For Class 8, 9 & 10
Master the basics - and everything after gets easier
Concept-first questions with clear model answers in Physics, Chemistry, Maths and Biology, all NCERT-aligned. Start early, build the habit, and walk into your boards, NEET and JEE already ahead.
Interactive lessons
learn by playingDrag, slide and build - watch each concept come alive, then reveal the answer.
200 interactive lessons
Ohm's law
Class 10 Physics
Slide V & R, watch the bulb glow
Open →pH scale
Class 10 Chemistry
Slide across acids and bases
Open →Atomic number and mass number
Class 9 Chemistry
Add protons & neutrons, build shells
Open →Laws of reflection
Class 8 Physics
Change the angle, watch it bounce
Open →Volume of a sphere
Class 9 Maths
Grow the radius, see the volume
Open →Area of a trapezium
Class 8 Maths
Drag the sides, read the area
Open →Power of a lens
Class 10 Physics
Move the object, trace the rays
Open →Food chain and energy flow
Class 10 Biology
Follow the energy as it flows
Open →Speed
Class 8 Physics
Slide distance & time, watch the speed
Open →Density
Class 9 Physics
Pack mass into volume, float or sink
Open →Work done
Class 9 Physics
Push harder or farther, watch work grow
Open →Kinetic energy
Class 9 Physics
Speed it up - energy grows with the square
Open →Power of a lens
Class 10 Physics
Shorten the focal length, boost the power
Open →Mole concept
Class 9 Chemistry
Weigh out grams, count the moles
Open →Avogadro's number
Class 9 Chemistry
Add moles, count the particles
Open →Microscope magnification
Class 8 Biology
Grow the image, read the magnification
Open →Population density
Class 10 Biology
Add individuals, shrink the land, see crowding
Open →Simple interest
Class 8 Maths
Slide money, rate & time, watch interest
Open →Pythagoras theorem
Class 9 Maths
Stretch the two sides, get the hypotenuse
Open →Probability of an event
Class 10 Maths
Change the outcomes, watch the odds
Open →Newton's second law
Class 9 Physics
Push a mass, pick an acceleration
Open →Momentum
Class 9 Physics
Slide mass & velocity, build momentum
Open →Pressure
Class 8 Physics
Shrink the area, feel the pressure rise
Open →Weight
Class 9 Physics
Change the planet's gravity, watch your weight
Open →Refractive index
Class 10 Physics
Slow light in the medium, raise the index
Open →Resistors in series
Class 10 Physics
Add two resistors in a line
Open →Mass percentage of a solution
Class 9 Chemistry
Dissolve solute, read the strength
Open →Concentration of a solution
Class 9 Chemistry
Pack solute into less liquid
Open →Population change
Class 10 Biology
Balance births against deaths
Open →Compound microscope
Class 8 Biology
Combine eyepiece & objective lenses
Open →Area of a circle
Class 8 Maths
Grow the radius, watch the area square
Open →Volume of a cuboid
Class 8 Maths
Stretch length, breadth & height
Open →Electronic configuration and valency
Class 9 Chemistry
Slide the atomic number, build the atom
Open →Homologous series (alkanes)
Class 10 Chemistry
Add carbons, name the compound
Open →Mass number
Class 9 Chemistry
Add protons & neutrons, get the mass number
Open →Power
Class 9 Physics
More work in less time = more power
Open →Potential energy
Class 9 Physics
Lift a mass higher, store energy
Open →Wave speed
Class 9 Physics
Tune frequency & wavelength, set the speed
Open →Electric current
Class 10 Physics
Push charge per second, get the current
Open →Percentage
Class 8 Maths
Compare part to whole as a %
Open →Electron dot structure
Class 9 Chemistry
Draw valence electrons as dots
Open →Acceleration
Class 9 Physics
Speed up over time, find acceleration
Open →Distance, speed and time
Class 8 Physics
Set speed & time, cover the distance
