m/s strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet. [Exam Favorite]
Answer:
================================================================================
NUMERICAL SOLUTION (BULLET RETARDATION & PENETRATION):
--------------------------------------------------------------------------------
GIVEN DATA:
• Mass of bullet (m) = 10 g = 10 / 1000 kg = 0.01 kg
• Initial Velocity (u) = 150 m/s
• Final Velocity (v) = 0 m/s (Comes to rest)
• Time Taken (t) = 0.03 s
STEP 1: Calculating Acceleration (a)
Formula: Acceleration a = (v - u) / t
a = (0 - 150) / 0.03 = -150 / 0.03 = -5000 m/s²
(Retardation of 5000 m/s²).
STEP 2: Calculating Distance of Penetration (s)
Using Kinematic Formula: v² - u² = 2 × a × s
(0)² - (150)² = 2 × (-5000) × s
-22500 = -10000 × s
s = -22500 / -10000 = 2.25 m
STEP 3: Calculating Retarding Force Exerted by Wood (F)
Formula: Force F = mass × acceleration
F = 0.01 kg × (-5000 m/s²) = -50 N
(Magnitude of resistive force = 50 N).
FINAL ANSWER:
• Distance of Penetration = 2.25 m
• Magnitude of Force = 50 N
================================================================================
Question 11 An object of mass 1 kg travelling in a straight line with a velocity of 10 m/s collides with, and sticks to, a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object. [Exam Favorite]
Answer:
================================================================================ NUMERICAL SOLUTION (CONSERVATION OF LINEAR MOMENTUM): -------------------------------------------------------------------------------- GIVEN DATA: • Mass of moving object (m₁) = 1 kg, Initial velocity (u₁) = 10 m/s • Mass of wooden block (m₂) = 5 kg, Initial velocity (u₂) = 0 m/s (Stationary) • Combined Mass (M) = m₁ + m₂ = 1 + 5 = 6 kg • Common Final Velocity = v STEP 1: Total Momentum Just Before Impact Initial Momentum = (m₁ × u₁) + (m₂ × u₂) Initial Momentum = (1 × 10) + (5 × 0) = 10 + 0 = 10 kg × m/s STEP 2: Total Momentum Just After Impact According to the Law of Conservation of Momentum: Total Momentum After Impact = Total Momentum Before Impact = 10 kg × m/s STEP 3: Calculating Combined Velocity (v) Final Momentum = (m₁ + m₂) × v 10 = 6 × v v = 10 / 6 = 5 / 3 = 1.67 m/s FINAL ANSWER: • Total Momentum Before Impact = 10 kg × m/s • Total Momentum After Impact = 10 kg × m/s • Velocity of Combined Object = 1.67 m/s (or 5/3 m/s) ================================================================================
Question 12 A tractor pulls a harrow of mass m1 with a force F producing an acceleration a1. The same tractor pulls a trolley of mass m2 with the same force F producing an acceleration a2. If the tractor now pulls the trolley with the harrow placed on it (with the same force F), obtain an expression for the resulting acceleration in terms of a1 and a2. (Ignore friction). [Exam Favorite]
Answer:
================================================================================
ALGEBRAIC DERIVATION (COMPOSITE ACCELERATION):
--------------------------------------------------------------------------------
STEP 1: Expressing Individual Masses from Newton's Second Law
• For Harrow : Force F = m1 × a1 ===> m1 = F / a1
• For Trolley : Force F = m2 × a2 ===> m2 = F / a2
STEP 2: Combined Total Mass (M_total)
M_total = m1 + m2 = (F / a1) + (F / a2)
M_total = F × (1/a1 + 1/a2) = F × [(a1 + a2) / (a1 × a2)]
STEP 3: Resulting Acceleration (a_combined) with Same Force F
Formula: a_combined = Force / M_total
a_combined = F / [ F × (a1 + a2) / (a1 × a2) ]
a_combined = (a1 × a2) / (a1 + a2)
FINAL ANSWER:
The resulting acceleration is: a = (a1 × a2) / (a1 + a2)
================================================================================
Question 13 When the pole of a bar magnet is brought close to a magnetic compass, the bar magnet and the compass needle (which is also a magnet) exert equal and opposite magnetic forces on each other as per Newton’s third law. However, the compass needle moves, whereas the bar magnet does not move. Explain why. [Exam Favorite]
Answer: According to Newton’s Third Law of Motion, the mutual magnetic forces of interaction between the bar magnet and the compass needle are strictly equal in magnitude and opposite in direction (Force on magnet = Force on needle).
However, the resulting acceleration of an object depends on its mass (Acceleration = Force / mass):
- Compass Needle: Has an extremely tiny mass (a few grams) and is freely pivoted on a low-friction jewel bearing. The magnetic force produces a large, immediately visible rotational acceleration.
- Bar Magnet: Has a much larger mass (hundreds of grams) and rests on a table where large static friction holds it firmly. The same magnitude of force produces an imperceptibly tiny acceleration that cannot overcome table friction. Hence, only the needle moves visibly.
Question 14 Why does a cricket player (fielder) pull their hands backward while catching a fast-moving cricket ball? Explain using the concept of momentum and Newton’s second law. [Exam Favorite]
Answer: A fielder pulls their hands backward to increase the time duration of the catch:
- Momentum Change: A fast-moving cricket ball has a large initial momentum (p = mass × velocity) that must be reduced to zero.
- Force Formula: According to Newton’s Second Law: Force = Change in momentum / Time.
