Short, simple notes by SciNCERT Decoder by Kuldeep S. Take g = 9.8 m/s² (or 10 m/s² for quick estimates).
These How Forces Affect Motion Class 9 Notes cover the whole of CBSE Class 9 Science Chapter 6 in easy words. First come the notes with examples, then solved numericals, important questions and MCQs. For the textbook answers, open our NCERT Solutions for this chapter. Read along with your NCERT Science textbook (also on the official NCERT website), and find more chapters on our Class 9 study material page.
How Forces Affect Motion Class 9 Notes at a Glance
- Force is a push or a pull. Unit: newton (N).
- Net force = the combined effect of all forces. Balanced forces: net force = 0. Unbalanced forces: net force ≠ 0, so the object accelerates.
- Friction opposes the sliding of two surfaces in contact.
- First law: no net force means no change in velocity (inertia).
- Second law: F = m a.
- Third law: forces come in equal and opposite pairs on two different objects.
1. Force
A force can start an object moving, stop it, change its speed, change its direction, or change its shape.
- Force has size (magnitude) and direction, so it is a vector. If either changes, the effect changes.
- SI unit: newton (N). Write newton with a small n, but the symbol with a capital N.
| Force | Direction |
|---|---|
| Gravitational | Towards the Earth (downwards) |
| Friction | Against the sliding of the surfaces |
| Buoyant | Upwards on an object in a liquid |
| Normal | Away from the surface, at 90° to it |
| Magnetic | Like poles repel, unlike poles attract |
| Electrostatic | Like charges repel, unlike charges attract |
Measuring a force
A spring balance measures force. The spring stretches more when the force is bigger. It shows weight (the pull of the Earth) or any force you pull with. A 100 g mass on your palm presses down with about 1 N.
2. Balanced and Unbalanced Forces
| Forces | Net force |
|---|---|
| Same direction | Add them, in that direction |
| Opposite directions | Subtract them, in the direction of the bigger force |
| Equal and opposite | Zero |
- Balanced forces can still change shape (squeeze a sponge from both sides).
- Equal and opposite forces at different points of an object can make it turn (a tap, a handlebar). You will study this later.
- Many forces may act on an object, but its motion depends only on the net force.
3. Friction
- It is bigger on rough surfaces and smaller on smooth surfaces.
- Pushing a heavy box: a small push does not move it, because friction balances the push. The box moves only when the push is greater than friction.
- Why do moving things stop? Friction. To keep an object moving at constant speed on a rough floor, push with a force equal to friction.
- Coin-stack activity: the smoother the surface, the farther the coins slide. A spring balance pulling a block gives an approximate measure of friction.
- Thought experiment: with zero friction, a moving object would keep moving forever at constant velocity.
- Air also gives friction, but for slow objects it is so small that we ignore it.
| Friction is useful | Friction is harmful | |
|---|---|---|
| Why | We can walk, grip, brake, write and hold things | Wear and tear, and energy wasted as heat |
| What we do | Increase it: grooves on shoes, tyre treads, grips on handles | Reduce it: oil or grease (lubricants), ball bearings, polishing |
When you walk, your foot pushes the ground back, and friction pushes you forward. That is why walking on wet, polished floors or ice is difficult.
Other forces on a pushed box: weight (down) and the normal force (up). They are equal and opposite, so the box does not move up or down. They act on the same object, so they are not an action–reaction pair.
4. Galileo and Newton
- Galileo (17th century): for ages people thought a force was needed to keep things moving. Galileo argued that if all resistance (friction) were removed, a body on a flat surface would move on forever.
- Newton: named this tendency inertia and gave the three laws of motion (1687). The unit of force is named after him.
5. Newton’s First Law and Inertia
- Net force = 0 means acceleration = 0.
- “At rest” means zero velocity. “Constant velocity” means the same speed and the same direction (a straight line).
- A force is needed to change motion, not to keep it going.
| Type | Meaning | Example |
|---|---|---|
| Inertia of rest | Resists starting to move | Passengers jerk backwards when a bus starts; dust flies off a beaten carpet; leaves fall when a tree is shaken |
| Inertia of motion | Resists stopping or slowing | Passengers jerk forward when a bus brakes |
| Inertia of direction | Resists turning | Passengers lean outwards on a curve; sparks fly off a grinding wheel in a straight line |
6. Momentum and Newton’s Second Law
- More force on the same object means more acceleration.
- The same force on a heavier object means less acceleration.
- 1 newton is the force that gives a 1 kg object an acceleration of 1 m/s². (1 N = 1 kg m/s²)
- A small unit: 1 dyne = 10⁻⁵ N.
Weight and mass
Near the Earth’s surface g = 9.8 m/s². It is the same for all objects if air resistance is ignored.
| Mass | Weight |
|---|---|
| Amount of matter in an object | Gravitational force on the object |
| Unit: kg | Unit: N |
| Same everywhere | Changes with g (less on the Moon) |
The complete form of the second law
Force is the rate of change of momentum: F = (mv − mu) ÷ t = m(v − u) ÷ t = m a. So F t = mv − mu. This form works even when mass changes, such as a rocket burning fuel.