Open →Frequency and time period
Class 9 Physics
Shorten the period, raise the frequency
Open →Heating effect of current
Class 10 Physics
Raise the current, watch heating soar
Open →Area of a triangle
Class 8 Maths
Set base & height, halve the rectangle
Open →Area of a rectangle
Class 8 Maths
Set length & breadth, fill the area
Open →Mean (average)
Class 9 Maths
Share the total equally across items
Open →Discount
Class 8 Maths
Slide price & % off, see the saving
Open →Heart rate
Class 10 Biology
Set heart rate & time, count the beats
Open →Resistors in parallel
Class 10 Physics
Wire two resistors side by side
Open →Electric charge
Class 10 Physics
Flow current over time, collect charge
Open →Electrical energy and units
Class 10 Physics
Run appliances, add up the units
Open →Kelvin temperature scale
Class 9 Chemistry
Slide Celsius, read the Kelvin
Open →Moles from number of particles
Class 9 Chemistry
Divide particles by Avogadro's number
Open →Ten percent law
Class 10 Biology
See 10% of energy reach the next level
Open →Area of a square
Class 8 Maths
Grow the side, square the area
Open →Volume of a cube
Class 8 Maths
Grow the edge, cube the volume
Open →Circumference of a circle
Class 8 Maths
Grow the radius, roll out the rim
Open →Surface area of a cube
Class 9 Maths
Grow the edge, cover six faces
Open →Potential difference
Class 10 Physics
Share work across charge, get volts
Open →Resistance from Ohm's law
Class 10 Physics
Divide voltage by current, get resistance
Open →Echo and SONAR
Class 9 Physics
Time the echo, find the distance
Open →Mass from moles
Class 9 Chemistry
Multiply moles by molar mass
Open →Breathing rate
Class 10 Biology
Set breathing rate & time
Open →Volume of a cylinder
Class 10 Maths
Set radius & height, fill the can
Open →Compound interest
Class 8 Maths
Compound money over years
Open →Profit and loss percentage
Class 8 Maths
Set cost & selling price, see profit %
Open →Perimeter of a rectangle
Class 8 Maths
Set length & breadth, walk the border
Open →Surface area of a sphere
Class 10 Maths
Grow the radius, wrap the ball
Open →Time period
Class 9 Physics
Raise the frequency, shrink the period
Open →Relative velocity
Class 9 Physics
Two objects approach - add their speeds
Open →Average velocity
Class 9 Physics
Average the start and end speeds
Open →Equations of motion (v = u + at)
Class 9 Physics
Accelerate from u for a time t
Open →Kelvin to Celsius
Class 9 Chemistry
Slide Kelvin, read the Celsius
Open →Population growth rate
Class 10 Biology
Balance births vs deaths per population
Open →Perimeter of a square
Class 8 Maths
Grow the side, walk four edges
Open →Perimeter of a triangle
Class 8 Maths
Add the three sides
Open →Area of a parallelogram
Class 8 Maths
Set base & height, slide the shape
Open →Area of a rhombus
Class 8 Maths
Set the two diagonals
Open →Equations of motion (distance)
Class 9 Physics
Start, accelerate, cover ground
Open →Joule's law of heating
Class 10 Physics
Raise current, resistance or time
Open →Electric power (P = VI)
Class 10 Physics
Multiply voltage by current
Open →Average atomic mass of isotopes
Class 9 Chemistry
Mix two isotopes by abundance
Open →Seed germination percentage
Class 9 Biology
Count sprouted seeds out of the total
Open →Volume of a cone
Class 9 Maths
Set radius & height, fill the cone
Open →Surface area of a cylinder
Class 9 Maths
Wrap the side and both ends
Open →Surface area of a cuboid
Class 9 Maths
Cover all six rectangular faces
Open →nth term of an AP
Class 10 Maths
Step from the first term by d
Open →Sum of an AP
Class 10 Maths
Add up the first n terms
Open →Focal length of a mirror
Class 10 Physics