- Reducing Impact: By pulling the hands backward along with the ball’s trajectory, the fielder increases the time interval over which the ball’s velocity drops to zero. As time increases, the rate of change of momentum decreases significantly, reducing the impact force on the player’s palms and preventing hand injury.
Question 15 State Newton’s three laws of motion and write the mathematical relation for Newton’s second law. [Exam Favorite]
Answer:
- Newton’s First Law of Motion: An object remains in its state of rest or of uniform motion in a straight line unless acted upon by an external net unbalanced force.
- Newton’s Second Law of Motion: The rate of change of momentum of a body is directly proportional to the applied unbalanced force and occurs in the direction of the force. Mathematical Equation: F = m × a (where F is force in Newtons, m is mass in kg, and a is acceleration in m/s²).
- Newton’s Third Law of Motion: To every action, there is an equal and opposite reaction; forces always act in pairs on two interacting bodies.
Frequently Asked Questions (FAQs) – Class 9 Science Chapter 6
Question 1: Define linear momentum (p). What is its SI unit? [Exam Favorite] Answer: Linear momentum is defined as the product of the mass (m) and velocity (v) of a moving body (p = m × v). It is a vector quantity possessing both magnitude and direction. Its SI unit is kilogram metre per second (kg · m/s).
Question 2: Define 1 Newton of force. [Exam Favorite] Answer: One Newton (1 N) is defined as the amount of net unbalanced force that produces an acceleration of 1 m/s² in an object of mass 1 kg (1 N = 1 kg × 1 m/s²).
Question 3: Why do action and reaction forces not cancel each other out? [Exam Favorite] Answer: Action and reaction forces do not cancel each other out because they always act on two different, distinct physical bodies simultaneously, never on the same body.
Question 4: State the Law of Conservation of Linear Momentum. [Exam Favorite] Answer: The Law of Conservation of Momentum states that the total linear momentum of an isolated system of interacting particles remains constant (conserved) if no external unbalanced force acts upon the system: (m1 × u1) + (m2 × u2) = (m1 × v1) + (m2 × v2).
Question 5: Why does a gun recoil backward when a bullet is fired? [Exam Favorite] Answer: Initially, the gun and bullet are at rest with zero total momentum. When fired, the chemical explosion exerts a forward force on the bullet, giving it forward momentum. To conserve total linear momentum, the gun experiences an equal backward momentum, causing it to recoil backward (Recoil Velocity = – mass of bullet × velocity of bullet / mass of gun).
Question 6: Why is a gun made much heavier than its bullet? [Exam Favorite] Answer: Recoil velocity is inversely proportional to the mass of the gun. Making the gun heavy ensures that its backward recoil velocity is small and safe for the shooter to absorb without injury.
Question 7: What is Impulse of a force? What is its relation to momentum? [Exam Favorite] Answer: Impulse is defined as the product of a large force and the short time duration for which it acts (Impulse = Force × time). According to the impulse-momentum theorem, impulse equals the total change in momentum of the body.
Question 8: Why do athletes in a high-jump event land on sand or cushioned foam? [Exam Favorite] Answer: Cushioned foam or loose sand compresses upon impact, increasing the time taken for the athlete to come to rest. Increasing the impact time significantly reduces the rate of change of momentum, minimizing the impact force exerted on the athlete’s body and preventing bone fractures.
Question 9: What is Friction? In which direction does it act? [Exam Favorite] Answer: Friction is the resistive contact force that arises between two surfaces in contact and always acts tangentially in the direction opposite to the relative motion (or impending motion) of the object.
Question 10: Which has more inertia: a rubber ball of mass 200 g or a stone of the same size with mass 2 kg? [Exam Favorite] Answer: The stone of mass 2 kg has more inertia because inertia is directly proportional to mass (2 kg is greater than 0.2 kg).
Question 11: Why does an equestrian rider fall forward when a galloping horse suddenly stops? [Exam Favorite] Answer: When the horse stops abruptly, the rider’s lower body in contact with the saddle stops with the horse, but their upper body tends to continue moving forward due to inertia of motion, throwing the rider forward over the horse’s head.
Question 12: Why do leaves and ripe fruits fall from tree branches when vigorously shaken? [Exam Favorite] Answer: Shaking moves the tree branches suddenly, but the attached leaves and fruits tend to remain in their original state of rest due to inertia of rest. The resulting mechanical shear stress breaks their delicate stalks, causing them to fall.
Question 13: How does a swimmer propel themselves forward in water? [Exam Favorite] Answer: A swimmer pushes the water backward with their hands and feet (Action force); in response, the water pushes the swimmer forward with an equal and opposite force (Reaction force), enabling forward propulsion.
Question 14: Can a body be in motion if the net force acting on it is zero? [Exam Favorite] Answer: Yes. If the net force is zero (Net Force = 0), acceleration is zero (a = 0), meaning the body moves with constant speed in a straight line (uniform velocity).
Question 15: What is the relationship between the Newton (SI unit) and the Dyne (CGS unit) of force? [Exam Favorite] Answer: 1 Newton = 1 kg × 1 m/s² = 1000 g × 100 cm/s² = 100,000 Dynes (10⁵ Dynes).
Mastering the NCERT Solutions for Class 9 Science Chapter 6 (Exploration), “How Forces Affect Motion”, equips students with the dynamical principles of Newton’s laws, force calculations, and momentum conservation required for top performance in CBSE physics evaluations. Review the numerical box solutions, the master summary tables, and the 15 high-yield FAQs above to secure full marks in your examinations.