Force is a push or pull that can change an object’s state of rest, speed, direction, or shape. Important Questions Class 9 Science Chapter 6 cover balanced and unbalanced forces, friction, Newton’s laws of motion, momentum, numericals, assertion-reason questions, and case studies.
A parked bus stays still because the forces on it balance. A moving bus throws passengers forward when it stops suddenly because their bodies resist the change. A rocket rises because the gases it pushes downward push it back upward.
Class 9 Science Chapter 6 How Forces Affect Motion is built around one idea: motion changes only when net force acts. Once students learn to check direction, object, and net force, Newton’s laws and momentum numericals become much easier to solve.
Key Takeaways
| Detail |
Information |
| Chapter |
Class 9 Science Chapter 6 |
| Topic |
How Forces Affect Motion |
| Syllabus |
CBSE 2026 |
| Question Types |
VSA, Short Answer, Long Answer, Numericals, Graph-Based, Assertion-Reason, Case Study |
| Key Topics |
Force, Balanced and Unbalanced Forces, Friction, Newton’s First Law, Newton’s Second Law, Momentum, Newton’s Third Law, Action-Reaction Pairs, Systems of Objects |
Class 9 Science Chapter 6 How Forces Affect Motion: Topics Covered
Before solving any question from this chapter, ask one thing first: is the net force zero or non-zero? That single check decides whether the object keeps its state or changes its motion.
- Concept of force: magnitude and direction
- SI unit of force: newton
- Measuring force using spring balance
- Balanced and unbalanced forces
- Net force
- Friction and normal force
- Newton’s first law: law of inertia
- Newton’s second law: F = ma
- Momentum and rate of change of momentum
- Newton’s third law: action-reaction pairs
- Forces acting on a system of objects
- Internal and external forces
Class 9 Science Chapter List
Important Class 9 Science Chapter 6 Questions with Answers
Important Questions Class 9 Science Chapter 6 with Answers are arranged from definitions to application. Begin with force and net force, then move to Newton’s laws, numericals, and real-life examples.
These how forces affect motion class 9 important questions help students identify which law applies before writing the answer.
Class 9 Science Chapter 6 Question Answer Practice by Topic
Each class 9 science chapter 6 question answer set is grouped by the concept it tests: force, friction, Newton’s laws, momentum, or systems of objects.
Very Short Answer Important Questions Class 9 Science Chapter 6
Q1. What is force?
Ans. Force is a push or pull that changes or tends to change the state of rest or motion of an object. Its SI unit is the newton (N).
Q2. What is the SI unit of force?
Ans. The SI unit of force is the newton (N). One newton gives a mass of 1 kg an acceleration of 1 m/s².
Q3. What is net force?
Ans. Net force is the vector sum of all forces acting on an object. If net force is zero, the object stays at rest or moves at constant velocity.
Q4. State Newton’s first law of motion.
Ans. An object at rest stays at rest and an object in motion stays in motion at constant velocity unless a net external force acts on it.
Q5. What is momentum?
Ans. Momentum is the product of mass and velocity. p = mv. The SI unit of momentum is kg m/s.
Q6. State Newton’s third law of motion.
Ans. For every action, there is an equal and opposite reaction. Action and reaction forces act on different objects.
Q7. What is friction?
Ans. Friction is the force that opposes relative motion between two surfaces in contact. It acts opposite to the direction of motion.
Q8. What is a spring balance?
Ans. A spring balance measures force by the extension of a spring. Greater force causes greater extension. The scale reading gives the force in newtons.
Short Answer Questions Class 9 Science Chapter 6 with Answers
Q1. Distinguish between balanced and unbalanced forces.
| Feature |
Balanced Forces |
Unbalanced Forces |
| Net force |
Zero |
Non-zero |
| Effect on motion |
No change in state |
Change in state |
| Example |
Book on table |
Kicked football |
| Acceleration |
Zero |
Non-zero |
Q2. Why does a high jump athlete land on a thick foam mat?
Ans. The foam mat increases the time over which momentum reduces to zero. By Newton’s second law, force = change in momentum / time.
A longer time means smaller force on the body. This reduces injury risk.
Q3. Why do action and reaction forces not cancel each other?
Ans. Action and reaction forces act on different objects. Cancellation happens only when two forces act on the same object.
A book pushes the table down. The table pushes the book up. These forces act on different objects, so they cannot cancel.
Q4. What is the relationship between force, mass, and acceleration?
Ans. Force equals mass multiplied by acceleration.
F = ma
A larger force on the same mass produces greater acceleration. The same force on a larger mass produces smaller acceleration.
