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Andhra Pradesh (BIEAP)Class 9 Physical Science★ AP Board (BSEAP) Class 9 Physical Science · ~12 marks · Board: AP Board

Laws of Motion

NEWTON'S THREE LAWS OF MOTION — Inertia, F=ma, Action-Reaction. Galileo's thought experiment on inertia. INERTIA and mass. MOMENTUM and its conservation with worked examples. Mass vs weight. Common misconceptions. AP Board Class 9 Physical Science.

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By the end of this chapter you'll be able to…

1State Newton's three laws of motion and give two everyday examples for each
2Explain inertia — its dependence on mass and its role in everyday situations
3Apply F = ma to find force, mass, or acceleration in numerical problems
4Distinguish mass from weight — units, constancy, measurement instruments
5Apply conservation of momentum to recoil and collision problems
6Identify action-reaction pairs in everyday situations

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Why this chapter matters

Laws of Motion is equal in marks to Motion and combines conceptual questions with numericals. Newton's First Law (inertia examples) generates 2-3 mark short answers. F = ma (Second Law) drives 4-5 mark numerical problems. Conservation of momentum (recoil, collisions) is a standard 4-5 mark numerical. The mass vs weight distinction is a reliable MCQ. Action-reaction pairs with examples are tested in both MCQ and short-answer formats. This chapter's content forms the foundation for Class 10 topics (gravitation, flotation).

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AP Board Class 9 Physical Science: Motion (equations of motion)

Laws of Motion — Class 9 Physical Science

"Newton's three laws govern EVERYTHING that moves — from falling apples to orbiting planets. They are the most powerful laws in all of physics."

1. Force — A Push or Pull

Force CHANGES or tends to change: the STATE of rest or motion of an object. The SPEED of a moving object. The DIRECTION of motion. The SHAPE of an object.

Force is a VECTOR — it has magnitude AND direction. Unit: NEWTON (N). 1 N = 1 kg·m/s². '1 Newton is approximately the WEIGHT of a small APPLE (100g). Sir Isaac Newton's name is immortalised in this unit.'

Balanced vs Unbalanced Forces

Balanced: Net force = 0. NO change in motion. Object at rest STAYS at rest. Object in motion CONTINUES with same velocity. Unbalanced: Net force ≠ 0. Causes ACCELERATION — change in speed or direction.

2. Galileo's Insight — The Thought Experiment

Before Galileo, Aristotle's view dominated: 'Objects need a FORCE to KEEP moving.' Galileo challenged this with a THOUGHT EXPERIMENT: Imagine a ball rolling on a perfectly smooth, horizontal surface with NO friction. The ball would NEVER stop. 'Galileo concluded: No force is needed to KEEP an object in motion. Force is needed only to CHANGE its motion. This was REVOLUTIONARY — it overturned 2,000 years of Aristotelian physics.' Newton built his First Law on Galileo's insight.

3. Newton's First Law — The Law of Inertia

"An object at rest stays at rest. An object in motion stays in uniform motion — UNLESS acted upon by an external UNBALANCED force."

What Is Inertia?

The TENDENCY of an object to RESIST change in its state of rest or motion. Inertia depends on MASS — greater mass = greater inertia. 'It is HARDER to push a loaded truck than an empty one. Harder to stop a moving train than a moving bicycle. This is INERTIA.'

Everyday Examples of Inertia

Bus stops suddenly: passengers LURCH FORWARD (upper body was in motion — wants to STAY in motion).
Bus starts suddenly: passengers FALL BACKWARD (body at rest — wants to STAY at rest).
Seatbelts: Prevent body from continuing forward when car stops.
Beating a carpet: Carpet moves, dust particles stay at rest and fall out.
Coin on a card: Flick card → coin drops into glass (coin stays at rest due to inertia).
Fruits fall from a tree when shaken: Branches move, fruits remain at rest and detach.
Athlete running a race: Continues past the finish line — cannot stop instantly.

4. Newton's Second Law — F = ma

"The force acting on an object is EQUAL to the mass multiplied by the acceleration."

F = ma. Force (N) = mass (kg) × acceleration (m/s²).

What This Law Tells Us

More force → more acceleration (same mass). Double force = double acceleration.
More mass → less acceleration (same force). Double mass = half acceleration.
Force and acceleration are in the SAME DIRECTION.

