For Class 8, 9 & 10
Foundation - a strong base in Physics, Chemistry, Maths & Biology
Concept-first questions with clear model answers across all four subjects, aligned to NCERT. Start early, build the habit, and walk into your board exams, NEET and JEE already ahead.
Showing 24 questions for Class 9. Tap a card to reveal the answer.
PhysicsMotionmediumA car starting from rest accelerates uniformly at 2 m/s^2 for 5 seconds. Find its final velocity and the distance travelled.
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What it is
The three equations of motion connect initial velocity, acceleration, time, distance and final velocity for steady acceleration.
Answer
Given u = 0, a = 2 m/s^2, t = 5 s. Final velocity v = u + at = 0 + 2 x 5 = 10 m/s. Distance s = ut + (1/2)at^2 = 0 + (1/2)(2)(5^2) = 25 m. So the car reaches 10 m/s and covers 25 m.
v = u + at ; s = ut + (1/2)at^2
- •v = u + at = 10 m/s
- •s = ut + (1/2)at^2 = 25 m
- •u = 0 (starts from rest)
Why learn this
They let engineers predict a rocket's speed, a car's braking distance and a ball's flight - the maths of all moving things.
💡 Memory trick
Use 'v = u + at' first (no distance), then 's = ut + 1/2 at^2'. Velocity gets a time-boost 'at'.
PhysicsForce and Laws of MotionmediumState Newton's second law of motion and define momentum.
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What it is
Force is what changes an object's momentum; for a fixed mass, force = mass x acceleration.
Answer
Newton's second law states that the rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force; for constant mass this gives F = ma. Momentum is the quantity of motion of a body, equal to the product of its mass and velocity, p = mv.
F = ma ; p = mv
- •Rate of change of momentum is proportional to force
- •F = ma (for constant mass)
- •Momentum p = mv
- •Change is in the direction of the force
Why learn this
It's why seatbelts and airbags save lives, and how rockets and cricket shots are designed.
💡 Memory trick
F = ma ('Force = Mass Accelerates'); momentum p = mv is 'mass in motion'.
PhysicsGravitationmediumState the universal law of gravitation and write its formula.
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What it is
Every mass attracts every other mass with a force that grows with mass and shrinks fast with distance.
Answer
The universal law of gravitation states that every body in the universe attracts every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. The force F = G m1 m2 / r^2, where G is the universal gravitational constant.
F = G m1 m2 / r^2
- •Force is proportional to m1 x m2
- •Force is inversely proportional to r^2
- •F = G m1 m2 / r^2
- •G is the gravitational constant
Why learn this
It keeps the Moon orbiting, satellites up and explains your weight - the law that runs the universe.
💡 Memory trick
More mass = more pull; double the distance = one-quarter the pull (inverse SQUARE).
PhysicsWork and EnergymediumDefine work done by a force and state the formula for kinetic energy.
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What it is
Work is force times the distance moved along it; a moving object stores that work as kinetic energy.
Answer
Work is done when a force acts on a body and the body moves in the direction of the force; work done W = F s cos(theta), where s is the displacement and theta is the angle between the force and the displacement. Kinetic energy is the energy possessed by a body due to its motion and is given by KE = (1/2) m v^2. The unit of both work and energy is the joule.
W = F s cos(theta) ; KE = (1/2) m v^2
- •Work needs force and displacement along it
- •W = F s cos(theta)
- •Kinetic energy KE = (1/2) m v^2
- •Unit is the joule (J)
Why learn this
It underlies electricity bills (energy) and how a dam, a bow-and-arrow or a moving car works.
💡 Memory trick
No movement, no work. KE = 1/2 m v^2 - double the speed, FOUR times the energy.
ChemistryMatter in Our SurroundingsmediumWhy does evaporation cause cooling?
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What it is
When the fastest particles escape a liquid's surface, the particles left behind are cooler.
Answer
During evaporation the fastest-moving (most energetic) particles at the surface of a liquid escape into the air. This lowers the average kinetic energy of the remaining particles, so the temperature of the liquid falls. The escaping particles also absorb latent heat from the surroundings, which is why evaporation produces a cooling effect - for example, sweating cools our body.