Longer time means a smaller force
For the same change in momentum, F = change in momentum ÷ time. So a longer stopping time gives a smaller force.
- A fielder pulls the hands back while catching a fast ball.
- Airbags and landing mats (high jump) give a longer stopping time.
- Bubble wrap and hay protect glass in the same way.
- A coconut hitting the ground stops in a very short time, so the force is huge and the shell breaks.
Real experiments give slightly different numbers from F = ma because of friction and measurement errors.
7. Newton’s Third Law
- Forces come in pairs (action–reaction).
- They are equal in size, opposite in direction, and act on two different objects, so they never cancel.
- The law works for contact forces and for magnetic, electrostatic and gravitational forces too.
| Situation | The pair of forces |
|---|---|
| Climbing a coconut tree | Legs push the trunk down; friction pushes you up |
| Jumping from a boat | You push the boat back; the boat pushes you forward |
| Fire hose | Hose pushes water out; water pushes the hose back, so it is hard to hold |
| Balloon | Balloon pushes air out; air pushes the balloon the other way |
Equal forces, different accelerations
Since a = F ÷ m, equal forces give a small acceleration to a heavy object and a big one to a light object.
- The Earth and a falling fruit pull each other equally. The Earth’s acceleration is far too small to notice.
- A gun (5 kg) and its bullet (0.1 kg) feel the same force, but the bullet’s acceleration is much bigger.
- A light compass needle moves towards a bar magnet, but the heavy magnet does not.
| Balanced forces | Action–reaction pair |
|---|---|
| Act on the same object | Act on different objects |
| Can cancel | Never cancel |
8. Forces on a System of Objects
Two boxes (m₁, m₂) are joined by a string, and a pull F acts on the first box.
- Internal force: the tension T in the string. It acts inside the system, so ignore it for the whole system.
- External forces: the pull F, the weight and the normal force. Only these change the motion of the system.
The two boxes move like one object of mass (m₁ + m₂). To find the tension, take the box behind: T = m₂ × a. The weight (m₁ + m₂)g is balanced by the normal force (N₁ + N₂), so the boxes do not move up or down.
Newton’s laws work for everyday motion, planets and stars. They need changes only near very massive objects, at speeds close to the speed of light, and at atomic sizes.
Formula Sheet
This sheet gives every key formula of the How Forces Affect Motion Class 9 Notes in one place. Revise it daily before the exam.
| Quantity | Formula | Unit |
|---|---|---|
| Net force (same direction) | F₁ + F₂ | N |
| Net force (opposite) | larger − smaller | N |
| Second law | F = m a | N |
| Weight | W = m g | N |
| Momentum | p = m v | kg m/s |
| Impulse | I = F t = mv − mu | N s |
| System acceleration | a = F ÷ (m₁ + m₂) | m/s² |
| Equations of motion | v = u + at; s = ut + ½at²; v² = u² + 2as |
Common Mistakes
Avoid these mistakes when you revise the How Forces Affect Motion Class 9 Notes and write your answers.
- Thinking a force is needed to keep an object moving. It is only needed to change the motion.
- Thinking action and reaction cancel. They act on different objects.
- Saying “bigger object, bigger force” in a collision. The forces are equal. Only the accelerations differ.
- Writing Newton for the unit. It is newton; the symbol is N.
- Mixing up mass (kg) and weight (N).
- Saying friction “always opposes motion”. It opposes the sliding of surfaces, and while walking it pushes you forward.
- Forgetting to convert grams to kg, and km/h to m/s (divide by 3.6).
Practise these numericals from the How Forces Affect Motion Class 9 Notes. Write the formula, put in the values with units, and then give the answer.
(400 + 350) − 200 = 550 N forward
a = 360 ÷ 120 = 3 m/s²
a = (10 − 0) ÷ 5 = 2 m/s². F = 400 × 2 = 800 N
m = F ÷ a = 36 ÷ 4 = 9 kg
On Earth: 50 × 9.8 = 490 N. On the Moon: 50 × 1.6 = 80 N. The mass stays 50 kg.
Runner: 60 × 8 = 480 kg m/s. Ball: 0.16 × 25 = 4 kg m/s
a = (0 − 20) ÷ 10 = −2 m/s². F = 5000 × (−2) = −10,000 N (opposite to the motion)
F = (0.16 × 30) ÷ 0.05 = 96 N. With 0.2 s: F = 4.8 ÷ 0.2 = 24 N (4 times smaller).
a₁ = 60 ÷ 50 = 1.2 m/s², a₂ = 60 ÷ 40 = 1.5 m/s², in opposite directions. The forces are equal, but the accelerations are not.
Net force = 12 − 4 = 8 N. a = 8 ÷ 2 = 4 m/s². s = ½ × 4 × 3² = 18 m. v = 4 × 3 = 12 m/s
F = m(v − u) ÷ t = 800 × (0 − 25) ÷ 10 = −2000 N
These are the most asked questions from the How Forces Affect Motion Class 9 Notes. Learn the answers in your own words. Textbook questions are solved in our NCERT Solutions.
