Force and Laws of Motion — Why things move the way they do
A complete, exam-ready guide to Newton's three laws, inertia, momentum and conservation of momentum — explained the way a good teacher would, with solved numericals, NCERT answers and practice MCQs.
Quick Jump
Chapter Overview
Everything that moves — a rolling ball, a moving bus, even the Earth around the Sun — obeys the same set of rules. This chapter builds those rules from scratch.
Think about a football lying still on the ground. It stays still until someone kicks it. Once kicked, it keeps rolling until friction and air slow it down. This everyday observation is the seed of one of the most powerful ideas in all of science — Isaac Newton's Laws of Motion.
In Class 9 Science Chapter 9, we study force — the push or pull that changes how objects move — and the three laws that govern this change. We'll also explore momentum, the quantity that stays conserved when objects collide, push off each other, or explode apart. From the recoil of a gun to the safety design of a car's airbags, this chapter explains motion happening all around you.
1Force
The push or pull that can change an object's state of rest or motion, shape or direction.
2Laws of Motion
Three principles by Newton that explain exactly how force relates to motion.
3Momentum
The "quantity of motion" an object carries — mass in motion — and how it's conserved.
Learning Objectives
- Define force and describe its effects on the state and shape of objects.
- Differentiate between balanced and unbalanced forces with real examples.
- Explain inertia and identify its three types — of rest, of motion, and of direction.
- State and apply Newton's First Law of Motion (the Law of Inertia).
- Derive the mathematical form of Newton's Second Law, F = ma.
- State Newton's Third Law and identify action-reaction pairs in daily life.
- Define momentum, write its SI unit, and calculate it in numerical problems.
- State the Law of Conservation of Momentum and use it to solve collision/recoil problems.
What is Force?
Force is simply a push or a pull acting on an object. You cannot see a force directly, but you can always see or feel its effect.
A force can:
- Move a stationary object (kicking a ball at rest)
- Stop a moving object (catching a thrown ball)
- Change the speed of a moving object (accelerating a bike)
- Change the direction of a moving object (a batsman deflecting a ball)
- Change the shape of an object (squeezing a sponge, stretching a rubber band)
Force is a vector quantity — it has both magnitude and direction.
Balanced and Unbalanced Forces
When two or more forces act on the same object, what matters is their net effect.
| Balanced Forces | Unbalanced Forces |
|---|---|
| Resultant (net) force = zero | Resultant (net) force ≠ zero |
| Object at rest stays at rest; object in motion continues at the same velocity | Object's speed and/or direction changes — it accelerates |
| Example: A book resting on a table (gravity balanced by normal force) | Example: Pushing a shopping trolley harder than friction resists |
| Can change the shape of an object without moving it (stretching a spring held at both ends) | Always produces or changes motion |
Inertia and Its Types
Inertia is the natural tendency of an object to resist any change in its state of rest or of uniform motion. Every object has inertia, and the mass of an object is a direct measure of its inertia — heavier objects are harder to start moving or stop.
🪑 Inertia of Rest
The tendency of a body to continue in its state of rest.
🏃 Inertia of Motion
The tendency of a body to continue in its state of uniform motion.
↪️ Inertia of Direction
The tendency of a body to resist a change in its direction of motion.
Newton's Three Laws of Motion
Three simple statements that describe every motion in the universe — from a falling apple to an orbiting satellite.
Newton's First Law of Motion
This law is also called the Law of Inertia because it simply describes the property of inertia we just studied. It tells us that objects don't change their motion by themselves — something outside must push or pull them.
- A book on a table stays there forever unless someone moves it.
- A spacecraft in deep space (with no forces acting) travels forever at constant velocity.
Newton's Second Law of Motion
Derivation: Let an object of mass m have initial velocity u. A force F acts on it for time t, changing its velocity to v.
Initial momentum, p₁ = mu | Final momentum, p₂ = mv
Rate of change of momentum = (mv − mu) / t
Since F ∝ rate of change of momentum:
Here, k = 1 when force is measured in newtons, mass in kilograms, and acceleration in m/s². This gives us the most-used equation in mechanics:
Newton's Third Law of Motion
Action and reaction forces are always equal in magnitude, opposite in direction, and act on different bodies — which is why they never cancel each other out.
- Walking: you push the ground backward (action); the ground pushes you forward (reaction).
- Firing a gun: the bullet is pushed forward; the gun recoils backward.
- A swimmer pushes water backward to move forward.
Momentum
Momentum is the quantity of motion possessed by a moving object. A moving object can exert a force on another object it collides with — momentum measures how strong that push would be.
SI Unit
kg·m/s (kilogram-metre per second)
Nature
Vector quantity — direction is the same as the velocity's direction.
