Everyday Physics Questions CBSE Class 9 Physics Worksheets Set 03

Questions with Answers

Question 1. Why are springs used in cars?
Answer:
Roads are never perfectly even. When a wheel meets a bump, the spring compresses and absorbs the shock and then slowly returns to its shape. This increases the time of the jerk, so the force transmitted to the passengers is small and the ride is comfortable.
In simple words: Springs absorb the bumps of the road.

Teacher's Note:
1. Longer time of impact means a smaller force.
2. Shock absorbers are used along with springs.

Question 2. Soap is used as a lubricant when iron nails are driven into wood. Why?
Answer:
Oil or water would be soaked up by wood. Soap is a solid, slippery material that reduces the friction between the nail and the wood, so the nail goes in more easily.
In simple words: Soap lowers friction and is not absorbed by wood.

Teacher's Note:
1. A lubricant reduces friction between surfaces in contact.
2. Graphite and wax are other solid lubricants.

Question 3. Glaciers melt at the bottom. Why?
Answer:
The huge weight of the glacier presses on the ice at the bottom. High pressure lowers the melting point of ice, so the bottom ice melts at a temperature below 0 °C. This water helps the glacier to slide.
In simple words: High pressure lowers the melting point of ice.

Teacher's Note:
1. Ice is one of the few substances that melt at a lower temperature under pressure.
2. The same effect lets a skater glide on a thin water film.

Question 4. If a body is taken from the earth to the moon, will there be any change in its mass and/or weight?
Answer:
The mass stays the same because it is the amount of matter in the body. The weight changes: since the moon's gravity is about one-sixth of the earth's, the weight on the moon is about one-sixth of that on the earth.
In simple words: Mass is unchanged; weight becomes about one-sixth.

Teacher's Note:
1. Weight = mass x acceleration due to gravity.
2. Mass is measured in kg, weight in newton.

Question 5. When a cube of ice floating on water in a beaker melts, the level of water in the beaker (a) rises, (b) falls, (c) remains the same, or (d) first rises and then falls?
Answer:
Floating ice displaces water equal to its own weight. When it melts, it becomes water of exactly the same weight, whose volume equals the volume of water that was displaced.
The level of water remains the same.
In simple words: The melted water fills exactly the space that the floating ice had displaced.

Teacher's Note:
1. A floating body displaces liquid equal to its weight.
2. If ice were resting on the bottom, the level would fall on melting.

Question 6. Steam produces greater burns than boiling water at the same temperature. Why?
Answer:
Steam at 100 °C contains extra heat, the latent heat of vaporisation (about 2260 J for every gram). When steam condenses on the skin, it gives out this heat in addition to the heat given by the hot water formed, so the burn is more severe.
In simple words: Steam carries extra hidden (latent) heat that it gives to the skin on condensing.

Teacher's Note:
1. Latent heat is absorbed or released without a change of temperature.
2. Water at 100 °C and steam at 100 °C have the same temperature but different heat contents.

Question 7. The flame from a Bunsen burner does not pass through a metal wire gauze placed across it for some time. Explain.
Answer:
Metal is a good conductor of heat. The gauze conducts heat away from the gas so quickly that the gas above the gauze does not reach its ignition temperature, and the flame does not pass through. After the gauze itself becomes hot, the flame may pass.
In simple words: The gauze removes heat so the gas above stays below its ignition temperature.

Teacher's Note:
1. The principle was used in the miner's safety lamp.
2. Burning needs the fuel to reach its ignition temperature.

Question 8. Why do stars twinkle?
Answer:
Light from a star passes through layers of air of different densities, which are always moving. The light is refracted irregularly, so the apparent position and brightness of the star keep changing slightly, and the star seems to twinkle. Planets are nearer and look like small discs, so they do not twinkle.
In simple words: Constantly changing refraction in the air makes the starlight flicker.

Teacher's Note:
1. Stars are point sources of light.
2. Atmospheric refraction is responsible for twinkling.

Question 9. Why does the sky appear blue?
Answer:
Sunlight is scattered by the molecules of air. Light of short wavelength (blue, violet) is scattered much more than light of long wavelength (red). The scattered light reaching our eyes is mainly blue, so the sky looks blue.
In simple words: Blue light is scattered more by the air than red light.