- •Fast (energetic) particles leave the surface
- •Remaining particles have lower average energy
- •Latent heat absorbed from surroundings
- •Example: sweating cools the body
Why learn this
It's why sweating, earthen pots (matka) and desert coolers keep things cold without a fridge.
💡 Memory trick
The 'hot-heads' leave first, so what's left behind is cooler.
ChemistryIs Matter Around Us PuremediumGive two differences between a mixture and a compound.
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What it is
A mixture is a physical blend you can separate; a compound is a fixed chemical union with new properties.
Answer
In a mixture the components are just mixed physically in any ratio, keep their own properties and can be separated by physical methods like filtration. In a compound the elements are chemically combined in a fixed ratio to form a new substance with new properties, and they can be separated only by chemical methods. For example, a mixture of iron and sulphur versus the compound iron sulphide.
- •Mixture: physical, any ratio, keeps properties, separated physically
- •Compound: chemical, fixed ratio, new properties, separated chemically
- •Example: iron+sulphur mix vs iron sulphide
Why learn this
It's how chemists tell air (mixture) from water (compound) and separate ores and medicines.
💡 Memory trick
Mixture = Mixed (any ratio, easy split); Compound = Chemically bonded (fixed, hard to split).
ChemistryAtoms and MoleculesmediumState the law of conservation of mass with an example.
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What it is
In any chemical reaction the total mass stays the same - atoms are only rearranged, never lost.
Answer
The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction; the total mass of the reactants equals the total mass of the products. For example, when 12 g of carbon burns completely in 32 g of oxygen, exactly 44 g of carbon dioxide is formed (12 + 32 = 44).
mass of reactants = mass of products
- •Mass is neither created nor destroyed
- •Total mass of reactants = total mass of products
- •Example: 12 g C + 32 g O2 -> 44 g CO2
Why learn this
It's why every chemical equation must be BALANCED - the foundation of all of chemistry.
💡 Memory trick
Atoms in = atoms out. Nothing vanishes, it just rearranges.
ChemistryStructure of the AtomeasyDefine the atomic number and the mass number of an atom.
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What it is
Atomic number = number of protons (the element's identity); mass number = protons + neutrons.
Answer
The atomic number (Z) is the number of protons in the nucleus of an atom, which also equals the number of electrons in a neutral atom; it decides the identity of the element. The mass number (A) is the total number of protons and neutrons in the nucleus. So the number of neutrons = A - Z.
A = Z + number of neutrons
- •Atomic number Z = number of protons (= electrons if neutral)
- •Mass number A = protons + neutrons
- •Neutrons = A - Z
- •Z identifies the element
Why learn this
These two numbers decide every element in the periodic table and its isotopes.
💡 Memory trick
Z = protons (Zee identity); A = All heavy bits (protons + neutrons). Neutrons = A - Z.
BiologyThe Fundamental Unit of LifemediumGive two differences between a prokaryotic and a eukaryotic cell.
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What it is
Prokaryotes (bacteria) have no true nucleus; eukaryotes (plants, animals) have a nucleus and organelles.
Answer
A prokaryotic cell has no true nucleus - its genetic material lies free in the cytoplasm - and it lacks membrane-bound organelles like mitochondria; bacteria are examples. A eukaryotic cell has a true, membrane-bound nucleus and membrane-bound organelles; plant and animal cells are examples. Eukaryotic cells are usually larger and more complex.
- •Prokaryote: no true nucleus, no membrane-bound organelles (bacteria)
- •Eukaryote: true nucleus + organelles (plants, animals)
- •Eukaryotic cells are larger and more complex
Why learn this
It's the deepest divide in the living world and why antibiotics can hit bacteria but not your own cells.
💡 Memory trick
PRO = primitive, NO true nucleus; EU = 'true', good nucleus.
BiologyTissuesmediumWhat is meristematic tissue and where is it found in plants?
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What it is
Meristem is a plant's growth tissue - young cells that keep dividing at tips and edges.
Answer
Meristematic tissue is made of small, thin-walled cells that keep dividing to help the plant grow. It is found at the growing regions - apical meristem at the tips of roots and shoots (which increases length) and lateral meristem or cambium (which increases thickness). Because its cells divide actively, they have dense cytoplasm and no large vacuoles.