Halve the radius to find the focus
Open →Speed of light in a medium
Class 10 Physics
Raise the index, slow the light
Open →Percentage purity
Class 9 Chemistry
Weigh the pure part of a sample
Open →Slope of a line
Class 10 Maths
Rise over run gives the steepness
Open →Percentage change
Class 8 Maths
Compare a new value to the old
Open →Volume of a hemisphere
Class 9 Maths
Grow the radius of half a ball
Open →Area of a sector
Class 10 Maths
Cut a slice of angle from a circle
Open →Length of an arc
Class 10 Maths
Measure the curved edge of a slice
Open →Slant height of a cone
Class 9 Maths
Combine radius & height for the slant
Open →Unit conversion (km/h to m/s)
Class 9 Physics
Convert km/h into m/s
Open →Equations of motion (v^2 = u^2 + 2as)
Class 9 Physics
Accelerate over a distance, find v
Open →Impulse
Class 9 Physics
Hit harder or longer, change momentum
Open →Wavelength
Class 9 Physics
Speed over frequency gives wavelength
Open →Oscillations
Class 9 Physics
Vibrate at a frequency for a time
Open →Cost of electricity
Class 10 Physics
Units times rate gives the bill
Open →Number of neutrons
Class 9 Chemistry
Take protons away from the mass number
Open →Curved surface area of a cone
Class 9 Maths
Wrap the slanted side of a cone
Open →Total surface area of a cone
Class 9 Maths
Add the base circle to the cone's side
Open →Curved surface area of a hemisphere
Class 9 Maths
Cover the dome of a hemisphere
Open →Diagonal of a square
Class 9 Maths
Cross a square corner to corner
Open →Unit conversion (m/s to km/h)
Class 9 Physics
Convert m/s into km/h
Open →Distance from velocities
Class 9 Physics
From two speeds, find the distance
Open →Diagonal of a rectangle
Class 9 Maths
Cross a rectangle corner to corner
Open →Diagonal of a cuboid
Class 9 Maths
The longest rod that fits in a box
Open →Area by Heron's formula
Class 9 Maths
Area from just the three sides
Open →Interior angle sum of a polygon
Class 8 Maths
Add up a polygon's inside angles
Open →Exterior angle of a regular polygon
Class 8 Maths
Share 360 among a polygon's corners
Open →Number of diagonals of a polygon
Class 8 Maths
Count the diagonals of a polygon
Open →Discriminant
Class 10 Maths
Test how many roots a quadratic has
Open →Sum of roots
Class 10 Maths
Sum of a quadratic's roots
Open →Punnett square (monohybrid cross)
Class 10 Biology
Cross two parents, predict the offspring
Open →Balancing chemical equations
Class 10 Chemistry
Slide coefficients until atoms balance
Open →Writing chemical formulae (valency)
Class 9 Chemistry
Criss-cross valencies into a formula
Open →Current from power
Class 10 Physics
Divide power by voltage for current
Open →Power (P = V^2 / R)
Class 10 Physics
Voltage squared over resistance
Open →Sine ratio
Class 10 Maths
Opposite over hypotenuse
Open →Cosine ratio
Class 10 Maths
Adjacent over hypotenuse
Open →Tangent ratio
Class 10 Maths
Opposite over adjacent
Open →Area of an equilateral triangle
Class 9 Maths
Area of an equilateral triangle
Open →Curved surface area of a cylinder
Class 9 Maths
Wrap only the curved side
Open →Loss percentage
Class 8 Maths
Sell below cost, find the loss %
Open →Amount with simple interest
Class 8 Maths
Principal plus its simple interest
Open →Distance formula
Class 10 Maths
Straight distance between two points
Open →States of matter
Class 9 Chemistry
Heat particles solid → liquid → gas
Open →Parts of a plant cell
Class 8 Biology
Tap a cell part to see its job
Open →Diagonal of a cube
Class 9 Maths
Longest diagonal through a cube
Open →Total surface area of a hemisphere
Class 9 Maths
Dome plus its flat circle
Open →Sum of first n natural numbers
Class 10 Maths
Add 1 + 2 + ... + n instantly
