Notes Class 9 Science Exploration Chapter 6 How Forces Affect Motion

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6.1 Introduction & The Concept of Force

In everyday life, we constantly experience forces — a cricket bat hitting a ball, pushing a door open, or a magnet attracting a pin. But what exactly is a force?

💡 What is Force?

A force (बल) is a push or a pull that can:

Make a stationary object start moving
Change the speed of a moving object (make it faster or slower)
Change the direction of a moving object
Change the shape of an object (e.g., squeezing a lemon)

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NCERT Definition

A force is a physical quantity that has both magnitude (how strong it is) and direction. It is a vector quantity.

📏 SI Unit of Force

SI Unit of Force = newton (N)

Symbol: N | 1 newton = 1 kg·m·s⁻²

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Remember This!

The unit is written as “newton” (small n), but the symbol is capital N. When a unit is named after a person, the full form is in lowercase.

🔭 Measuring Force

A spring balance is used to measure the magnitude of a force. When you pull or push the balance, it measures the force in newtons. The weight of an object is the gravitational force the Earth pulls it with — it can also be measured by a spring balance.

Contact Forces

Forces that act only when two objects touch each other — e.g., push, pull, friction, normal force.

Non-Contact Forces

Forces that act even without touching — e.g., gravitational force, magnetic force, electrostatic force.

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Fun Fact!

In everyday life, the smallest force we can feel is about 1 millinewton (like a light touch). But scientists in specialised labs can measure forces as tiny as a yoctonewton — that’s 10⁻²⁴ N!

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6.2 Balanced and Unbalanced Forces

In real life, more than one force usually acts on an object at a time. What matters is the net force — the combined effect of all forces.

⚖️ Balanced Forces (संतुलित बल)

When two or more forces act on an object and the net force is zero, the forces are called balanced forces. The object does NOT accelerate — it either stays at rest or moves at constant velocity.

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Key Idea

Balanced forces = Equal magnitude, Opposite direction → Net force = 0 → No change in state of motion.

Example: In a Tug of War (रस्साकशी), if both teams pull with equal force, the rope does not move — the forces are balanced!

Team A100 N →ROPE← 100 NTeam BBalanced Forces — Rope does not move

🚀 Unbalanced Forces (असंतुलित बल)

When the net force on an object is not zero, the forces are unbalanced. An unbalanced force causes acceleration — it changes the object’s speed or direction.

Situation	Net Force Formula	Direction
Both forces in SAME direction	F_net = F₁ + F₂	Same as both forces
Forces in OPPOSITE direction	F_net = \|F₁ − F₂\|	Towards larger force
Forces balanced (equal & opposite)	F_net = 0	No motion change

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Solved Example

Two forces 10 N and 6 N act on a block.

(a) Same direction → Net = 10+6 =16 N →

(b) 10 N right, 6 N left → Net = 10−6 =4 N →

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Common Mistake

Multiple forces can act on an object, but motion depends ONLY on the net force, not individual forces separately.

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6.3 The Force of Friction (घर्षण बल)

When you push a box on the floor, it doesn’t move immediately — why? That’s because of friction (घर्षण)! Friction is always trying to stop (or slow down) motion.

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Definition

The force of friction acts between two surfaces in contact and always acts in a direction opposite to the direction of motion or attempted motion.

🔬 What Affects Friction?

Nature of surfaces in contact — rough surfaces have MORE friction; smooth (polished) surfaces have LESS friction
Normal force (how hard surfaces press together)
NOT on the area of contact (mostly)

🔭 Types of Friction

Static Friction

Acts on an object that is NOT yet moving. It prevents motion from beginning. Maximum static friction must be overcome to start motion.

Kinetic Friction

Acts on an object that IS already moving. It slows the object down. Generally less than static friction.

🌍 Friction in Daily Life

You walk because friction between your shoes and the ground pushes you forward
A bicycle slows down when you stop pedalling — friction does this
Grooves on shoe soles and tyre treads increase friction for safety
Ice rinks and polished floors are slippery because they have very low friction

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Exam Tip

If friction between all surfaces disappeared → Objects at rest would never start, moving objects would never stop! This is related to Newton’s 1st Law.