- •Dividing cells responsible for growth
- •Apical meristem at root/shoot tips (length)
- •Lateral meristem/cambium (thickness)
- •Dense cytoplasm, actively dividing
Why learn this
It's why plants grow taller and thicker all their lives, and how grafting and pruning work.
💡 Memory trick
Apical meristem at the tips (length); lateral meristem sideways (girth).
BiologyImprovement in Food ResourcesmediumWhat is the difference between mixed cropping and intercropping?
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What it is
Mixed cropping blends crops with no set pattern; intercropping plants them in definite rows.
Answer
In mixed cropping two or more crops are grown together on the same field at the same time with no fixed pattern, mainly to reduce the risk of total crop failure. In intercropping two or more crops are grown together in a definite row pattern (for example, a few rows of one crop then a few of another) to use nutrients and space efficiently and to help control pests.
- •Mixed cropping: crops mixed, no pattern, reduces risk
- •Intercropping: crops in a definite row pattern
- •Both grown together to improve yield
Why learn this
Farmers use both to raise yield and cut the risk of losing everything to one pest or failure.
💡 Memory trick
Mixed = Muddled together; Inter = In neat rows (a definite pattern).
BiologyNatural ResourceseasyWhy is the atmosphere important for life on Earth?
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What it is
The atmosphere is Earth's protective blanket of air that supplies gases and steadies temperature.
Answer
The atmosphere is a blanket of air that supports life in several ways: it provides oxygen for breathing and carbon dioxide for photosynthesis, keeps the temperature of the Earth steady by preventing sudden day-to-night changes, and shields us from the Sun's harmful ultraviolet rays through the ozone layer. Winds and the water cycle also depend on it.
- •Provides oxygen and carbon dioxide
- •Keeps temperature steady (day and night)
- •Ozone layer blocks harmful UV rays
- •Drives winds and the water cycle
Why learn this
It gives us oxygen, blocks harmful UV rays (ozone) and drives weather - no atmosphere, no life.
💡 Memory trick
The air blanket does 3 B's: Breathe, Block UV, Balance temperature.
MathematicsNumber SystemsmediumRationalise the denominator of 1 / sqrt(2).
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What it is
Rationalising clears a surd (root) from the denominator by multiplying top and bottom by it.
Answer
Multiply the numerator and denominator by sqrt(2): (1/sqrt(2)) x (sqrt(2)/sqrt(2)) = sqrt(2) / 2. So 1/sqrt(2) = sqrt(2)/2, which is about 0.707. Rationalising removes the irrational number (surd) from the denominator.
1/sqrt(a) = sqrt(a)/a
- •Multiply top and bottom by sqrt(2)
- •= sqrt(2)/2
- •Removes the surd from the denominator
Why learn this
It makes fractions easier to compute and compare - important in exams and in engineering maths.
💡 Memory trick
Multiply top and bottom by the same surd - the root moves upstairs.
MathematicsPolynomialsmediumUsing the remainder theorem, find the remainder when p(x) = x^3 + 3x^2 + 3x + 1 is divided by (x + 1).
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What it is
The remainder theorem says the remainder of p(x) divided by (x - a) is simply p(a).
Answer
By the remainder theorem, the remainder when p(x) is divided by (x + 1) is p(-1). p(-1) = (-1)^3 + 3(-1)^2 + 3(-1) + 1 = -1 + 3 - 3 + 1 = 0. Since the remainder is 0, (x + 1) is a factor of p(x).
Remainder theorem: remainder = p(a) when dividing by (x - a)
- •Remainder = p(-1)
- •p(-1) = -1 + 3 - 3 + 1 = 0
- •Remainder 0 -> (x + 1) is a factor
Why learn this
It checks factors of big polynomials in one step - a shortcut used all through algebra.
💡 Memory trick
Dividing by (x + 1)? Just plug in x = -1. Set the bracket to zero, then substitute.
MathematicsLinear Equations in Two VariableseasyWhere does the line x + y = 5 meet the x-axis and the y-axis?