Open →Range of data
Class 9 Maths
Spread from smallest to largest
Open →Class mark
Class 9 Maths
Midpoint of a class interval
Open →Selling price from profit percent
Class 8 Maths
Mark up cost by a profit %
Open →Perimeter of a sector
Class 10 Maths
Two radii plus the curved arc
Open →Circumference from diameter
Class 8 Maths
Circumference straight from diameter
Open →Power (P = F x v)
Class 9 Physics
Force times velocity gives power
Open →Percentage of a number
Class 8 Maths
Find a percentage of a number
Open →Series and parallel circuits
Class 10 Physics
Break a bulb in series vs parallel
Open →Symbols of elements
Class 9 Chemistry
Match each element to its symbol
Open →Free fall (velocity)
Class 9 Physics
Drop from a height, hit this speed
Open →Free fall (time)
Class 9 Physics
How long a drop takes
Open →Free fall (distance)
Class 9 Physics
Distance fallen in a given time
Open →Complement of an event
Class 10 Maths
Chance an event does NOT happen
Open →Product of roots
Class 10 Maths
Product of a quadratic's roots
Open →Exterior angle theorem
Class 9 Maths
Exterior angle = sum of remote interiors
Open →Complementary angles
Class 10 Maths
What adds to 90 degrees
Open →Supplementary angles
Class 9 Maths
What adds to 180 degrees
Open →Perimeter of a semicircle
Class 10 Maths
Curved half plus the diameter
Open →Area of a semicircle
Class 10 Maths
Half the area of a circle
Open →Turning effect (moments)
Class 9 Physics
Balance the see-saw with moments
Open →Reflex arc
Class 10 Biology
Step through a reflex, stimulus to action
Open →Buoyant force (upthrust)
Class 9 Physics
Displace liquid, feel the upthrust
Open →Relative density
Class 9 Physics
Compare a density to water's
Open →Power in lifting a load
Class 9 Physics
Lift a load, faster needs more power
Open →Cosecant ratio
Class 10 Maths
Hypotenuse over opposite
Open →Secant ratio
Class 10 Maths
Hypotenuse over adjacent
Open →Cotangent ratio
Class 10 Maths
Adjacent over opposite
Open →Height from angle of elevation
Class 10 Maths
Height from an angle of elevation
Open →Area of a quadrant
Class 10 Maths
A quarter of a circle's area
Open →Interior angle of a regular polygon
Class 8 Maths
One inside angle of a regular polygon
Open →Sum of first n odd numbers
Class 10 Maths
Add the first n odd numbers
Open →Sum of first n even numbers
Class 10 Maths
Add the first n even numbers
Open →Quadratic formula (a root)
Class 10 Maths
Larger root of a quadratic
Open →LCM from HCF
Class 10 Maths
LCM from the product and HCF
Open →Depreciation
Class 8 Maths
Value drops by a % each year
Open →Cost price from selling price
Class 8 Maths
Work back to the cost price
Open →Downstream speed
Class 8 Maths
Row with the current
Open →Upstream speed
Class 8 Maths
Row against the current
Open →Average speed for a round trip
Class 8 Maths
Average speed there and back
Open →Sales tax / GST
Class 8 Maths
Tax added on a price
Open →Area of a ring (annulus)
Class 10 Maths
Area of a ring between two circles
Open →Edge of a cube from volume
Class 9 Maths
Edge back from the volume
Open →Radius from area
Class 10 Maths
Radius back from a circle's area
Open →Side from area of a square
Class 8 Maths
Side back from a square's area
Open →Height of a triangle from area
Class 9 Maths
Height back from area and base
Open →Rate from simple interest
Class 8 Maths
Rate back from the interest
Open →Time from simple interest
Class 8 Maths
Time back from the interest
Open →Principal from simple interest
Class 8 Maths
Principal back from the interest
Open →Mean proportional
Class 10 Maths
Geometric mean of two numbers
Open →Fourth proportional
Class 8 Maths
Complete the proportion a : b = c : ?
