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ICSE Class 9 Science★ ICSE Class 9 Physics · Full Year Coverage · Board: ICSE

Physics — Motion, Laws of Motion, Fluids, Heat, Light & Electricity

ICSE Class 9 Physics: MOTION in one dimension. NEWTON'S three laws. PRESSURE in liquids. Why things FLOAT (upthrust/Archimedes). HEAT and energy. REFLECTION of light at plane and spherical surfaces. And STATIC electricity. ICSE Class 9 Science.

⏱ 18 min readMedium difficulty✓ Free · NCERT-aligned

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

1Apply the 3 equations of motion for uniform acceleration: v=u+at, s=ut+½at², v²=u²+2as
2Interpret displacement-time and velocity-time graphs
3State Newton's 3 Laws and define inertia; apply F=ma
4Calculate pressure using P=F/A and hydrostatic pressure P=hρg
5State Pascal's Law and explain the hydraulic press
6State Archimedes' Principle and the conditions for floating vs sinking
7Convert between temperature scales: K=°C+273, °F=(9/5)°C+32
8Distinguish specific heat capacity from latent heat
9Apply the laws of reflection; state properties of plane mirror images
10State uses of concave and convex mirrors with image characteristics

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Why this chapter matters

This ICSE Class 9 Physics chapter is the most calculation-heavy part of the science paper. The 3 equations of motion (v=u+at, s=ut+½at², v²=u²+2as) appear in every paper as 3-4 mark numerical problems. Newton's Second Law (F=ma) and the mirror formula (1/f=1/u+1/v) are reliable numerical questions. Archimedes' Principle and the floatation condition (weight=upthrust) are 3-mark conceptual questions. Latent heat (change of state without temperature change) vs specific heat capacity is a conceptual MCQ trap. The v-t graph (slope=acceleration, area=displacement) is tested in diagram-interpretation questions every year.

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Physics — Motion, Forces, Fluids, Heat, Light & Electricity

1. Motion in One Dimension

Key Equations of Motion (Uniform Acceleration)

Equation	Relates
v = u + at	v, u, a, t
s = ut + ½at²	s, u, a, t
v² = u² + 2as	v, u, a, s

u = initial velocity. v = final velocity. a = acceleration. t = time. s = displacement.

Displacement-Time and Velocity-Time Graphs

v-t graph: slope = acceleration. Area under = displacement.

2. Newton's Laws of Motion

Law	Statement
First (Inertia)	An object remains at rest or in UNIFORM MOTION unless acted upon by an external UNBALANCED force.
Second	F = ma. Force = mass × acceleration. Unit: Newton (N). 1 N = 1 kg m/s².
Third	For every action, there is an EQUAL and OPPOSITE reaction. Action and reaction act on DIFFERENT bodies.

Inertia

The TENDENCY of an object to RESIST change in its state of motion. Mass is the measure of inertia. More mass = more inertia.

3. Pressure in Fluids and Upthrust (Buoyancy)

Pressure

P = F/A (Pressure = Force/Area). Unit: Pascal (Pa). Liquids: P = hρg (pressure at depth h).

Pascal's Law

Pressure applied to an ENCLOSED fluid is transmitted EQUALLY in all directions. Hydraulic press.

Archimedes' Principle

'When a body is PARTIALLY or WHOLLY immersed in a fluid, it experiences an UPWARD FORCE (upthrust/buoyancy) EQUAL to the WEIGHT of the FLUID DISPLACED.'

Floatation

Object SINKS: Weight > Upthrust. Density > fluid.
Object FLOATS: Weight = Upthrust. Density < fluid.

4. Heat and Energy

Temperature Scales

Celsius (°C). Kelvin (K = °C + 273). Fahrenheit (°F = (9/5)°C + 32).

Thermal Expansion

Solids, liquids, and gases EXPAND when heated. 'Railway tracks have gaps. Bridges have expansion joints. This is physics — not poor construction.'

Specific Heat Capacity

Amount of heat needed to raise 1 kg of a substance by 1°C. Water has a HIGH specific heat → used as a COOLANT.

Latent Heat

Heat absorbed/released during a CHANGE OF STATE — without change in temperature.

Latent heat of fusion: Solid ↔ Liquid (ice → water: 336 J/g at 0°C)
Latent heat of vaporisation: Liquid ↔ Gas (water → steam: 2260 J/g at 100°C)

Energy Sources

Renewable (solar, wind, hydro, biomass) vs. Non-renewable (coal, oil, natural gas — fossil fuels). 'The future is RENEWABLE.'