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What it is
A straight line meets the axes at its intercepts - put y = 0 for the x-axis, x = 0 for the y-axis.
Answer
On the x-axis, y = 0, so x + 0 = 5 gives x = 5; the line meets the x-axis at (5, 0). On the y-axis, x = 0, so 0 + y = 5 gives y = 5; it meets the y-axis at (0, 5). These two intercept points are enough to draw the straight line.
Intercepts: set y = 0 for x-axis, x = 0 for y-axis
- •x-axis: put y = 0 -> (5, 0)
- •y-axis: put x = 0 -> (0, 5)
- •Two points define the straight line
Why learn this
Two intercepts are the fastest way to draw any line - used in graphs, economics and coding.
💡 Memory trick
Zero the other one: y = 0 gives the x-cut, x = 0 gives the y-cut.
MathematicsHeron's FormulamediumFind the area of a triangle whose sides are 3 cm, 4 cm and 5 cm using Heron's formula.
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What it is
Heron's formula finds a triangle's area from just its three sides, using the semi-perimeter.
Answer
First find the semi-perimeter s = (3 + 4 + 5)/2 = 6 cm. Heron's formula gives Area = sqrt[s(s - a)(s - b)(s - c)] = sqrt[6(6 - 3)(6 - 4)(6 - 5)] = sqrt[6 x 3 x 2 x 1] = sqrt[36] = 6 square cm.
Area = sqrt[s(s-a)(s-b)(s-c)], s = (a+b+c)/2
- •s = (a + b + c)/2 = 6 cm
- •Area = sqrt[s(s-a)(s-b)(s-c)]
- •= sqrt[36] = 6 cm^2
Why learn this
It measures land, plots and any triangle where you cannot easily find the height.
💡 Memory trick
Half the perimeter is 's'; Area = sqrt of s times the three (s - side) gaps.
PhysicsSoundmediumWhat is an echo, and what is the minimum distance needed to hear one clearly?
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What it is
An echo is sound bouncing back off a surface and reaching you a moment later.
Answer
An echo is the sound heard again after it reflects off a hard surface and returns to the listener. To hear a distinct echo the reflected sound must reach the ear at least 0.1 second after the original, which needs the reflecting surface to be at least about 17.2 m away (since sound travels about 344 m/s in air).
distance = (speed x time) / 2 = (344 x 0.1) / 2 = 17.2 m
- •Echo = reflected sound heard again
- •Ear needs a 0.1 s gap to separate the two sounds
- •Minimum distance about 17.2 m (speed 344 m/s)
Why learn this
It's the principle behind SONAR, ultrasound scans and measuring ocean depth.
💡 Memory trick
Need a 0.1 s gap -> surface at least about 17 m away, or the echo merges with the sound.
ChemistryIs Matter Around Us PuremediumWhat is the Tyndall effect, and what does it tell us about a mixture?
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What it is
The Tyndall effect is light scattering off colloid particles, revealing the beam's path.
Answer
The Tyndall effect is the scattering of a beam of light by the tiny particles of a colloid, which makes the path of the light visible. It shows that the mixture is a colloid (like milk or fog), because a true solution's particles are too small to scatter light while a colloid's particles are just the right size.
- •Scattering of light by colloidal particles makes the beam visible
- •Seen in colloids (milk, fog), not in true solutions
- •Shows the particle size is in the colloidal range
Why learn this
It's why you see sunbeams through fog or a projector beam in a dusty room.
💡 Memory trick
If you can SEE the light beam, it's a colloid (Tyndall). Crystal-clear = true solution.
BiologyThe Fundamental Unit of LifemediumWhat is osmosis? What happens to a cell placed in pure water?
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What it is
Osmosis is water moving across a membrane to balance the concentration on both sides.
Answer
Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration. A cell placed in pure water (a hypotonic solution) gains water by osmosis and swells; an animal cell may even burst, while a plant cell becomes firm (turgid) because of its cell wall.
- •Water moves across a selectively permeable membrane
- •From higher to lower water concentration
- •In pure water a cell swells (animal may burst; plant turns turgid)
Why learn this
It's how roots absorb water, how our cells stay hydrated and why salt preserves food.
