Class 9 Physics — Important Questions with Answers

6 concept-first Class 9 Physics questions across 5 chapters — each with a clear model answer, the why behind it, and a memory trick to make it stick. NCERT-aligned and free. Build the base now, and your board exams, NEET and JEE feel a whole lot easier later.

6 questions•5 chapters•Concept-first•NCERT-aligned•For boards, NEET & JEE

Motion (1)Force and Laws of Motion (1)Gravitation (2)Work and Energy (1)Sound (1)

Motion1 question

PhysicsEquations of motionmedium

A car starting from rest accelerates uniformly at 2 m/s^2 for 5 seconds. Find its final velocity and the distance travelled.

Reveal answer▾

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'.

Force and Laws of Motion1 question

PhysicsNewton's second law and momentummedium

State Newton's second law of motion and define momentum.

Reveal answer▾

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'.

Gravitation2 questions

PhysicsUniversal law of gravitationmedium

State 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).

PhysicsBuoyancy and Archimedes' principlemedium

State 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.

Work and Energy1 question

PhysicsWork and kinetic energymedium

Define work done by a force and state the formula for kinetic energy.

Reveal answer▾

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.

Sound1 question

PhysicsEchomedium

What is an echo, and what is the minimum distance needed to hear one clearly?

Reveal answer▾

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.

Start early. Stay ahead.

A few Class 9 Physics questions a day and the concepts become second nature. Explore every subject for Class 9 in the interactive Foundation bank, or ask the AI tutor any doubt the moment it shows up.

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