📐 Forces on an Object Being Pushed

BOXFloor / GroundApplied →Force (F)← Friction↓ Weight (mg)↑ Normal Force (N)

Forces acting on a box being pushed on the floor

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Real Life — Indian Context

Ever noticed how bullock carts move slowly on muddy village roads but easily on paved highways? Mud increases surface roughness → more friction → slower movement! This is why smooth tar roads changed rural transport completely.

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6.4 Newton’s First Law of Motion & Inertia

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Historical Background

In ancient times, people believed a constant force was needed to keep an object moving. In the 17th century,Galileo Galileiargued through thought experiments that objects moving on a perfectly smooth surface would move forever! Isaac Newton built on this idea and gave us the First Law of Motion in 1687.

Newton’s First Law (Law of Inertia):

“An object at rest remains at rest and an object in motion continues to move with a constant velocity, unless a net force acts upon the object.”

In simple words: Objects are lazy! They don’t want to change their state on their own. A stationary cricket ball stays still until you kick it. A moving train keeps going until brakes (friction force) stop it.

🧲 What is Inertia (जड़ता)?

Inertia is the tendency of an object to resist any change in its state of rest or uniform motion. The more mass an object has, the greater its inertia.

Inertia of Rest

A stationary object resists being set in motion. Example: When a bus suddenly starts, passengers fall backward.

Inertia of Motion

A moving object resists being stopped. Example: When a bus brakes suddenly, passengers lurch forward.

📈 Graphs for Newton’s First Law

At Rest: Position-TimextHorizontal line (x constant)

Constant Vel: x-t x Straight diagonal line

Constant Vel: v-t v Horizontal line (v = constant)

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Exam Tip — Quick Trick!

Newton’s 1st Law says: If net force = 0, acceleration = 0. Either object is at rest OR moving with constant velocity. Both are correct!

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Common Mistake

Students often think “constant velocity” means the object is at rest. WRONG! Constant velocity means no change in speed AND no change in direction. It can be non-zero.

🚌 Real-Life Examples of Inertia

When a DTC bus (Delhi) suddenly brakes, standing passengers fall forward — inertia of motion
A coin placed on a card falls into a glass when the card is flicked — inertia of rest
Dust falls from a carpet when you beat it — inertia of rest of dust particles
An athlete runs before a long jump — to use inertia of motion for a bigger leap

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6.5 Newton’s Second Law of Motion (F = ma)

Newton’s First Law tells us what happens when net force is zero. What happens when a net force does act on an object? That’s what the Second Law answers!

Newton’s Second Law:

“When a net force acts on an object, the object accelerates in the direction of the net force. The magnitude of acceleration is proportional to the net force and inversely proportional to the mass.”

Mathematical Form:

F = m × a

Where: F = Net Force (N) | m = Mass (kg) | a = Acceleration (m/s²)

📊 What does F = ma mean?

Force ↑ → Acceleration ↑

Same mass, more force = more acceleration. A harder kick sends a ball farther.

Mass ↑ → Acceleration ↓

Same force, more mass = less acceleration. Pushing a truck is harder than pushing a cycle!

📏 Definition of 1 Newton

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Definition — Learn This!

One newton (1 N) is defined as the force that produces an acceleration of 1 m/s² on an object of mass 1 kg.

Therefore: 1 N = 1 kg × 1 m/s²= 1 kg·m·s⁻²

⬇️ Gravitational Force and g

Gravitational Force: F = mg

where g = 9.8 m/s² (acceleration due to gravity)

For quick calculations, use g ≈ 10 m/s²

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Important Note

The acceleration due to gravity (g) does NOT depend on the mass of the object. A 1 kg stone and a 10 kg stone both fall with the same acceleration g = 9.8 m/s² (ignoring air resistance)!