Open →Marked price from selling price
Class 8 Maths
Marked price back from the sale price
Open →Chambers of the human heart
Class 10 Biology
Tap a heart chamber to see its job
Open →Equation of a line (y = mx + c)
Class 9 Maths
Read y off a straight line
Open →Average term of an AP
Class 10 Maths
Average of first and last term
Open →Number of terms in an AP
Class 10 Maths
How many terms in an AP
Open →Midpoint of two points
Class 10 Maths
x-coordinate of a midpoint
Open →Empirical mode
Class 10 Maths
Estimate the mode from mean & median
Open →Length of a shadow
Class 10 Maths
Shadow from height and sun angle
Open →Train crossing a pole
Class 8 Maths
Speed to cross a pole
Open →Time and work
Class 8 Maths
More workers, fewer days
Open →Dividing in a ratio
Class 8 Maths
Split a total in a ratio
Open →Unitary method
Class 8 Maths
Cost of a single item
Open →Your progress — Foundation - Class 8 to 10
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Showing 15 questions for Class 9. Tap a card to reveal the answer.
PhysicsGravitationmediumState the universal law of gravitation and write its formula.
Reveal answer ↓
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).
PhysicsGravitationmediumState Archimedes' principle. Why does an iron nail sink but an iron ship float?
Reveal answer ↓
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.
PhysicsGravitationmediumA block has a mass of 200 g and a volume of 50 cm^3. Find its density and decide if it floats in water.
Reveal answer ↓
What it is
Density is how much mass is packed into a given volume.
Answer
Density = mass / volume = 200 / 50 = 4 g/cm^3. Since 4 g/cm^3 is greater than the density of water (1 g/cm^3), the block sinks. Packing more mass into the same volume raises the density.
density = mass / volume
- •Density = mass / volume
- •Unit: g/cm^3 or kg/m^3
- •Denser than water (1 g/cm^3) -> sinks
Why learn this
It decides what floats or sinks and helps us identify materials.
💡 Memory trick
Density = mass / volume. Denser than water (1 g/cm^3) -> it sinks.
PhysicsGravitationeasyFind the weight of a 10 kg body on Earth (g = 9.8 m/s^2), then try the Moon. Slide g to explore.
Reveal answer ↓
What it is
Weight is the force of gravity on a body: its mass times the gravitational acceleration g.
Answer
Weight = mass x gravity = 10 x 9.8 = 98 newton (N) on Earth. On the Moon g is about 1.6 m/s^2, so the same body weighs only about 16 N, even though its mass is still 10 kg.
W = m x g
- •Weight = mass x g
- •Unit: newton (N)
- •Mass is constant; weight changes with g
Why learn this
It's why you weigh less on the Moon even though your mass is unchanged.
💡 Memory trick
W = m x g. Mass is fixed; weight follows g.
PhysicsGravitationmediumHow fast is a body moving after falling 20 m (g = 9.8 m/s^2)? Slide the height to explore.
Reveal answer ↓
What it is
A body dropped from rest gains a landing speed equal to the square root of two times g times the height.
Answer
v = sqrt(2 x g x h) = sqrt(2 x 9.8 x 20) = sqrt(392) = 19.8 m/s. This comes from v^2 = u^2 + 2as with the initial speed u = 0 and acceleration a = g. All objects gain the same speed regardless of mass (ignoring air resistance).
v = sqrt(2 g h)
- •v = sqrt(2 g h)
- •From v^2 = 2gh (u = 0)
- •Independent of mass
Why learn this
It shows how dangerous a fall from height can be, whatever the object's mass.
💡 Memory trick
v = sqrt(2gh), from v^2 = u^2 + 2as with u = 0 and a = g.
PhysicsGravitationmediumHow long does a body take to fall 20 m (g = 9.8 m/s^2)? Slide the height to explore.
Reveal answer ↓
What it is
The time a body takes to fall from rest is the square root of two times the height divided by g.