Q5. Why does a boat move backward when a sailor jumps forward?
Ans. When the sailor jumps forward, the sailor exerts a backward force on the boat. By Newton’s third law, the boat moves backward.
Action and reaction forces are equal and opposite.
Q6. What is the role of friction in daily life?
Ans. Friction allows walking by preventing feet from sliding. It helps vehicles brake and stop.
It also keeps nails and screws in place. Without friction, grip, braking, and traction would not work.
Long Answer Important Questions Class 9 Science Chapter 6
Q1. State and explain Newton’s second law of motion. Derive the mathematical expression.
Ans. Newton’s second law states that the net force on an object equals the rate of change of its momentum.
Let an object have mass m, initial velocity u, and final velocity v over time t.
Initial momentum = mu
Final momentum = mv
Change in momentum = mv - mu = m(v - u)
Rate of change of momentum = m(v - u) / t
Since acceleration, a = (v - u) / t
Therefore, F = ma.
Force is directly proportional to acceleration for fixed mass. Force is directly proportional to mass for fixed acceleration. The direction of force is the same as the direction of acceleration.
Q2. Explain Newton’s third law with three real-life examples. Why do action-reaction forces never cancel?
Ans. Newton’s third law states that for every action, there is an equal and opposite reaction on a different object.
Example 1: Rocket launch
Fuel pushes gases downward. Gases push the rocket upward. The rocket rises.
Example 2: Swimming
A swimmer pushes water backward. Water pushes the swimmer forward.
Example 3: Balloon activity
Air moves backward out of the balloon. The balloon moves forward.
Action-reaction forces never cancel because they act on different objects. Cancellation requires two forces on the same object.
Numericals Class 9 Science Chapter 6 How Forces Affect Motion
Newton second law numericals class 9 and momentum questions class 9 are the most important numerical types in Chapter 6.
Always write the formula, substitute values with units, calculate clearly, and write the final unit.
Net Force Numericals Class 9 Science Chapter 6
Q1. Two forces act on a box: 30 N east and 10 N west. Find net force and direction.
Ans. Net force = 30 - 10 = 20 N east
Q2. Three forces act on an object: 40 N right, 15 N right, and 25 N left. Find net force.
Ans. Net force = 40 + 15 - 25 = 30 N right
Q3. A book rests on a table. Weight = 5 N downward. Normal force = 5 N upward. Find net force.
Ans. Net force = 5 - 5 = 0 N.
The book stays at rest.
Newton’s Second Law Numericals Class 9 Science Chapter 6
Q1. A force of 24 N acts on a 6 kg mass. Find acceleration.
Ans. a = F / m
a = 24 / 6
a = 4 m/s²
Q2. A car of mass 1000 kg accelerates at 3 m/s². Find net force.
Ans. F = ma
F = 1000 × 3
F = 3000 N
Q3. A force of 50 N produces acceleration of 5 m/s². Find the mass.
Ans. m = F / a
m = 50 / 5
m = 10 kg
Q4. Two boxes have masses 4 kg and 8 kg. The same force acts on both. Which accelerates more?
Ans. a = F / m.
For 4 kg: a = F / 4
For 8 kg: a = F / 8
The 4 kg box accelerates twice as much.
Momentum Numericals Class 9 Science Chapter 6
Q1. A ball of mass 0.5 kg moves at 10 m/s. Find momentum.
Ans. p = mv
p = 0.5 × 10
p = 5 kg m/s
Q2. A car of mass 800 kg moves at 20 m/s. Find momentum.
Ans. p = mv
p = 800 × 20
p = 16000 kg m/s
Q3. A force of 10 N acts on a 2 kg object for 5 seconds. Find change in momentum.
Ans. Change in momentum = F × t
Change in momentum = 10 × 5
Change in momentum = 50 kg m/s
Q4. A 0.1 kg ball moving at 15 m/s stops in 0.05 seconds. Find the force applied.
Ans. Change in momentum = 0.1 × 15 = 1.5 kg m/s
Force = change in momentum / time
Force = 1.5 / 0.05
Force = 30 N
Force and Acceleration Numericals Class 9 Science Chapter 6
Q1. A 5 kg object starts from rest. A force of 20 N acts for 4 seconds. Find final velocity.
Ans. a = F / m
a = 20 / 5
a = 4 m/s²
v = u + at
v = 0 + 4 × 4
v = 16 m/s
Q2. A 10 kg box decelerates from 20 m/s to rest in 5 seconds. Find braking force.