🧮 Solved Examples

Example 1: A block of mass 25 kg. Max friction = 50 N. Force applied = 55 N. Find acceleration and displacement in 2 s.

Net Force = Applied Force − Friction = 55 − 50 = 5 N

Using F = ma:

a = F/m = 5/25 = 0.2 m/s²

Displacement: s = ut + ½at² = 0 + ½ × 0.2 × 4

✅ Displacement = 0.4 m in the forward direction

Example 2: Weightlifter holds a 30 kg barbell steady. What force does she apply?

Gravitational Force on barbell = mg = 30 × 9.8 = 294 N (downward)

For steady hold, she applies equal upward force:

✅ Force applied = 294 N upward

🏏 Real-Life Applications

Cricket fielder pulling hands back while catching a ball — increases time of impact → reduces force on hands → avoids injury
Airbags in cars — increase the time of impact during collision → reduce force on passengers
Cracking a coconut — brought down at high speed, stops quickly → large force breaks the shell
Bubble wrap & hay for packing fragile items — increases time, reduces impact force

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Fun Fact!

A formula 1 racing car can accelerate from 0 to 100 km/h in under 2 seconds because it has enormous engine force and relatively low mass compared to trucks. This is Newton’s Second Law in action at its finest!

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6.6 Newton’s Third Law of Motion

When you push a wall, does the wall push back? Yes! Every force comes in a pair. Newton’s Third Law explains this beautifully.

Newton’s Third Law:

“Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object.”

In short: Every action has an equal and opposite reaction.

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CRITICAL — Don’t Confuse This!

Action and reaction forces are ALWAYS on two DIFFERENT objects. They never cancel each other because they act on different objects!

📌 Action-Reaction Pairs

Action	Reaction
You push the ground backward with your foot (walking)	Ground pushes you forward (friction)
Paddle pushes water backward (rowing a boat)	Water pushes paddle (and boat) forward
Rocket expels gas downward	Gas pushes rocket upward
You kick a ball	Ball pushes back on your foot
Earth pulls fruit downward (gravity)	Fruit pulls Earth upward (imperceptible)

🚀 Rocket Launch (Chandrayaan-3 Connection!)

A rocket’s engine burns fuel and expels gas at very high speed in the downward direction. By Newton’s Third Law, the exhaust gas exerts an equal force on the rocket in the upward direction. This upward force exceeds the rocket’s weight → net upward force → rocket lifts off!

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Indian Pride — Chandrayaan-3!

India’s Vikram lander used retro-firing (burning engines in the direction of motion) to slow down and achieve a soft landing near the Moon’s south pole in August 2023. This slowing-down used the reaction force from the exhaust gas — Newton’s Third Law in deep space!

🧮 Solved Example — Gun Recoil

Example: A 0.1 kg bullet is fired from a 5 kg gun with a force of 2 N. Find accelerations.

By Newton’s 3rd Law: Recoil force on gun = 2 N (equal and opposite)

Acceleration of bullet: a = F/m = 2/0.1 = 20 m/s²

Acceleration of gun: a = F/m = 2/5 = 0.4 m/s²

✅ Bullet accelerates much more than gun because it has much smaller mass!

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Key Insight

Even though action = reaction in magnitude, the accelerations produced are different because the masses of the two objects are different! Earth pulls fruit with same force as fruit pulls Earth, but Earth’s mass is so huge that its acceleration is negligible.

🌐 Newton’s 3rd Law is Universal

Newton’s Third Law applies to ALL forces — contact forces (friction, normal) AND non-contact forces (gravity, magnetic, electrostatic). Two magnets repelling each other, two charged balloons pushing apart — all obey Newton’s Third Law.

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6.7 Forces Acting on a System of Objects

Newton’s laws don’t just apply to single objects. We can treat a group of connected objects as a single system and apply Newton’s laws to the whole system!

🧩 Internal vs External Forces

Internal Forces

Forces that act between objects within the system. Example: Tension in the string connecting two boxes. These CANCEL OUT within the system — we ignore them.

