Force and Laws of Motion Class 9 Notes: Understanding force and motion is one of the most important parts of Class 9 Physics. The chapter Force and Laws of Motion explains how objects move, stop, speed up, or change their direction when a force acts on them. These concepts form the foundation of mechanics and help students understand many real-life situations, such as kicking a football, riding a bicycle, or applying brakes to a vehicle.
These Force and Laws of Motion Class 9 Science Notes are designed to help students learn the chapter in a simple and structured way. The notes cover important topics such as force, balanced and unbalanced forces, inertia, momentum, Newton’s First Law, Newton’s Second Law, and Newton’s Third Law of Motion. Along with theory, students will also find force and laws of motion class 9 notes questions answers, solved examples, and exam-focused explanations for better preparation.
To make learning easier, these notes include force and laws of motion class 9 notes numericals with step-by-step solutions based on the latest syllabus. Students looking for force and laws of motion class 9 notes PDF can use these notes for quick revision before tests and exams. In addition, important concepts are explained in line with force and laws of motion class 9 notes NCERT solutions, helping learners build a strong conceptual understanding.
By studying this chapter carefully, students can improve their problem-solving skills and gain a better understanding of motion, acceleration, net force, mass, action-reaction forces, and other key physics principles. Some topics may look difficult at first, but with regular practice and clear concepts, they become much easier to understand and apply in numericals and daily life examples.
What is Force?
Imagine a ball resting on a table. You can move it by pushing or pulling. If you push it harder in the direction it is already moving, its speed increases. Push it in the opposite direction, the speed decreases. Push it sideways, and the direction of motion changes. Squeeze a soft rubber ball between your palms, and even its shape changes.
In every one of these situations, you have applied a force.
Definition of Force
Force is an external push or pull that can change or attempt to change the state of rest, state of uniform motion, direction of motion, or shape of a body.
In a more scientific form:
"Force is the cause that produces acceleration in the body on which it acts."
The SI unit of force is the newton (N), named after Sir Isaac Newton.
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Effects of Force
A single force (or a set of forces) acting on a body can do one or more of the following:
- Change the speed of the body (start a stationary body, speed it up, or slow it down).
- Change the direction of motion of a moving body.
- Change the shape or size of the body (e.g., compressing a spring, denting a car).
- Rotate the body about a fixed axis (turning effect, e.g., opening a door).
Balanced vs Unbalanced Forces
| Type | Net Force | Effect on Body |
|---|---|---|
| Balanced Forces | Zero | No change in state of motion (body stays at rest or moves uniformly) |
| Unbalanced Forces | Non-zero | Produces acceleration changes speed, direction, or both |
Galileo's Experiments – The Foundation of the Laws of Motion
Long before Newton, the Italian scientist Galileo Galilei laid the groundwork through two ingenious thought-experiments with inclined planes and smooth surfaces.
Experiment 1 – The Inclined Plane
- A ball rolled down an inclined plane → speed increases.
- A ball rolled up an inclined plane → speed decreases.
- A ball rolled on a perfectly horizontal, frictionless plane → speed remains constant.
Experiment 2 – The Double Incline
When a ball is released on the inner surface of a smooth hemispherical bowl, it climbs up the opposite side to the same height before momentarily coming to rest. If the opposite side is made gentler, the ball travels a longer distance to reach the same height. If the opposite side is made perfectly horizontal, the ball will theoretically never stop it will keep moving with uniform velocity.
Galileo's Conclusions
- A body at rest, with no unbalanced force acting on it, remains at rest.
- A body in motion, with no unbalanced force acting on it, continues to move with a constant speed in a straight line.
- An unbalanced force is needed to produce acceleration.
These conclusions were later formalised by Newton into the First Law of Motion.
Newton's First Law of Motion (Law of Inertia)
"A body at rest will remain at rest, and a body in uniform motion will continue in uniform motion in a straight line, unless acted upon by an unbalanced external force."
An equivalent statement: A body remains unaccelerated if and only if the net (resultant) force on it is zero. In this condition, the body is said to be in equilibrium.