Ans. a = (v - u) / t
a = (0 - 20) / 5
a = -4 m/s²
F = ma
F = 10 × 4
F = 40 N opposing motion
Important Questions on Force Class 9 Science
Force questions class 9 depend on magnitude and direction. A force answer is incomplete if it gives only the value but not the direction.
Magnitude and Direction of Force Questions Class 9
Q1. Why is force a vector quantity?
Ans. Force has both magnitude and direction.
Pushing a door right and pushing it left may involve the same force value, but the effects are opposite. A quantity that needs both magnitude and direction is a vector.
Q2. Can two forces of different magnitudes produce zero net force?
Ans. No. To produce zero net force, opposite forces must have equal magnitudes.
A 10 N force east and a 10 N force west produce zero net force.
SI Unit of Force Questions Class 9 Science
Q1. Define one newton.
Ans. One newton is the force that gives a mass of one kilogram an acceleration of one metre per second squared.
1 N = 1 kg m/s²
Spring Balance Questions Class 9 Science
Q1. How does a spring balance measure force?
Ans. A spring stretches when force is applied. Greater force causes greater extension.
A calibrated scale reads the extension in newtons. The reading gives the magnitude of the applied force.
Q2. What does a spring balance read when no force is applied?
Ans. The reading is zero. The spring returns to its natural length, and the pointer rests at the zero mark.
Balanced and Unbalanced Forces Questions Class 9
Balanced and unbalanced forces questions class 9 depend on net force. Zero net force does not always mean rest. It can also mean constant velocity.
Net Force Questions Class 9 Science
Q1. Name the forces on a book resting on a table. Are they balanced?
Ans. Weight pulls the book downward. Normal force pushes the book upward.
Both forces are equal in magnitude and opposite in direction on the same object. Net force is zero, so the forces are balanced.
Q2. Why does a moving box stop on a rough surface with no applied force?
Ans. Friction acts opposite to motion. Friction is an unbalanced force.
It reduces velocity. The box decelerates and stops.
Tug of War Questions Class 9 Science
Q1. Both teams pull with equal force in a tug of war. What happens to the rope?
Ans. The rope stays still. Net force is zero, so no acceleration occurs.
This is an example of balanced forces.
Q2. One team pulls with 500 N and the other with 400 N. Find net force.
Ans. Net force = 500 - 400 = 100 N toward the stronger team.
The rope accelerates in that direction.
Balanced Forces vs Unbalanced Forces Questions Class 9
Q1. Give two examples each of balanced and unbalanced forces.
Ans. Balanced forces:
- Book on table: weight and normal force cancel.
- Car at constant velocity: driving force and friction cancel.
Unbalanced forces:
- Car accelerating: driving force exceeds friction.
- Stone falling: gravity acts with no equal opposing force.
Friction Important Questions Class 9 Science
Friction questions class 9 should always mention direction. Friction acts opposite to relative motion, but it can still help in walking, braking, writing, and gripping.
Force of Friction Questions Class 9 Science
Q1. What factors affect the force of friction between two surfaces?
Ans. The nature of surfaces affects friction. Rough surfaces produce more friction.
The normal force also affects friction. Greater normal force produces greater friction.
Q2. Why does friction oppose but not always prevent motion?
Ans. Friction is a resistive force. It reduces speed over time.
If the applied force is greater than friction, the object still accelerates. Friction stops motion only when it balances the applied force.
Normal Force and Friction Questions Class 9
Q1. What is normal force? How does it relate to friction?
Ans. Normal force is the contact force a surface exerts perpendicular to an object.
Greater normal force means greater friction. Pushing a box downward increases normal force and friction.
Spring Balance Friction Experiment Questions Class 9
Q1. A spring balance reads 8 N when a block moves at constant velocity. What is friction?
Ans. At constant velocity, net force is zero.
Applied force equals friction. Friction force = 8 N opposing motion.
Newton’s First Law Important Questions Class 9
Newton first law questions class 9 test inertia. The key idea is simple: objects resist change unless a net external force acts.
Law of Inertia Questions Class 9 Science
Q1. What is inertia? How does mass relate to it?
Ans. Inertia is the tendency of an object to resist a change in its state of rest or motion.
Greater mass means greater inertia. A heavy truck is harder to start and stop than a bicycle.
Q2. Why does a passenger lurch forward when a bus stops suddenly?
Ans. The passenger’s body has inertia.
The bus stops, but the body continues moving forward until a seat or seatbelt applies a backward force.
Zero Net Force Questions Class 9 Science
Q1. A hockey puck slides on frictionless ice at constant velocity. What is the net force?
Ans. Net force is zero.