Worked Example 1

'A force of 20 N acts on a body of mass 5 kg at rest. Find (i) acceleration, (ii) velocity after 10 s, (iii) distance covered in 10 s.' (i) a = F/m = 20/5 = 4 m/s². (ii) v = u + at = 0 + 4×10 = 40 m/s. (iii) s = ut + ½at² = 0 + ½×4×100 = 200 m.

Worked Example 2 — Impulse and Cricket

'A cricket ball of mass 150 g moving at 20 m/s is hit back at 30 m/s. Bat contact = 0.01 s. Find force.' Mass = 0.15 kg. u = 20 m/s, v = −30 m/s. Δv = −30−20 = −50 m/s. a = −50/0.01 = −5000 m/s². F = ma = 0.15×(−5000) = −750 N. 'Force magnitude = 750 N — equivalent to the weight of a 75 kg person!'

Worked Example 3 — Car Braking

'A 1000 kg car moving at 20 m/s stops in 5 s. Find the braking force.' a = (v−u)/t = (0−20)/5 = −4 m/s². F = ma = 1000×(−4) = −4000 N. 'The negative sign means force is OPPOSITE to motion. Braking force = 4000 N.'

5. Mass vs Weight

	Mass	Weight
Definition	Amount of MATTER in a body	Force of GRAVITY on the mass
Unit	kg	Newton (N)
Measured by	Beam balance	Spring balance
Constancy	CONSTANT everywhere	Varies — less on Moon (1/6 of Earth)
Formula	—	W = mg (g = 9.8 m/s² on Earth)

'Mass is SCALAR. Weight is a FORCE (vector). Your mass is 50 kg everywhere. Your weight is 490 N on Earth — but only ~82 N on the Moon.'

6. Newton's Third Law — Action and Reaction

"For every action, there is an EQUAL and OPPOSITE reaction."

The Critical Point

Action and reaction act on DIFFERENT BODIES. They do NOT cancel. 'You push DOWN on the ground. The ground pushes YOU up. Forces are EQUAL. But they act on DIFFERENT bodies — so you JUMP.'

Examples

Walking: Push ground backward → ground pushes you forward.
Swimming: Push water backward → water pushes you forward.
Rocket: Gases ejected downward → rocket propelled upward.
Recoil of a gun: Bullet forward, gun backward.

7. Momentum

Momentum (p) = mv. Unit: kg·m/s. VECTOR. Conservation: In absence of external force, total momentum BEFORE = total momentum AFTER.

Worked Example — Recoil

'A 4 kg gun fires a 50 g bullet at 400 m/s. Find recoil velocity.' Before: total momentum = 0. After: 4v + 0.05×400 = 0 → v = −5 m/s. Gun recoils at 5 m/s opposite to bullet.

8. Common Mistakes

'Action and reaction cancel each other' — They act on DIFFERENT bodies. Cannot cancel.
'If no force acts, objects STOP' — Objects CONTINUE in uniform motion (First Law). They stop only due to FRICTION.
'Heavier objects fall FASTER' — In vacuum, all fall at g = 9.8 m/s². Air resistance affects lighter objects more.
'Mass and weight are the same' — Mass = matter (kg, constant). Weight = force (N, varies with gravity).

9. AP SSC Exam Focus

Topic	Marks	Type
First Law / Inertia	2-3	MCQ or Short
F = ma numericals	4-5	Numerical
Conservation of momentum	4-5	Numerical
Action-Reaction pairs	2-3	MCQ or Short
Mass vs Weight	2-3	MCQ

Key formulas & results

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Newton's Laws and Momentum

FORCE: Vector. Unit: Newton (N). 1 N = 1 kg·m/s². Balanced forces → no change in motion. Unbalanced forces → acceleration. NEWTON'S FIRST LAW (LAW OF INERTIA): Object at rest stays at rest; object in motion stays in uniform motion UNLESS acted on by unbalanced external force. INERTIA = resistance to change in state. Inertia ∝ mass. EXAMPLES: Bus stops → passengers lurch forward (body stays in motion). Bus starts → passengers fall back (body stays at rest). Coin on card flicked → coin falls into glass (coin stays at rest). NEWTON'S SECOND LAW: F = ma. Force (N) = mass (kg) × acceleration (m/s²). Also: F = Δp/t = m(v−u)/t (rate of change of momentum). IMPULSE = F × t = Δp (change in momentum). MASS vs WEIGHT: Mass = matter in body, kg, constant, measured by beam balance. Weight W = mg (N), varies with g, measured by spring balance. g_Earth = 9.8 m/s². g_Moon = g/6. On Moon: mass same, weight = 1/6 of Earth weight. NEWTON'S THIRD LAW: For every action, there is an EQUAL and OPPOSITE reaction. Forces act on DIFFERENT bodies (so they cannot cancel). EXAMPLES: Walking (push ground back → ground pushes forward). Swimming. Rocket (gases down → rocket up). Gun recoil (bullet forward → gun back). MOMENTUM: p = mv (kg·m/s, vector). CONSERVATION OF MOMENTUM: Total momentum before = Total momentum after (no external force). m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. RECOIL: gun+bullet system → initial p=0. After: M×V + m×v = 0 → V = −mv/M. WORKED EXAMPLE: Gun (4 kg) fires bullet (50g = 0.05kg) at 400 m/s. Recoil: 4×V + 0.05×400 = 0 → V = −5 m/s.