5. Light — Reflection

Laws of Reflection

Angle of incidence (i) = Angle of reflection (r)
Incident ray, reflected ray, and normal all lie in the SAME PLANE.

Plane Mirror

Image: VIRTUAL, ERECT, SAME SIZE, LATERALLY INVERTED, as far BEHIND the mirror as the object is in front.

Spherical Mirrors

	Concave	Convex
Shape	Curved INWARD	Curved OUTWARD
Images	Real/Virtual. Magnified/Diminished.	Always VIRTUAL, DIMINISHED, ERECT.
Uses	Torches, shaving mirrors, dentists' mirrors	Rear-view mirrors. Shop security.

Mirror Formula: 1/f = 1/u + 1/v. Magnification: m = —v/u = hᵢ/hₒ.

f = focal length. u = object distance (NEGATIVE). v = image distance.

6. Static Electricity

Electric Charges

TWO types: POSITIVE and NEGATIVE. Like charges REPEL. Unlike ATTRACT.
Charging by: FRICTION (rubbing). CONDUCTION (contact). INDUCTION (without contact).

The Electroscope

Device to DETECT electric charge. Gold-leaf electroscope — leaves DIVERGE when charged.

Lightning and Lightning Conductors

Lightning = MASSIVE static discharge between clouds and earth. Lightning conductor = metal rod on building roof, connected to earth — provides a safe PATH for the charge.

7. Magnetism

Magnetic Field Lines

Originate from NORTH pole. Enter at SOUTH pole.
NEVER intersect. CLOSER lines = STRONGER field.

Induced Magnetism

A magnetic material becomes a MAGNET when brought NEAR a permanent magnet. The NEAR end acquires the OPPOSITE polarity.

Electromagnetism

Current through a coil (solenoid) → MAGNETIC FIELD. Strength increased by: more turns, more current, soft iron CORE.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Motion, Newton's Laws, Fluids, Heat, Light, and Electricity

EQUATIONS OF MOTION (uniform acceleration): v = u + at. s = ut + ½at². v² = u² + 2as. Where: u = initial velocity. v = final velocity. a = acceleration. t = time. s = displacement. GRAPHS: Displacement-time graph: slope = VELOCITY. Velocity-time graph: slope = ACCELERATION. Area under v-t graph = DISPLACEMENT. NEWTON'S THREE LAWS: First (INERTIA LAW): Object stays at rest or in uniform motion unless external unbalanced force acts. Second: F = ma (Force = mass × acceleration). 1 N = 1 kg·m/s². Third: Every action has equal and opposite reaction; they act on DIFFERENT bodies. INERTIA: Tendency to resist change in motion. MASS = measure of inertia. More mass → more inertia. PRESSURE: P = F/A (Pascal, Pa). Hydrostatic pressure: P = hρg (h=depth, ρ=density of fluid, g=9.8 m/s²). PASCAL'S LAW: Pressure applied to enclosed fluid transmitted EQUALLY in all directions. Application: hydraulic press (small force → large force via area difference). ARCHIMEDES' PRINCIPLE: A body wholly/partially immersed in a fluid experiences an UPWARD FORCE (upthrust/buoyancy) = WEIGHT of FLUID DISPLACED. FLOATATION: Object SINKS → Weight > Upthrust → density of object > density of fluid. Object FLOATS → Weight = Upthrust → density of object ≤ density of fluid. TEMPERATURE SCALES: Kelvin: K = °C + 273. Fahrenheit: °F = (9/5)°C + 32. SPECIFIC HEAT CAPACITY (c): amount of heat to raise 1 kg by 1°C. Water has HIGH specific heat (4200 J/kg°C) → used as COOLANT. Q = mcΔT. LATENT HEAT: Heat absorbed/released during CHANGE OF STATE with NO temperature change. Latent heat of FUSION (solid↔liquid, ice→water: 336 J/g at 0°C). Latent heat of VAPORISATION (liquid↔gas, water→steam: 2260 J/g at 100°C). LAWS OF REFLECTION: Angle of incidence = Angle of reflection. Incident ray, reflected ray, and normal are coplanar. PLANE MIRROR IMAGE: Virtual. Erect. Same size. Laterally inverted. Distance behind mirror = distance of object in front. SPHERICAL MIRRORS: CONCAVE (curved inward): can form real/virtual, magnified/diminished images. Uses: torches (parallel rays), shaving/dentist's mirrors (magnified virtual image). CONVEX (curved outward): ALWAYS virtual, erect, diminished. Uses: rear-view mirrors (wide field of view), shop security. MIRROR FORMULA: 1/f = 1/v + 1/u. Magnification: m = −v/u = hᵢ/hₒ. (Convention: distances measured from pole; u is NEGATIVE for real objects.) STATIC ELECTRICITY: Two charges: positive and negative. Like charges REPEL. Unlike ATTRACT. Charging methods: friction, conduction, induction. Electroscope: detects charge (leaves diverge). LIGHTNING: massive static discharge; lightning conductor = safe path to earth. MAGNETISM: Field lines go N→S outside magnet. NEVER intersect. Closer = stronger field. Induced magnetism: near end acquires OPPOSITE polarity. Electromagnet: current in solenoid + soft iron core → strong, switchable magnet.