This is also known as the Law of Inertia, because it captures a body's natural reluctance its inertia to alter its state of motion.
Inertia – The Property Behind the First Law
Inertia is the inherent property of a body by virtue of which it resists any change in its state of rest or uniform motion in a straight line.
Inertia Depends on Mass
It is harder to push a loaded truck than an empty trolley. It is harder to stop a moving bus than a moving bicycle. Therefore:
Mass is a quantitative measure of inertia. Greater the mass, greater the inertia.
Types of Inertia
(a) Inertia of Rest
The tendency of a body to remain at rest unless compelled by an external force.
Everyday Examples:
- A passenger sitting in a stationary bus jerks backwards when the bus suddenly starts. The lower body moves with the bus, but the upper body tends to stay at rest.
- Dust is removed from a carpet by beating it with a stick. The carpet moves, but the dust particles remain at rest and separate.
- Fruits fall down when a tree branch is shaken vigorously.
- A coin placed on a stiff card over a glass falls into the glass when the card is flicked away rapidly.
(b) Inertia of Motion
The tendency of a body to continue in motion in a straight line.
Everyday Examples:
- A person stepping off a moving bus falls forward the feet stop with the ground, but the upper body continues moving.
- Athletes take a running start before a long jump to utilise the inertia of motion.
- A passenger jolts forward when a moving vehicle suddenly brakes.
- Mud or snow flies off the shoes when struck against a wall.
(c) Inertia of Direction
The tendency of a body to resist any change in its direction of motion.
Everyday Examples:
- Passengers in a car taking a sharp turn lean outward their bodies try to continue in a straight line.
- When a stone tied to a string is whirled in a horizontal circle and the string snaps, the stone flies off tangentially, not radially.
- Water droplets fly off tangentially from the rim of a rotating bicycle tyre this is why mudguards work.
- Sparks fly off tangentially when a knife is sharpened on a grinding wheel.
Momentum – Combining Mass and Velocity
Momentum (p) of a body is the product of its mass (m) and its velocity (v).
p= m×v
Momentum is a vector quantity its direction is the same as that of velocity.
- A heavy truck moving slowly and a fast bullet can both have large momentum.
Units of Momentum
| System | Unit |
|---|---|
| SI (MKS) | kg·m/s (or N·s) |
| CGS | g·cm/s (or dyne·s) |
Why Momentum Matters
A small bullet has very little mass, but its enormous velocity gives it a momentum capable of doing significant damage. A loaded truck moving at low speed has comparable momentum and can flatten objects on collision. Momentum, therefore, is the "quantity of motion" contained in a body.
Newton's Second Law of Motion
Statement
The rate of change of momentum of a body is directly proportional to the applied unbalanced force, and the change takes place in the direction of the applied force.
Mathematical Derivation
Let a body of mass m be moving with initial velocity u. A force F is applied for time t, after which the velocity becomes v.
- Initial momentum, p₁ = mu
- Final momentum, p₂ = mv
- Change in momentum = mv − mu = m(v − u)
By Newton's Second Law:
F ∝ m (v−u)/t
Since (v − u) / t = acceleration (a):
F ∝ m⋅ a ⇒ F = k⋅ m⋅a
By choosing units such that k = 1 (this is exactly how the newton is defined):
F = m⋅a
Definition of 1 Newton
One newton is the force that produces an acceleration of 1 m/s² in a body of mass 1 kg.
1 N = 1 kg⋅m/s2
Units of Force
| Unit | System | Value |
|---|---|---|
| Newton (N) | SI | 1 kg·m/s² |
| Dyne | CGS | 1 g·cm/s² |
| Kilogram-force (kgf) | Gravitational | 1 kgf = 9.8 N |
| Gram-force (gf) | Gravitational | 1 gf = 981 dyne |
Conversion: 1 N = 10⁵ dyne
How the First Law Follows from the Second Law
From F = ma, if F = 0, then a = 0 (since m ≠ 0). This means (v − u)/t = 0, hence v = u. The velocity does not change confirming the First Law: without an unbalanced force, a body's state of motion remains unchanged.