AP EXAM KEY TRAPS: (1) Action and reaction are EQUAL and OPPOSITE but act on DIFFERENT bodies — they do NOT cancel (a common misconception). (2) Mass is in kg, weight is in Newton. Never say 'mass = 50 N' or 'weight = 50 kg.' (3) When using F = ma, ensure F is NET force (resultant of all forces). (4) Galileo's contribution: 'force is NOT needed to maintain motion — only to CHANGE it.' This contradicts Aristotle and leads to Newton's First Law. (5) Conservation of momentum applies in ABSENCE of external forces. Friction is an external force — if friction is mentioned, momentum is NOT conserved.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Saying action and reaction forces cancel each other out, so objects never accelerate

✓ Action and reaction forces act on DIFFERENT BODIES — they CANNOT cancel each other. Example: You push a wall with 50 N (action). The wall pushes YOU with 50 N in the opposite direction (reaction). These two forces act on different objects (wall and you) — so they are not 'balanced forces' on any single object. The net force on YOU is 50 N (from the wall) → you accelerate backward. For forces to cancel, they must act on the SAME object. Newton's 3rd law forces always act on DIFFERENT objects. This is why rockets can propel themselves in space — exhaust gases go backward (action) → rocket goes forward (reaction) — no cancellation.

Practice problems

Work through this chapter's problems as a readiness check — reveal each solution, mark yourself honestly, and get your gap report at the end.

Readiness check

Are you exam-ready for Laws of Motion?

1 problems from this chapter. Try each one, reveal the worked solution, mark yourself honestly — get your gap report at the end.

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5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•FORCE: A push or pull that changes (or tends to change) the state of motion of a body. Vector quantity. SI unit: Newton (N). 1 N = force needed to accelerate 1 kg at 1 m/s². Balanced forces → no acceleration. Unbalanced forces → acceleration.
•NEWTON'S FIRST LAW (Law of Inertia): A body at rest stays at rest; a body in motion continues with uniform velocity in a straight line — UNLESS acted upon by an external unbalanced force. INERTIA = property of a body to resist any change in its state of motion. Inertia is directly proportional to MASS — heavier objects have more inertia.
•INERTIA EXAMPLES: (1) Passengers lurch forward when a bus suddenly stops (body continues in motion). (2) Passengers fall back when bus suddenly starts (body resists starting motion). (3) Coin placed on a card over a glass — card flicked horizontally, coin drops into glass (coin's inertia keeps it in place). (4) Dust beaten out of a carpet (carpet moves, dust resists due to inertia). (5) Athletes run past the finish line (body in motion stays in motion).
•NEWTON'S SECOND LAW: F = ma. Force = mass × acceleration. Equivalent form: F = Δp/Δt = m(v−u)/t (rate of change of momentum). The greater the force, the greater the acceleration for a given mass. The greater the mass, the smaller the acceleration for a given force.
•MOMENTUM: p = mv. SI unit: kg·m/s. Vector quantity. A bullet (small mass, high velocity) and a truck (large mass, low velocity) can have similar momentum. Change in momentum Δp = m(v−u). Impulse = F × t = Δp.
•MASS vs WEIGHT: MASS = quantity of matter, SI unit kilogram (kg), CONSTANT everywhere, measured by BEAM BALANCE. WEIGHT = gravitational force on a body, W = mg, SI unit NEWTON (N), VARIES with location (g changes), measured by SPRING BALANCE. On Moon: weight is 1/6 of Earth weight; mass is same.
•NEWTON'S THIRD LAW: For every action, there is an EQUAL and OPPOSITE reaction. Action and reaction forces act on DIFFERENT bodies — they cannot cancel each other. EXAMPLES: (1) Walking — feet push ground back, ground pushes feet forward. (2) Swimming — hands push water back, water pushes swimmer forward. (3) Rocket propulsion — gases ejected backward, rocket accelerates forward. (4) Gun recoil — bullet forward, gun backward.
•CONSERVATION OF MOMENTUM: In the ABSENCE of external forces, the total momentum of a system remains constant. m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. Used in collision problems, recoil of guns, rocket propulsion, explosions. This law is a consequence of Newton's Third Law.
•RECOIL OF GUN: Initial momentum = 0 (both at rest). After firing: bullet has momentum mv (forward), gun must have momentum −MV (backward) so total is still zero. V = −mv/M. Heavier guns have less recoil. Example: bullet 50 g at 400 m/s, gun 4 kg → V = (0.05 × 400)/4 = 5 m/s backward.
•IMPULSE: F × t = m(v−u). Same change in momentum can come from large force × small time (sudden hit) or small force × long time (gentle catch). Example: catching a cricket ball — moving hands backward INCREASES time → DECREASES force on hand → less injury. Padded helmets reduce force by increasing collision time.

