Chapter 6: How Forces Affect Motion
Understand Newton’s three laws of motion, friction, and how forces change the way objects move in the world around us.
Balanced & Unbalanced Forces
Friction (घर्षण)
Newton’s 1st Law
Newton’s 2nd Law
Newton’s 3rd Law
F = ma
Inertia (जड़ता)
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)
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
Symbol: N | 1 newton = 1 kg·m·s⁻²
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.
Forces that act only when two objects touch each other — e.g., push, pull, friction, normal force.
Forces that act even without touching — e.g., gravitational force, magnetic force, electrostatic force.
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!
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.
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!
🚀 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 |
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 →
(c) 6 N right, 10 N left → Net = 10−6 = 4 N ←
Multiple forces can act on an object, but motion depends ONLY on the net force, not individual forces separately.
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.
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
Acts on an object that is NOT yet moving. It prevents motion from beginning. Maximum static friction must be overcome to start motion.
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
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
Forces acting on a box being pushed on the floor
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.
6.4 Newton’s First Law of Motion & Inertia
In ancient times, people believed a constant force was needed to keep an object moving. In the 17th century, Galileo Galilei argued 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.
“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.
A stationary object resists being set in motion. Example: When a bus suddenly starts, passengers fall backward.
A moving object resists being stopped. Example: When a bus brakes suddenly, passengers lurch forward.
📈 Graphs for Newton’s First Law
Newton’s 1st Law says: If net force = 0, acceleration = 0. Either object is at rest OR moving with constant velocity. Both are correct!
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
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!
“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.”
F = m × a
Where: F = Net Force (N) | m = Mass (kg) | a = Acceleration (m/s²)
📊 What does F = ma mean?
Same mass, more force = more acceleration. A harder kick sends a ball farther.
Same force, more mass = less acceleration. Pushing a truck is harder than pushing a cycle!
📏 Definition of 1 Newton
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
where g = 9.8 m/s² (acceleration due to gravity)
For quick calculations, use g ≈ 10 m/s²
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
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
Gravitational Force on barbell = mg = 30 × 9.8 = 294 N (downward)
For steady hold, she applies equal upward force:
🏏 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
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!
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.
“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.
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!
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
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
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.
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
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.
Forces that act on the system from outside. Example: The applied force F pulling the boxes. These determine the system’s acceleration.
a = F / (m₁ + m₂)
Total mass = 3 + 2 = 5 kg
Acceleration: a = F/M = 10/5 = 2 m/s²
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!
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!
Quick Revision Summary
A push or pull. Vector quantity. SI unit = newton (N). Measured using spring balance.
Equal & opposite forces. Net force = 0. No change in motion. Object at rest stays at rest.
Opposes motion. Depends on surface nature. Helps in walking, causes wear. Can be reduced by smooth surfaces or lubricants.
Objects resist change. No net force → no acceleration. Inertia ∝ mass. The “Law of Inertia.”
F = ma. More force → more acceleration. More mass → less acceleration. Defines 1 newton.
Every action has equal & opposite reaction. Forces act on DIFFERENT objects. Explains rockets, walking, rowing.
F = mg, g = 9.8 m/s². Same for all masses. Weight ≠ Mass.
Treat connected objects as one. a = F_ext / (m₁+m₂). Internal forces cancel out.
F = ma | F = mg | a = F/m | Net F (same dir.) = F₁+F₂ | Net F (opposite dir.) = |F₁−F₂|
Important Exam Questions with Answers
(ii) Opposite directions: Net Force = 8 − 5 = 3 N in the direction of the 8 N force.
Chapter 6 is a favourite for CBSE board exams! Always remember: (1) State the law first, (2) Explain with formula, (3) Give a real-life example. Questions involving F = ma calculations and Newton’s 3rd Law conceptuals appear every year. Practice all numerical problems from the “Revise, Reflect, Refine” section!
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