Impulse – Force Acting for a Short Time
Impulse is the product of the force applied and the time interval for which it acts.
Impulse (J) = F × t
Impulse–Momentum Theorem
From Newton's Second Law:
F = p2 −p1 /t ⇒ F⋅t = p2 − p1
Impulse = Change in momentum
SI unit: newton-second (N·s)
CGS unit: dyne-second (dyne·s)
Real-Life Applications of Impulse
The principle here is simple: to produce a given change in momentum, a small force applied over a long time has the same impulse as a large force applied for a short time. We exploit this everywhere:
| Application | How it Works |
|---|---|
| Cricketer pulling hands back while catching | Increases time of catch → reduces force on hands → prevents injury |
| Jumping onto a heap of sand | Sand increases stopping time → reduces impact force on legs |
| Getting off a moving bus carefully | Running along reduces rate of momentum change → less force on body |
| Springs in vehicle suspension | Increase duration of jolt impulses → reduce force felt by passengers |
| Soft packaging for glassware | Prolongs collision time → reduces force of impact → glass survives |
| Athletes slowing down gradually after a sprint | Avoids sudden momentum change → prevents muscular injury |
| Airbags in cars | Increase the time of stoppage during a crash → drastically reduce force on occupants |
Newton's Third Law of Motion
Statement
To every action, there is an equal and opposite reaction. Moreover, action and reaction act on two different bodies.
Features
- Action and reaction are equal in magnitude but opposite in direction.
- They act simultaneously not one after the other.
- They act on two different bodies, which is why they never cancel out.
- They form an inseparable pair no isolated force exists in nature.
Demonstration with Spring Balances
When two spring balances A and B are hooked together and one end is pulled, both balances show the same reading. The force exerted by A on B (action) equals the force exerted by B on A (reaction), but in the opposite direction.
Classic Examples
| Action | Reaction |
|---|---|
| Swimmer pushes water backward | Water pushes swimmer forward |
| Walking man pushes ground backward | Ground pushes man forward |
| Rocket exhausts hot gases downward | Gases push the rocket upward |
| Gun pushes the bullet forward | Bullet pushes the gun backward (recoil) |
| Man pushes a boat backward to step off | Boat pushes man forward onto the bank |
| A bird pushes air downward with wings | Air pushes the bird upward |
Important: Not Every Equal-and-Opposite Pair is Action–Reaction
Consider a book resting on a table. The table pushes the book up (normal force, N), and the Earth pulls the book down (weight, W). Although N = W and they are opposite, they are NOT an action–reaction pair because:
- They both act on the same body (the book).
- The reaction to N is the force of the book pressing down on the table.
- The reaction to W is the gravitational pull of the book on the Earth.
Rule of thumb: An action–reaction pair always involves two different bodies interacting with each other.
Law of Conservation of Linear Momentum
Statement
In the absence of an external unbalanced force, the total linear momentum of a system of bodies remains constant (conserved).
Derivation Using Newton's Third Law
Consider two bodies A (mass m₁, initial velocity u₁) and B (mass m₂, initial velocity u₂), where u₁ > u₂. They collide and, after time t, move with velocities v₁ and v₂ respectively.
Force on B by A (action):
F1 = m2v2 − m2u2/t
Force on A by B (reaction):
F2 = m1v1−m1u1/t
By Newton's Third Law, F₁ = −F₂:
m2v2−m2u2/t = −m1v1− m1u1/t
Simplifying:
m1u1 + m2u2 = m1v1 + m2v2
Total initial momentum = Total final momentum
Famous Applications
(i) Recoil of a Gun
Before firing, the gun-and-bullet system is at rest, so the total momentum is zero. After firing, by conservation:
0 = M⋅V + m⋅v ⇒ V = −m⋅v/M
The negative sign indicates the gun recoils backward. Because the gun's mass M is much greater than the bullet's mass m, the recoil velocity V is small but noticeable.