No friction acts, and no other horizontal force acts. By Newton’s first law, zero net force means constant velocity.
Position-Time and Velocity-Time Graph Questions Class 9
Q1. A velocity-time graph shows a horizontal line at 5 m/s. What does this say about net force?
Ans. Constant velocity means zero acceleration.
F = ma. If a = 0, then F = 0. Net force is zero.
Q2. A position-time graph shows a straight line with positive slope. What does this tell you about net force?
Ans. A straight line means constant velocity.
Constant velocity means zero acceleration. Zero acceleration means zero net force.
Newton’s Second Law Important Questions Class 9
Newton second law numericals class 9 are formula-based, but direction still matters. Write F = ma first, then identify which quantity is missing.
F = ma Questions Class 9 Science
Q1. State Newton’s second law in mathematical form. Define each term.
Ans. F = ma.
F is net force in newtons.
m is mass in kilograms.
a is acceleration in m/s².
The direction of force is the same as the direction of acceleration.
Q2. Why is a cricket ball harder to stop than a tennis ball at the same speed?
Ans. A cricket ball has greater mass.
At the same speed, it has greater momentum. Greater momentum needs greater force to stop in the same time.
Force, Mass and Acceleration Questions Class 9
Q1. If mass doubles but force stays the same, how does acceleration change?
Ans. a = F / m.
If mass doubles, acceleration becomes half. Mass and acceleration are inversely proportional when force is constant.
Q2. If force doubles and mass stays the same, how does acceleration change?
Ans. a = F / m.
If force doubles, acceleration doubles. Force and acceleration are directly proportional when mass is constant.
Rate of Change of Momentum Questions Class 9
Q1. A 3 kg object’s velocity changes from 4 m/s to 10 m/s in 3 seconds. Find the force.
Ans. Change in momentum = m(v - u)
Change in momentum = 3 × (10 - 4)
Change in momentum = 18 kg m/s
Force = change in momentum / time
Force = 18 / 3
Force = 6 N
Q2. Explain why rate of change of momentum equals force.
Ans. F = ma
a = (v - u) / t
So, F = m(v - u) / t
F = (mv - mu) / t
Therefore, force equals rate of change of momentum.
Newton’s Third Law Important Questions Class 9
Newton third law questions class 9 become easy when you check the object. Action and reaction are equal and opposite, but they act on different objects.
Action and Reaction Pair Questions Class 9
Q1. Identify the action-reaction pair when a ball hits a wall.
Ans. Action: the ball exerts force on the wall.
Reaction: the wall exerts an equal and opposite force on the ball.
The ball bounces back because the reaction force acts on the ball.
Q2. Why do action and reaction forces not cancel?
Ans. Action and reaction forces act on different objects.
Cancellation requires two forces on the same object. Since these forces act on different objects, they do not cancel.
Rocket Launch Questions Class 9 Science
Q1. Explain rocket motion using Newton’s third law.
Ans. Burning fuel produces hot gases.
The rocket pushes gases downward. The gases push the rocket upward. This upward reaction force helps the rocket accelerate.
Balloon Activity Questions Class 9 Science
Q1. A released balloon flies across the room. Explain this using Newton’s third law.
Ans. The balloon pushes air backward.
The air pushes the balloon forward. The balloon moves opposite to the escaping air.
Momentum Questions Class 9
Momentum questions class 9 need mass, velocity, and direction. For stopping problems, write initial momentum and final momentum separately before calculating force.
Momentum Formula Questions Class 9
Q1. Write the formula for momentum.
Ans. Momentum = mass × velocity
p = mv
Its SI unit is kg m/s.
Q2. How does mass affect momentum?
Ans. Momentum increases when mass increases, if velocity remains the same.
A heavier object moving at the same speed has greater momentum.
Q3. How does velocity affect momentum?
Ans. Momentum increases when velocity increases, if mass remains the same.
A faster object has greater momentum than a slower object of the same mass.
Momentum Change Questions Class 9
Q1. Why does a fast-moving cricket ball hurt more than a slow-moving ball?
Ans. A fast-moving cricket ball has greater momentum.
When it stops suddenly, the change in momentum is large. This produces a larger force.
Q2. Why is a foam mat used in high jump?
Ans. The foam mat increases the time taken to stop the athlete.
A longer stopping time reduces the force on the body.
Forces Acting on a System of Objects Questions Class 9
A system is a group of objects studied together. Internal forces act between objects inside the system. External forces act from outside the system.
Connected Boxes Questions Class 9 Science
Q1. Two boxes of 3 kg and 5 kg are connected by a rope on a frictionless surface. A 16 N force pulls the system. Find acceleration and tension.