Andhra Pradesh (BIEAP) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Car safety and Newton's laws

Modern cars are designed using Newton's laws of motion. Seat belts apply the necessary external force (Newton's First Law) to decelerate passengers during sudden braking. Airbags increase the collision time, reducing peak force (Impulse principle). Crumple zones at the car's front absorb energy, again reducing the force on passengers. Anti-lock braking systems (ABS) ensure brakes apply optimal deceleration. Every AP car driver benefits from these applications of Class 9 physics.

ISRO rocket launches and conservation of momentum

Every ISRO rocket launch (Sriharikota, AP — Satish Dhawan Space Centre) is a real-world demonstration of conservation of momentum. The rocket ejects hot gases downward at speeds of 4–5 km/s; by conservation of momentum, the rocket accelerates upward. The PSLV rocket can deliver multi-tonne payloads to orbit through staged combustion — each stage ejecting gases and gaining velocity. AP's Sriharikota is India's primary launch site and every launch is Class 9 physics in action.

Sports — cricket, badminton, athletics

Every sport applies Newton's laws. Cricket bowlers swing their arms over to generate the bat speed needed for fast deliveries (F = ma). Sprinters drive their feet down hard on the starting blocks (action) and accelerate forward (reaction). High jumpers convert horizontal kinetic energy into vertical motion. Even the curve of a cricket ball (Magnus effect) involves understanding force and motion. AP's Hyderabad cricket academies use motion physics for batting and bowling technique analysis.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

F = ma numerical (4 marks): identify the THREE quantities, write the formula explicitly, substitute with units, solve, state answer with correct unit (Newton or m/s²). Check: if force is in N and mass in kg, acceleration comes out in m/s². Unit checking earns method marks.

Conservation of momentum (4-5 marks): write the conservation equation explicitly (m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂). Substitute given values. Solve for unknown. ALWAYS state the answer with direction (positive or negative — backward or forward). For recoil problems, initial total momentum = 0.

Inertia examples (2-3 marks): give 2 or 3 SPECIFIC examples and EXPLAIN each one in terms of inertia. Don't just list — explain WHY each happens. 'When the bus stops suddenly, passengers lurch forward BECAUSE their bodies were in motion and resist the change to rest.'

Mass vs weight (2 marks): give a TABLE with 4 rows comparing — what it is, SI unit, where it varies, measuring instrument. Tables are clearer than paragraphs for comparison questions.

Newton's Third Law examples (2 marks): always identify BOTH the action and the reaction force EXPLICITLY. 'Action: feet push ground backward. Reaction: ground pushes feet forward.' Identifying both halves of the pair earns the full marks.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

STRETCH

Research the historical development of mechanics — from Aristotle (who believed continuous force was needed to maintain motion) to Galileo (who proved that motion continues without force if friction is removed) to Newton (who formalised the three laws). Each step required rejecting accepted intuitions. Research Galileo's inclined plane experiments and how they refuted 2,000 years of Aristotelian physics.

STRETCH

Investigate non-inertial reference frames — Newton's laws apply only in INERTIAL FRAMES (frames not accelerating). In a rotating frame (like Earth) or an accelerating frame (like a turning car), 'pseudo forces' (centrifugal force, Coriolis force) appear to act on objects. The Coriolis force is why cyclones spin counter-clockwise in the Northern Hemisphere (including AP's Bay of Bengal cyclones) and clockwise in the Southern Hemisphere.