(ii) Rocket and Jet Propulsion
A rocket ejects burnt gases downward at very high velocity. The gases carry momentum in one direction; by conservation, the rocket gains an equal momentum in the opposite (upward) direction. This is the working principle of every rocket and jet aircraft.
(iii) Collisions and Explosions
In any collision (elastic or inelastic) and in any explosion where only internal forces act, the total linear momentum of the system before and after is exactly the same a cornerstone principle in atomic physics, astrophysics, and engineering.
Force and Laws of Motion Class 9 Science Solved Examples
Q. A force F₁ acts on a body of mass 2 kg producing an acceleration of 2.5 m/s². Another force F₂ acts on a body of mass 5 kg producing an acceleration of 2 m/s². Find the ratio F₂ : F₁.
Solution:
F1 = m1a1 = 2 x 2.5 = 5N
F2 = m2a2 = 4 x 2 = 10 N
F2/F1 = 10/5 = 2
∴ F₂ : F₁ = 2 : 1
Q. A force of 20 N acting on mass m₁ produces an acceleration of 4 m/s². The same force on mass m₂ produces 0.5 m/s². What acceleration will this force produce if both masses are tied together?
Solution:
m1 = F/a1 = 20/4 = 5k
m2 = F/a2 = 20/0.5 = 40 kg
Total mass = 45 kg
a = F/m1 + m2 = 20/45 ≈ 0.44 m/s2
∴ Acceleration = 0.44 m/s²
Q. A field gun of mass 1.5 t fires a shell of mass 15 kg with a velocity of 150 m/s. Calculate the recoil velocity of the gun.
Solution:
Mass of gun, M = 1500 kg ; Mass of shell, m = 15 kg ; Velocity of shell, v = 150 m/s
By conservation of momentum: 0 = MV + mv
V = - mu/M = - 15 x 150 / 1500 = −1.5 m/s
∴ Recoil velocity = 1.5 m/s (in the opposite direction to the shell).
Q. A hunter of mass 45 kg standing on frictionless ice fires a bullet of 100 g with a velocity of 500 m/s from a gun of mass 5 kg. Find the recoil velocity of the hunter (with the gun).
Solution:
Initial momentum = 0 (everything at rest).
Let V be the common recoil velocity of the hunter + gun system.
By conservation of momentum:
0 = (45 + 5) x V + 0.1 x 500
0 = 50V + 50 ⇒ V = -1 m/s
∴ The hunter (with the gun) recoils at 1 m/s in the direction opposite to the bullet.
Why This Chapter Matters
The three laws of motion are not just textbook rules they describe how every car brakes, every rocket launches, every ball bounces, and every athlete sprints. Understanding force, inertia, momentum, and impulse equips Class 9 students with the conceptual foundation required for:
- Class 10 Physics (work, energy, and gravitation)
- Class 11 Physics (laws of motion in two dimensions, friction, circular motion)
- Competitive exams like NTSE, NSO, JEE Foundation, and Olympiads
Master this chapter thoroughly, practise the numerical problems, and revisit the real-life applications because in physics, intuition built from everyday examples is as important as the equations.
Practice Problems for Self-Assessment
- A car of mass 1500 kg accelerates from rest to 20 m/s in 10 seconds. Calculate the force exerted by the engine.
- A ball of mass 0.5 kg moving at 10 m/s is brought to rest by a fielder in 0.2 s. Find the average force exerted by the fielder.
- Two trolleys of masses 4 kg and 2 kg moving towards each other at 3 m/s and 4 m/s respectively, stick together after collision. Find their common velocity.
- State Newton's three laws of motion in your own words and give one daily-life example for each.
- Explain why a karate expert can break a stack of tiles with a single blow. (Hint: Impulse and time of impact.)
About the Author & Review
Shiksha Nation Physics Faculty a panel of experienced CBSE and ICSE physics educators.
- Reviewed by: Senior subject matter expert in secondary-school physics, with over a decade of teaching experience aligned to NCERT and CBSE curricula.
- Aligned with: NCERT Class 9 Science (Chapter 9), CBSE Syllabus 2025–26.
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