Ans. Total mass = 3 + 5 = 8 kg
Acceleration = Force / total mass
Acceleration = 16 / 8
Acceleration = 2 m/s²
Tension on 3 kg box:
T = ma
T = 3 × 2
T = 6 N
Q2. Why does the rope between connected boxes experience tension?
Ans. The rope transmits force from one box to the other.
The leading box pulls the trailing box through the rope. The rope experiences pulling force from both sides.
Internal and External Forces Questions Class 9
Q1. Distinguish between internal and external forces in a system.
Ans. Internal forces act between objects within the system. They come in action-reaction pairs.
External forces act on the system from outside. Only external forces change the total momentum of a system.
Class 9 Science Chapter 6 Assertion Reason Questions
Class 9 science chapter 6 assertion reason questions test whether students understand the reason behind a force concept. Read both statements independently before choosing the answer.
Directions:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Q1. Assertion (A): A book on a table remains at rest even though gravity acts on it.
Reason (R): The normal force from the table equals and opposes the weight, making net force zero.
Ans. (a) Zero net force means no change in motion. The reason correctly explains the assertion.
Q2. Assertion (A): A rocket moves upward in space even though there is no air to push against.
Reason (R): By Newton’s third law, the rocket pushes gases downward and gases push the rocket upward.
Ans. (a) Newton’s third law does not require a medium. The reason correctly explains the assertion.
Q3. Assertion (A): A heavier object has more inertia than a lighter object.
Reason (R): Inertia depends on mass. Greater mass means greater resistance to change in motion.
Ans. (a) Inertia is directly proportional to mass. The reason correctly explains the assertion.
Q4. Assertion (A): Action and reaction forces cancel each other.
Reason (R): Action and reaction are equal in magnitude and opposite in direction.
Ans. (d) A is false, but R is true.
Action and reaction act on different objects, so they do not cancel.
Q5. Assertion (A): A high jump athlete lands on a foam mat to reduce injury.
Reason (R): The foam mat increases time of impact, reducing force on the body.
Ans. (a) Force = change in momentum / time. Greater time means smaller force.
Case Study Questions Class 9 Science Chapter 6
Case study questions in Chapter 6 apply Newton’s laws to real-world situations. Identify which law or formula each question tests before writing the answer.
Case Study 1: Chandrayaan-3 Soft Landing
India’s Chandrayaan-3 soft-landed on the Moon in 2023. The lander used thrusters to slow its descent. The thrusters fired downward, pushing hot gases toward the Moon’s surface. This created an upward force on the lander. The lander slowed gradually and touched down safely.
Q1. Which Newton’s law explains the thruster action?
Ans. Newton’s third law explains the thruster action.
The thrusters push gases downward. The gases push the lander upward.
Q2. Why did the lander need to reduce velocity before landing?
Ans. A high-velocity landing creates a large impact force.
Reducing velocity reduces momentum. A smaller momentum change at landing means a smaller impact force.
Q3. If the lander had mass 1752 kg and decelerated at 2 m/s², find the net braking force.
Ans. F = ma
F = 1752 × 2
F = 3504 N upward
Case Study 2: Snake Boat Race
In Kerala’s traditional snake boat races, over 100 rowers pull their oars together. Each rower pushes water backward. Water pushes the rower and boat forward.
Q1. Which Newton’s law explains how rowing moves the boat forward?
Ans. Newton’s third law explains rowing.
Each rower pushes water backward. Water pushes the boat forward.
Q2. A snake boat of mass 2000 kg accelerates at 0.5 m/s². Find the net forward force.
Ans. F = ma
F = 2000 × 0.5
F = 1000 N
Q3. As rowers increase stroke rate, boat speed increases. Which Newton’s law explains this?
Ans. Newton’s second law explains this.
Greater force on the same mass produces greater acceleration. Greater acceleration builds greater speed.
Important Definitions and Formulas Class 9 Science Chapter 6
| Term |
Definition / Formula |
| Force |
F = ma; push or pull that changes state of rest or motion |
| Newton (N) |
SI unit of force; 1 N = 1 kg m/s² |
| Net force |
Vector sum of all forces on an object |
| Momentum |
p = mv |
| Newton’s first law |
Object continues in its state unless net external force acts |
| Newton’s second law |
F = ma = rate of change of momentum |
| Newton’s third law |
Every action has equal and opposite reaction on a different object |
| Inertia |
Resistance to change in state; proportional to mass |
| Friction |
Force opposing relative motion between surfaces in contact |
| Normal force |
Contact force perpendicular to a surface |