External Forces

Forces that act on the system from outside. Example: The applied force F pulling the boxes. These determine the system’s acceleration.

For a system of two connected masses m₁ and m₂ with external force F:

a = F / (m₁ + m₂)

Example: Two boxes, m₁ = 3 kg and m₂ = 2 kg, connected by a string. External force F = 10 N.

Total mass = 3 + 2 = 5 kg

Acceleration: a = F/M = 10/5 = 2 m/s²

✅ Both boxes accelerate at 2 m/s² together!

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Exam Tip — Systems Shortcut!

When objects are connected and move together, treat them as ONE object with total mass = sum of all masses. Apply F = ma directly with external force only!

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Everyday Example

When you walk, your arms and legs move in complex patterns. But scientists can study your overall motion by treating your entire body as a single object. This simplification is the power of the systems approach!

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Quick Revision Summary

Force (बल)

A push or pull. Vector quantity. SI unit = newton (N). Measured using spring balance.

Balanced Forces

Equal & opposite forces. Net force = 0. No change in motion. Object at rest stays at rest.

Friction (घर्षण)

Opposes motion. Depends on surface nature. Helps in walking, causes wear. Can be reduced by smooth surfaces or lubricants.

Newton’s 1st Law

Objects resist change. No net force → no acceleration. Inertia ∝ mass. The “Law of Inertia.”

Newton’s 2nd Law

F = ma. More force → more acceleration. More mass → less acceleration. Defines 1 newton.

Newton’s 3rd Law

Every action has equal & opposite reaction. Forces act on DIFFERENT objects. Explains rockets, walking, rowing.

Gravitational Force

F = mg, g = 9.8 m/s². Same for all masses. Weight ≠ Mass.

System of Objects

Treat connected objects as one. a = F_ext / (m₁+m₂). Internal forces cancel out.

Key Formulas

F = ma | F = mg | a = F/m | Net F (same dir.) = F₁+F₂ | Net F (opposite dir.) = |F₁−F₂|

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Important Exam Questions with Answers

Q1. State Newton’s First Law of Motion. Give one real-life example. (CBSE Board / 3 Marks)

Newton’s First Law states: “An object at rest remains at rest and an object in motion continues to move with a constant velocity, unless a net force acts upon it.” This is also called the Law of Inertia. Example: When a bus suddenly brakes, passengers lurch forward because of the inertia of their bodies — they tend to continue in their state of motion.

Q2. A force of 5 N acts on a body of mass 2 kg. What is the acceleration produced? (2 Marks)

Using Newton’s Second Law: F = ma → a = F/m = 5/2 = 2.5 m/s². The acceleration produced is 2.5 m/s² in the direction of the applied force.

Q3. Explain why a fielder in cricket pulls their hands backwards while catching a fast ball. (CBSE / 3 Marks)

When a fielder pulls their hands back while catching, the time of contact between the ball and hands increases. By Newton’s Second Law, F = m(v−u)/t. As time (t) increases, the force (F) required to reduce the ball’s velocity to zero decreases. This smaller force causes less pain and reduces the risk of injury to the fielder.

Q4. State Newton’s Third Law of Motion and explain why the Earth does not seem to move towards a falling apple. (CBSE / 3 Marks)

Newton’s Third Law: “Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.” The apple falls towards Earth because Earth pulls it with gravitational force mg. By Newton’s 3rd Law, the apple also pulls Earth with the same force mg. However, acceleration of Earth = Force/Mass of Earth = mg/M_Earth. Since M_Earth is extremely large (~6×10²⁴ kg), the acceleration of Earth is negligibly small — too tiny to notice.

Q5. Two forces of 8 N and 5 N act on an object. What is the net force when they act (i) in the same direction, and (ii) in opposite directions? (2 Marks)

(i) Same direction: Net Force = 8 + 5 = 13 N in the direction of both forces.

(ii) Opposite directions: Net Force = 8 − 5 = 3 N in the direction of the 8 N force.