A. Very short answer (1 mark)
- Define force. A push or a pull.
- What is the SI unit of force? Newton (N).
- What is the net force when two equal forces act in opposite directions? Zero.
- What is the unit of momentum? kg m/s.
- What measures the inertia of a body? Its mass.
- Which force helps a person to walk? Friction.
- Do action and reaction act on the same body? No, on two different bodies.
- What is the value of g near the Earth’s surface? 9.8 m/s².
B. Short answer (2–3 marks)
What is the difference between balanced and unbalanced forces?
Why does a ball rolling on the ground slowly stop?
Why do shoes and tyres have grooves?
State Newton’s second law and write its formula.
Why does a fielder pull the hands back while catching a fast ball?
Action and reaction are equal and opposite. Why do they not cancel each other?
C. Long answer (5 marks)
State Newton’s third law. Explain it with rowing a canoe and a rocket launch.
Canoe: The paddle pushes the water backwards. The water pushes the paddle and canoe forwards with an equal force. A harder push gives a bigger forward force.
Rocket: The engine pushes hot gas downwards. The gas pushes the rocket upwards with an equal force. When this is larger than the rocket’s weight, the net force is upward and the rocket lifts off.
Describe the coin-stack activity. What does it show about friction?
Derive F = m a from the momentum form of the second law.
D. Case-based questions (4 marks)
In a collision, an airbag inflates and the passenger’s head pushes into it instead of hitting the hard dashboard. The time of stopping increases.
- Name the law used to explain this.
- How does a longer time change the force?
- What happens if the time of contact is very short?
A stack of coins is released with the same push on a wooden table, a laminated top and polished marble. It travels the shortest distance on wood and the longest on marble.
- Which force stops the coins?
- On which surface is this force the smallest?
- How can a spring balance estimate this force?
Test yourself on the How Forces Affect Motion Class 9 Notes with these 20 MCQs. Tap Show answer after you choose.
Multiple choice questions
The SI unit of force is:
- joule
- newton
- watt
- pascal
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Two forces of 8 N and 5 N act on a body in the same direction. The net force is:
- 3 N
- 13 N
- 40 N
- 0 N
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A force of 8 N acts towards the right and a force of 5 N towards the left. The net force is:
- 3 N to the right
- 3 N to the left
- 13 N to the right
- zero
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A body is moving with constant velocity. The net force on it is:
- equal to its weight
- zero
- equal to its velocity
- increasing with time
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Friction acts:
- always along the motion
- against the sliding of the surfaces
- at 90° to the surface
- only in liquids
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The inertia of a body depends on its:
- shape
- speed
- mass
- colour
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When a bus suddenly starts, the passengers fall backwards because of:
- inertia of rest
- inertia of motion
- inertia of direction
- friction
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The momentum of a 2 kg ball moving at 5 m/s is:
- 2.5 kg m/s
- 7 kg m/s
- 10 kg m/s
- 3 kg m/s
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A force of 20 N acts on a 4 kg body. Its acceleration is:
- 80 m/s²
- 5 m/s²
- 16 m/s²
- 0.2 m/s²
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One newton is the force that gives a 1 kg mass an acceleration of:
- 1 m/s
- 1 m/s²
- 9.8 m/s²
- 10 m/s²
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The weight of a 10 kg object (g = 9.8 m/s²) is:
- 10 N
- 98 N
- 9.8 N
- 0.98 N
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A fielder pulls the hands back while catching a ball to:
- increase the stopping time and reduce the force
- decrease the stopping time
- increase the force
- increase the ball’s mass
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The action and reaction forces:
- act on the same object
- are equal and opposite and act on different objects
- are unequal
- cancel each other
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A rocket moves upwards because:
- the air pushes it
- the exhaust gas pushes it up
- the Earth attracts it
- its mass decreases only
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Two boxes m₁ and m₂ on a smooth floor are pulled by a force F. The acceleration of the system is:
- F ÷ m₁
- F ÷ (m₁ + m₂)
- F × (m₁ + m₂)
- F ÷ m₂
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Which quantity is the same for an object on the Earth and on the Moon?
- weight
- mass
- g
- gravitational force
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Assertion–Reason questions
Options: (a) A and R are true and R explains A. (b) A and R are true but R does not explain A. (c) A is true but R is false. (d) A is false but R is true.
A: A passenger lurches forward when a bus brakes. R: The body tends to continue its state of motion.
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A: Action and reaction forces do not cancel each other. R: They act on different bodies.
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A: A heavy truck and a light car in a collision exert equal forces on each other. R: Forces come in equal and opposite pairs (third law).
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A: Friction is always harmful. R: Friction wastes energy as heat and wears out parts.
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Summary
A force is a push or a pull, and only a net force changes the motion of an object. Friction opposes sliding. Newton’s three laws tell us that objects keep their motion without a net force, that F = m a, and that forces come in pairs on different objects. Revise these How Forces Affect Motion Class 9 Notes with the diagrams and examples, and then solve the NCERT questions. Next, read the notes for Work, Energy and Simple Machines.