Teacher's Note:
1. Scattering is greater for shorter wavelengths.
2. On the moon there is no atmosphere, so the sky looks black.

Question 10. What is a rainbow?
Answer:
A rainbow is a band of the seven colours, VIBGYOR, seen in the sky after rain when the Sun is behind the observer. Sunlight is refracted, dispersed and internally reflected by tiny water droplets in the air.
In simple words: A natural spectrum formed by sunlight passing through raindrops.

Teacher's Note:
1. Dispersion splits white light into its colours.
2. The observer must stand with the Sun behind him.

Question 11. A radio receiver using valves or the picture tube of a T.V. set does not start immediately on switching on, whereas a set having transistors operates immediately. Why?
Answer:
Valves and picture tubes depend on thermionic emission: their filaments must first become hot to release electrons. This takes some time. Transistors and other solid-state devices work without heating, so they operate immediately.
In simple words: Valves need time to heat up; transistors do not.

Teacher's Note:
1. Thermionic emission is the release of electrons from a hot metal.
2. Solid-state devices also use less power.

Question 12. What is Brownian movement?
Answer:
It is the continuous, rapid and random zig-zag motion of very small particles suspended in a liquid or a gas, seen under a microscope. It is caused by the unequal bombardment of the particles by the moving molecules of the liquid.
In simple words: Tiny particles in a fluid move at random because molecules keep hitting them.

Teacher's Note:
1. It is evidence that matter is made of moving particles.
2. It was first noticed in pollen grains in water.

Question 13. What are communication satellites?
Answer:
They are artificial satellites placed in geostationary orbit that receive signals from a ground station, amplify them and send them back to another place on the earth. They carry solar panels for power. They are used for telephone, television, radio and internet signals over long distances.
In simple words: Satellites that relay signals over long distances.

Teacher's Note:
1. A geostationary orbit is about 36,000 km above the equator.
2. India's INSAT series is a set of communication satellites.

Question 14. What is the difference between a star and a planet?
Answer:
A star is a huge, hot ball of gas which produces its own heat and light by nuclear fusion, for example the Sun. A planet is much smaller, has no light of its own, shines by reflected sunlight and moves round a star.
In simple words: Stars make their own light; planets only reflect it.

Teacher's Note:
1. The Earth is a planet of the Sun.
2. Stars twinkle, planets generally do not.

Question 15. Which is the most massive planet and who gave the laws of planetary motion?
Answer:
Jupiter is the most massive planet of the solar system. The laws of planetary motion were given by Johannes Kepler.
In simple words: Jupiter is the heaviest planet; Kepler gave the laws of planetary motion.

Teacher's Note:
1. Kepler's laws describe how planets orbit the Sun.
2. Jupiter's mass is more than twice the mass of all other planets together.

Question 16. What are asteroids?
Answer:
Asteroids are small rocky bodies which revolve round the Sun mostly in a belt between the orbits of Mars and Jupiter.
In simple words: Rocky bodies in orbit between Mars and Jupiter.

Teacher's Note:
1. Asteroids are also called minor planets.
2. Meteors are small pieces that burn up in the atmosphere.

Question 17. What is meant by Mach number?
Answer:
Mach number is the ratio of the speed of an object to the speed of sound in the same medium. A speed equal to the speed of sound (about 330 m/s in air) is Mach 1, and twice that speed is Mach 2.
In simple words: Speed of the object divided by the speed of sound.

Teacher's Note:
1. Aircraft faster than Mach 1 are supersonic.
2. Mach number has no unit.

Question 18. How can we find direction at sea?
Answer:
Sailors use a magnetic compass whose needle always points along the north-south direction, and in big ships also a gyrocompass. Pole star and other stars are also used for finding direction.
In simple words: A compass shows the north-south direction.

Teacher's Note:
1. The needle aligns with the earth's magnetic field.
2. A bar magnet suspended freely also settles north-south.

Question 19. What is the name of the central region of the Sun which gives out light?
Answer:
The visible surface of the Sun that emits the light we receive is called the photosphere.
In simple words: The shining surface layer of the Sun is the photosphere.

Teacher's Note:
1. The core of the Sun is where nuclear fusion takes place.
2. The corona is the outer layer seen during a total solar eclipse.