💡 Memory trick
Water follows the salt. Cell in pure water swells; in salty water it shrinks.
MathematicsSurface Areas and VolumesmediumFind the volume of a sphere of radius 7 cm. (Take pi = 22/7.)
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What it is
A sphere's volume grows with the cube of its radius: V = (4/3) pi r^3.
Answer
Volume of a sphere = (4/3) x pi x r^3 = (4/3) x (22/7) x 7^3 = (4/3) x (22/7) x 343 = (4 x 22 x 49)/3 = 4312/3, which is about 1437.3 cubic cm.
V(sphere) = (4/3) pi r^3
- •V = (4/3) pi r^3
- •= (4/3)(22/7)(343)
- •= 4312/3 = about 1437.3 cm^3
Why learn this
It measures balls, planets, bubbles and tanks - anywhere something is round.
💡 Memory trick
Four-thirds pi r-cubed: V = (4/3) pi r^3 - the roundest formula in maths.
PhysicsGravitationmediumState Archimedes' principle. Why does an iron nail sink but an iron ship float?
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What it is
Any object in a fluid feels an upward buoyant force equal to the weight of fluid it displaces.
Answer
Archimedes' principle states that when a body is fully or partly immersed in a fluid, it experiences an upward buoyant force (upthrust) equal to the weight of the fluid it displaces. An iron nail sinks because it displaces only a little water, whose weight is less than the nail's weight. An iron ship is hollow and wide, so it displaces a large volume of water whose weight equals the ship's weight, and so it floats.
Buoyant force = weight of displaced fluid
- •Upthrust = weight of fluid displaced
- •Nail displaces little water -> sinks
- •Hollow ship displaces lots of water -> floats
Why learn this
It's why ships float, balloons rise and you feel lighter in water - core to shipbuilding and submarines.
💡 Memory trick
Archimedes: upthrust = weight of fluid pushed aside. Float if you displace enough water.
ChemistryStructure of the AtommediumWhat are isotopes? Give one example.
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What it is
Isotopes are atoms of the same element with the same protons but different numbers of neutrons.
Answer
Isotopes are atoms of the same element that have the same atomic number (the same number of protons) but different mass numbers (different numbers of neutrons). Because they have the same number of protons and electrons, they show the same chemical properties. For example, carbon-12 and carbon-14 are isotopes of carbon.
Same Z, different A
- •Same element, same protons (same atomic number)
- •Different neutrons (different mass number)
- •Same chemical properties
- •Example: carbon-12 and carbon-14
Why learn this
Isotopes are used in cancer treatment, carbon dating and nuclear energy.
💡 Memory trick
Same element (same protons), different mass (different neutrons). ISO = same place in the table.
BiologyTissuesmediumWhat are xylem and phloem? State the function of each.
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What it is
Xylem and phloem are the transport tissues of plants - xylem carries water, phloem carries food.
Answer
Xylem and phloem are the two conducting (vascular) tissues in plants. Xylem carries water and dissolved minerals upward from the roots to the leaves. Phloem transports the food (sugar) made in the leaves during photosynthesis to all other parts of the plant. Together they form the plant's transport system.
- •Xylem: carries water and minerals up from the roots
- •Phloem: carries food from leaves to the whole plant
- •Both are conducting (vascular) tissues
Why learn this
They are the plant's plumbing - how water rises from the roots and food reaches every part.
💡 Memory trick
Xylem = water UP from roots; Phloem = Food (from leaves) to the whole plant.
MathematicsStatisticseasyFind the mean of the data: 10, 15, 20, 25, 30.
Reveal answer ↓
What it is
The mean (average) is the total of all values divided by how many values there are.
Answer
Mean = (sum of all observations) / (number of observations) = (10 + 15 + 20 + 25 + 30) / 5 = 100 / 5 = 20.
Mean = (sum of observations) / (number of observations)
- •Mean = sum of values / number of values
- •Sum = 100, count = 5
- •Mean = 20
Why learn this
It summarises data in one number - used in marks, cricket averages, economics and science.
💡 Memory trick
Add them all up, divide by how many. Mean = sum / count.