Answer
t = sqrt(2h / g) = sqrt(2 x 20 / 9.8) = sqrt(4.08) = 2.02 s. This is the fall equation h = 1/2 g t^2 rearranged for the time. Because of the square root, quadrupling the height only doubles the time.
t = sqrt(2h / g)
- •t = sqrt(2h / g)
- •From h = 1/2 g t^2
- •Time grows with the square root of height
Why learn this
It predicts how long a dropped object takes to reach the ground.
💡 Memory trick
t = sqrt(2h / g), from h = 1/2 g t^2.
PhysicsGravitationmediumHow far does a dropped body fall in 3 s (g = 9.8 m/s^2)? Slide the time to explore.
Reveal answer ↓
What it is
A body dropped from rest falls a distance equal to half g times the time squared.
Answer
s = 1/2 x g x t^2 = 1/2 x 9.8 x 3^2 = 1/2 x 9.8 x 9 = 44.1 m. Since the time is squared, the object covers much more distance in later seconds than in the first second.
s = 1/2 g t^2
- •s = 1/2 g t^2
- •For a drop from rest (u = 0)
- •Distance grows with time squared
Why learn this
It shows how quickly the distance builds up during a fall.
💡 Memory trick
s = 1/2 g t^2 (u = 0).
PhysicsGravitationmediumFind the upthrust on an object that displaces 10 L of water (density 1000 kg/m^3, g = 9.8). Slide to explore.
Reveal answer ↓
What it is
The buoyant force (upthrust) on a submerged object equals the weight of the liquid it displaces.
Answer
Upthrust = density x volume x g = 1000 x (10 / 1000) x 9.8 = 1000 x 0.01 x 9.8 = 98 N. By Archimedes' principle this equals the weight of the displaced liquid, so displacing more liquid gives a larger upthrust.
upthrust = density x volume x g
- •Upthrust = density x volume x g
- •Equals the weight of liquid displaced
- •Archimedes' principle
Why learn this
It's why ships float and why you feel lighter in a swimming pool.
💡 Memory trick
Upthrust = density x volume x g (Archimedes' principle).
PhysicsGravitationeasyFind the relative density of aluminium (density 2700 kg/m^3). Slide the density to explore.
Reveal answer ↓
What it is
Relative density is the density of a substance compared with the density of water.
Answer
Relative density = density of substance / density of water = 2700 / 1000 = 2.7. It has no unit because it is a ratio of two densities. A value above 1 means the substance sinks in water; below 1 it floats.
relative density = substance density / water density
- •Relative density = density / 1000
- •It has no unit (a ratio)
- •Above 1 sinks, below 1 floats
Why learn this
It quickly tells whether something floats or sinks in water.
💡 Memory trick
Relative density = substance density / water density (1000).
PhysicsGravitationmediumState Newton's universal law of gravitation and write its formula.
Reveal answer ↓
What it is
Every two masses attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Answer
Newton's 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 formula is F = G m1 m2 / r^2, where G is the universal gravitational constant, m1 and m2 are the masses and r is the distance between them.
F = G m1 m2 / r^2
- •F is proportional to m1 x m2
- •F is inversely proportional to r^2
- •F = G m1 m2 / r^2
- •G = 6.67 x 10^(-11) N m^2/kg^2
Why learn this
It explains the orbits of planets, the tides and why things fall to Earth.
💡 Memory trick
F = G m1 m2 / r^2. Double the distance -> one quarter the force (inverse square).
PhysicsGravitationmediumDifferentiate between mass and weight. Why is weight on the Moon less than on Earth?
Reveal answer ↓
What it is
Mass is the amount of matter in a body and stays constant; weight is the gravitational force on it and changes with g.
Answer
Mass is the quantity of matter contained in a body; it is a scalar, measured in kilograms, and remains constant everywhere. Weight is the force with which a body is attracted towards the centre of a planet; it is a vector given by W = mg, measured in newtons, and changes from place to place. On the Moon the acceleration due to gravity is about one-sixth of that on Earth, so an object's weight on the Moon is one-sixth of its weight on Earth, although its mass stays the same.