ICSE CLASS 9 PHYSICS KEY TRAPS: (1) SIGN CONVENTION: u is always NEGATIVE (object on left, real). v is negative for real image (in front of concave), positive for virtual image (behind mirror). f is negative for concave, positive for convex. (2) LATENT HEAT: during melting or boiling, temperature does NOT change even though heat is being added. All heat goes into BREAKING BONDS. This is why boiling water stays at 100°C until all water has evaporated. (3) ARCHIMEDES: The upthrust = weight of FLUID DISPLACED, NOT the weight of the object. If an iron ship displaces a large enough volume of water, the weight of that water can equal the ship's weight — and it floats. (4) NEWTON'S THIRD LAW: The reaction force acts on a DIFFERENT BODY. You push the floor → floor pushes YOU. They are equal in magnitude but opposite in direction — and they do NOT cancel because they act on different objects. (5) v-t GRAPH AREA = displacement. v-t graph SLOPE = acceleration. These two facts appear in MCQs every year.

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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

✗ Confusing latent heat (temperature unchanged) with specific heat capacity, or applying the mirror formula with wrong signs

✓ TWO CORRECTIONS: (1) LATENT vs SPECIFIC HEAT: SPECIFIC HEAT CAPACITY = how much energy to change the TEMPERATURE of 1 kg of a substance by 1°C. Temperature CHANGES. Q = mcΔT. LATENT HEAT = how much energy to change the STATE (solid→liquid or liquid→gas) of 1 kg of a substance at CONSTANT TEMPERATURE. Temperature does NOT change during state change. The graph of 'temperature vs heat added' for water shows: rising slope (ice warming), FLAT LINE at 0°C (ice melting — latent heat of fusion), rising slope (water warming), FLAT LINE at 100°C (water boiling — latent heat of vaporisation). (2) MIRROR FORMULA SIGN CONVENTION: Object is always placed on the LEFT (in front of the mirror). Distance of object (u) is always NEGATIVE. Distance of image (v): NEGATIVE if image is in front of mirror (real image — concave mirror). POSITIVE if image is behind mirror (virtual image — concave or convex). Focal length (f): NEGATIVE for concave mirror. POSITIVE for convex mirror. Formula: 1/f = 1/v + 1/u. Always substitute with signs.

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1MEDIUM· equations-of-motion-archimedes

A stone is dropped from a height of 80 m (take g = 10 m/s²). Find: (a) time to reach the ground, (b) velocity on impact. Then: A wooden block of mass 500 g and volume 800 cm³ is placed in water (density 1 g/cm³). Find the upthrust. Does the block float or sink?

▶ Show solution

STONE DROP: u = 0 (dropped from rest), a = g = 10 m/s² (downward), s = 80 m. (a) Using s = ut + ½at²: 80 = 0×t + ½×10×t². 80 = 5t². t² = 16. t = 4 seconds. (b) Using v = u + at: v = 0 + 10×4 = 40 m/s. [Check: v² = u² + 2as = 0 + 2×10×80 = 1600. v = 40 m/s ✓]. ARCHIMEDES / FLOATATION: Mass of block = 500 g = 0.5 kg. Volume of block = 800 cm³ = 800 × 10⁻⁶ m³ = 8 × 10⁻⁴ m³. UPTHRUST = Weight of water displaced = Volume × density of water × g = 800 cm³ × 1 g/cm³ × g = 800 g = 0.8 kg weight (if fully submerged). Weight of block = 500 g. Since weight of block (500 g) < weight of water displaced if fully submerged (800 g), the block FLOATS. It will submerge only until upthrust = weight of block. Volume submerged = mass/density of water = 500 g / 1 g/cm³ = 500 cm³. So 500 cm³ is submerged (62.5% of the block), 300 cm³ is above water. UPTHRUST (actual) = 500 g-force (= weight of block). The block FLOATS with 500/800 = 62.5% of its volume submerged.

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