For Class 8, 9 & 10
Foundation - a strong base in Physics, Chemistry, Maths & Biology
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Showing 6 questions in Physics 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.
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'.
PhysicsForce and Laws of MotionmediumState 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'.
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).
PhysicsWork and EnergymediumDefine 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.
PhysicsSoundmediumWhat 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.
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.