STRETCH

Explore the work-energy theorem and the energy formulation of mechanics — Newton's F = ma can be reformulated in terms of energy: Work done by net force = Change in kinetic energy. This Lagrangian/Hamiltonian formulation of mechanics is more powerful for complex problems than force-based Newtonian mechanics. Research how 18th–19th century mathematicians (Lagrange, Hamilton) reformulated Newton's work.

STRETCH

Research relativistic momentum — at very high velocities (close to the speed of light), Newton's p = mv is incorrect. Einstein's correction: p = γmv where γ = 1/√(1−v²/c²). For everyday speeds, γ ≈ 1 and Newtonian formula works. But for particles in CERN's LHC moving at 99.99% of light speed, γ ≈ 7000 — momentum is enormously larger than Newton predicts. This is why particle accelerators must use relativistic corrections.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

AP Board SSC (Class 10) — PhysicsVery High — Laws of Motion concepts are extended in Class 10 Force, Gravitation, Work-Energy, and Momentum

JEE Main and Advanced (Mechanics)Very High — Newton's laws, momentum, impulse, and collisions are core JEE Physics topics

NEET (Physics)High — Newton's laws and momentum conservation appear in NEET physics

AP EAPCET (Engineering)Very High — Newton's laws and dynamics form a major chapter in EAPCET Physics

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Questions students ask

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Action and reaction forces are EQUAL in magnitude and OPPOSITE in direction — BUT they ACT ON DIFFERENT OBJECTS. For two forces to cancel, they must act on the SAME body. Example: When you push a wall with 100 N, the wall pushes back on YOU with 100 N. The 100 N on the wall and 100 N on you cannot cancel because they are NOT on the same object. The force on the wall is the action; the force on you is the reaction. This is why rockets work — the rocket pushes hot gases out the back (action), the gases push the rocket forward (reaction); these forces are on different bodies (gases and rocket), so the rocket accelerates forward.

This is a direct demonstration of Newton's First Law (Inertia). Before the bus stops, the passengers AND the bus are moving forward together with some velocity. When the bus suddenly stops (the brakes apply a backward force on the bus), the passengers' bodies are not directly connected to the brakes — they are sitting on or standing in the bus by friction and contact. The passengers' bodies CONTINUE to move forward with their previous velocity (because nothing stopped them) while the bus has decelerated. Result: relative to the bus, they lurch forward. This is INERTIA OF MOTION — bodies in motion tend to stay in motion. Wearing seat belts solves this — the belt applies the stopping force to the passenger's body when the bus brakes.

MASS is the AMOUNT OF MATTER in your body — your atoms and molecules. This quantity does NOT change based on where you are. Whether you are on Earth, Moon, Mars, or in space, your mass remains the same. WEIGHT is the GRAVITATIONAL FORCE pulling you down: W = mg. The 'g' (acceleration due to gravity) DEPENDS on the planet/moon: g_Earth = 9.8 m/s², g_Moon ≈ 1.6 m/s² (about 1/6 of Earth's). Therefore, your WEIGHT on the Moon = m × g_Moon = (1/6) × your Earth weight. Astronauts feel 'lighter' on the Moon not because they have less mass, but because the gravitational force on them is weaker. They can jump higher and lift heavier objects.

Initially, the rocket (with fuel) is at rest. Total momentum = 0. The rocket burns fuel and EJECTS hot gases at high velocity DOWNWARD (or backward in space). The exhaust gases gain momentum DOWNWARD. By conservation of momentum (total must remain 0), the rocket gains EQUAL and OPPOSITE momentum UPWARD. m_gas × v_gas (downward) = m_rocket × v_rocket (upward). As fuel is continuously ejected, the rocket continuously accelerates upward. This works in SPACE (no air needed for 'pushing against') because the gases themselves provide the reaction. ISRO's PSLV and GSLV rockets work on exactly this principle — and they all originate conceptually from Newton's third law and conservation of momentum.

This applies the concept of IMPULSE: F × t = Δp (change in momentum). When the ball arrives, it has momentum mv. The catcher must reduce this to zero, so Δp = mv (a fixed value). The CATCHER CAN CHOOSE: stop the ball QUICKLY (small t) — this requires a LARGE force F (which would hurt the hand). OR stop the ball SLOWLY by moving hands backward (large t) — this requires a SMALLER force F (less painful). By moving hands backward, the catcher INCREASES the catching time t → DECREASES the average force on the hand. Same principle: car airbags increase collision time, padded gym mats cushion falls, packaging foam protects fragile items during shipping.

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