Weight W = m x g
- •Mass = matter, constant, kg
- •Weight = mg, varies with g, newton
- •Moon's g is about 1/6 of Earth's
- •Weight on Moon = 1/6 of weight on Earth
Why learn this
It explains why your weight on the Moon is one-sixth of that on Earth, but your mass is the same.
💡 Memory trick
Mass = matter (kg, constant). Weight = W = mg (newton, changes with gravity).
PhysicsGravitationmediumA stone is dropped from rest from a height. Find its velocity after 3 s. (Take g = 9.8 m/s^2.)
Reveal answer ↓
What it is
When a body falls only under gravity, it is in free fall with acceleration g (about 9.8 m/s^2).
Answer
In free fall the stone starts from rest, so u = 0, and its acceleration is g = 9.8 m/s^2. Using v = u + gt = 0 + 9.8 x 3 = 29.4 m/s. So the velocity after 3 seconds is 29.4 metres per second, directed downward.
v = u + g t (free fall)
- •Free fall: only gravity acts
- •a = g = 9.8 m/s^2, u = 0
- •v = u + gt = 9.8 x 3
- •Velocity = 29.4 m/s
Why learn this
It is why all objects, heavy or light, fall together in the absence of air.
💡 Memory trick
In free fall use the equations of motion with a = g = 9.8 m/s^2 and u = 0.
PhysicsGravitationeasyDefine thrust and pressure. State the SI unit of pressure.
Reveal answer ↓
What it is
Thrust is the force acting perpendicular to a surface; pressure is thrust per unit area.
Answer
Thrust is the force acting on a body perpendicular to its surface; its SI unit is the newton (N). Pressure is the thrust acting per unit area of the surface, given by pressure = thrust / area. The SI unit of pressure is the pascal (Pa), where 1 Pa = 1 N/m^2.
Pressure = thrust / area
- •Thrust = perpendicular force (N)
- •Pressure = thrust / area
- •SI unit of pressure = pascal (Pa)
- •1 Pa = 1 N/m^2
Why learn this
It explains why a sharp needle pierces easily but a wide strap spreads a load comfortably.
💡 Memory trick
Same thrust, smaller area -> larger pressure. Pressure = thrust / area.
PhysicsGravitationmediumState Archimedes' principle and define buoyancy.
Reveal answer ↓
What it is
A body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.
Answer
Buoyancy is the upward force exerted by a fluid on a body immersed in it, also called upthrust. Archimedes' principle states that when a body is immersed fully or partially in a fluid, it experiences an upward buoyant force that is equal to the weight of the fluid displaced by the body. This is why objects appear to lose weight in a liquid.
Buoyant force = weight of fluid displaced
- •Buoyancy = upward force (upthrust) of a fluid
- •Buoyant force = weight of fluid displaced
- •Objects seem lighter in a fluid
- •Explains floating and sinking
Why learn this
It explains why ships float and why we feel lighter in water.
💡 Memory trick
Buoyant force = weight of the fluid displaced (Archimedes' principle).
PhysicsGravitationmediumThe density of a substance is 800 kg/m^3. Find its relative density and state if it floats on water.
Reveal answer ↓
What it is
Relative density is the ratio of the density of a substance to the density of water; it decides floating or sinking.
Answer
Relative density = density of the substance / density of water = 800 / 1000 = 0.8. Since the relative density (0.8) is less than 1, the substance is less dense than water, so it will float on water. Relative density has no unit because it is a ratio of two densities.
Relative density = density of substance / density of water
- •RD = density of substance / density of water
- •800/1000 = 0.8
- •RD < 1 -> floats
- •Relative density has no unit
Why learn this
An object floats if its relative density is less than 1 and sinks if it is more than 1.
💡 Memory trick
RD < 1 floats, RD > 1 sinks. Relative density has no unit (it is a ratio).
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