Law of Conservation of Momentum
This law follows directly from Newton's Third Law. When two objects A and B collide, the force A exerts on B equals and opposes the force B exerts on A. Since these forces act for the same time, the momentum lost by one equals the momentum gained by the other — so the total momentum before collision = total momentum after collision.
Real-Life Applications
🔫 Recoil of a Gun
The forward momentum of the bullet is balanced by the backward momentum (recoil) of the gun.
🚀 Rocket Propulsion
Gases ejected backward at high speed give the rocket forward momentum.
🎱 Collision of Billiard Balls
Momentum transfers from the striking ball to the struck ball.
🛶 Jumping off a Boat
The boat moves backward as a person jumps forward off it.
Solved Numericals
Step-by-step solutions in the exact format expected in SEBA / CBSE board exams.
Memory Tricks
NCERT Solutions (Intext & Exercise Questions)
Acceleration, a = (v − u)/t = (5 − 25)/4 = −5 m/s²
Change in momentum = m(v − u) = 1200 × (5 − 25) = −24000 kg·m/s
Force, F = ma = 1200 × (−5) = −6000 N (i.e., 6000 N opposing the motion)
Previous Year & Important Board Questions
| Question | Marks |
|---|---|
| State and explain Newton's three laws of motion with one example each. | 5 |
| Derive the mathematical expression for Newton's second law of motion. | 3 |
| Why does a passenger in a moving bus fall forward when the bus suddenly stops? | 2 |
| State the law of conservation of momentum. Explain with the example of a gun and bullet. | 3 |
| Define momentum. Give its SI unit and explain why it is a vector quantity. | 2 |
| A hammer of mass 500 g moving at 50 m/s strikes a nail. The nail stops the hammer in 0.01 s. Find the force exerted on the nail. | 3 |
| Why is it dangerous to jump off a moving bus? | 2 |
| Explain, with reason, why a karate player can break a pile of tiles with a single blow. | 2 |
MCQs with Answers
Tap "Show Answer" to reveal the correct option and explanation.
1. The SI unit of force is:
- A. Joule
- B. Newton
- C. Watt
- D. Pascal
2. Which law of motion gives the definition of force?
- A. First Law
- B. Second Law
- C. Third Law
- D. Law of Gravitation
3. A passenger in a moving bus falls forward when the bus suddenly stops due to:
- A. Inertia of rest
- B. Inertia of motion
- C. Inertia of direction
- D. Newton's third law
4. Momentum is defined as:
- A. Mass × Acceleration
- B. Mass × Velocity
- C. Force × Time
- D. Force / Mass
5. The recoil of a gun is an example of:
- A. Newton's First Law
- B. Newton's Second Law
- C. Newton's Third Law / Conservation of Momentum
- D. Law of Gravitation
6. Which of the following has the greatest inertia?
- A. A cricket ball
- B. A bicycle
- C. A truck
- D. A pencil
7. 1 Newton is equal to:
- A. 10⁵ dyne
- B. 10⁻⁵ dyne
- C. 10² dyne
- D. 10⁷ dyne
8. A karate player can break a pile of tiles with a single blow because:
- A. of high momentum delivered in a very short time, producing a large force
- B. of Newton's First Law only
- C. tiles have low inertia
- D. of gravity
Concept Map
Frequently Asked Questions
Mass is the quantity of matter in a body and is measured in kilograms. Inertia is the property of resisting a change in motion. Mass is the actual measure of inertia — greater the mass, greater the inertia — but they are not the same physical concept.
Because it describes the natural tendency (inertia) of objects to resist changes in their state of rest or uniform motion unless an external unbalanced force acts on them.
Momentum is a vector quantity. It has both magnitude (mass × speed) and direction, which is the same as the direction of the object's velocity.
Pulling the hands back increases the time taken for the ball's momentum to become zero. Since force = change in momentum / time, a longer time reduces the force on the fielder's hands, preventing injury.
Yes. Momentum is always conserved in a collision with no external force, but kinetic energy is only conserved in elastic collisions. In inelastic collisions (like two objects sticking together), momentum is conserved but kinetic energy is not.
The second law relates force to the mass and acceleration of a single object (F = ma). The third law describes a pair of equal and opposite forces acting on two different objects that interact with each other.
Chapter Summary & Revision Notes
- Force is a push or pull that can move, stop, or change the speed, direction, or shape of an object.
- Balanced forces produce no change in motion; unbalanced forces cause acceleration.
- Inertia is the resistance to a change in state of motion; mass measures inertia. Three types: rest, motion, direction.
- Newton's First Law: An object stays at rest or in uniform motion unless acted on by an unbalanced force.
- Newton's Second Law: F = ma; force equals rate of change of momentum.
- Newton's Third Law: Every action has an equal and opposite reaction, acting on different bodies.
- Momentum, p = mv, is a vector quantity measured in kg·m/s.
- Law of Conservation of Momentum: Total momentum before collision = total momentum after collision, when no external force acts.
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