Everyday Physics Questions CBSE Class 9 Physics Worksheets Set 02

Questions with Answers

Question 1. The frozen Arctic seas are navigable. How?
Answer:
Ice forms only as a layer on the surface and the upper part of the sea. Below it there is liquid sea water. Powerful ships called icebreakers cut and break through the ice layer and clear a channel for other ships.
In simple words: Only the top layer of the sea freezes, so strong ships can break through it.

Teacher's Note:
1. Ice is less dense than water, so it floats on top.
2. Sea water freezes at a temperature lower than 0 °C because of the dissolved salts.

Question 2. Why are ventilators provided high up on the walls of a room near the ceiling?
Answer:
Air in a room gets warm by the breathing of people and by other sources. Warm air is lighter and rises, so it goes out through the ventilators near the ceiling, while cool fresh air enters from the doors and windows below. This sets up convection currents that keep the air fresh.
In simple words: Warm, used air rises and escapes through the high openings; fresh air comes in from below.

Teacher's Note:
1. Convection currents are caused by differences in density.
2. Ventilators should be near the ceiling, not near the floor.

Question 3. Two glass tumblers have stuck to each other. Suggest an easy way of separating them.
Answer:
Pour cold water into the inner tumbler and keep the outer one in warm water (or run warm water over the outer tumbler). The inner tumbler contracts and the outer one expands slightly, so the two come apart.
In simple words: Cool the inner glass and warm the outer glass so that they come loose.

Teacher's Note:
1. Solids contract on cooling and expand on heating.
2. Do this gently as glass can crack with sudden temperature changes.

Question 4. Does water expand or contract when heated from 0 °C to 4 °C?
Answer:
Water contracts when heated from 0 °C to 4 °C, so its density increases and is maximum at 4 °C. Above 4 °C it expands like other liquids.
In simple words: Water shrinks between 0 °C and 4 °C.

Teacher's Note:
1. This unusual behaviour is called the anomalous expansion of water.
2. The bottom of a frozen lake stays at about 4 °C, so fish can survive.

Question 5. A steam engine and a motor car engine have the same mechanical arrangement of cylinder and piston. What is the point of difference between them?
Answer:
In a steam engine fuel is burnt outside the cylinder (in a boiler) to make steam, which then pushes the piston: it is an external combustion engine. In a motor car engine the fuel is burnt inside the cylinder itself: it is an internal combustion engine.
In simple words: Steam engines burn fuel outside the cylinder; car engines burn it inside.

Teacher's Note:
1. Internal combustion engines are lighter and more efficient.
2. Both change heat energy into mechanical energy.

Question 6. What type of lens is used to correct myopia (short-sightedness)?
Answer:
A concave (diverging) lens. In myopia the image of a distant object forms in front of the retina. A concave lens of suitable power diverges the rays slightly so that the image is formed on the retina.
In simple words: A concave lens corrects short-sightedness.

Teacher's Note:
1. Hypermetropia (long sight) is corrected by a convex lens.
2. A myopic eye can see near objects clearly but not distant ones.

Question 7. Why is a dam built across a river thicker at the bottom than at the top?
Answer:
The pressure of water increases with depth, so the water exerts a much larger force on the lower part of the wall. The dam is made thicker at the bottom to withstand this large pressure.
In simple words: Pressure rises with depth, so the bottom of the wall must be stronger.

Teacher's Note:
1. Pressure in a liquid = depth x density x g.
2. The same reasoning is used for the walls of water tanks.

Question 8. Why is there a vacuum between the two walls of a thermos flask?
Answer:
Heat cannot travel through a vacuum by conduction or convection. The vacuum between the double walls therefore prevents heat from passing in or out. The walls are also silvered to reflect radiation, so hot things stay hot and cold things stay cold.
In simple words: A vacuum stops heat conduction and convection.

Teacher's Note:
1. A thermos flask reduces all three modes of heat transfer.
2. The silver coating reduces heat loss by radiation.

Question 9. Why are detergents preferred to soaps for cleaning?
Answer:
Soaps form insoluble scum with the calcium and magnesium salts in hard water and do not lather well. Detergents do not react with these salts, so they give lather and clean even in hard water.
In simple words: Detergents work in hard water, soaps do not.

Teacher's Note:
1. Hard water contains calcium and magnesium salts.
2. Some detergents are not biodegradable and can pollute water.

Question 10. Though hydrogen is lighter than helium, the latter is preferred for filling balloons. Why?
Answer:
Hydrogen is highly inflammable and forms explosive mixtures with air. Helium is inert and does not burn, so it is safe, though a little heavier than hydrogen (twice as dense), and it still gives good lift.
In simple words: Helium is safe because it does not burn; hydrogen can explode.

Teacher's Note:
1. Weather balloons and advertising balloons use helium.
2. Safety matters more than the small gain in lift.

Question 11. A cup full of water at 4 °C overflows when heated or cooled. Explain.
Answer:
Water has its maximum density, and therefore minimum volume, at 4 °C. If it is heated above 4 °C or cooled below 4 °C, its volume increases. Since the cup is already full, the extra water overflows.
In simple words: Water has least volume at 4 °C, so any temperature change makes it expand.

Teacher's Note:
1. Water behaves anomalously between 0 °C and 4 °C.
2. The overflow is due to expansion of water more than of the cup.

Question 12. Why does blotting paper absorb ink?
Answer:
Blotting paper is made of fibres with a large number of very fine pores that act as capillary tubes. Ink rises through these tiny tubes by capillary action and is absorbed.
In simple words: The tiny pores draw ink up by capillary action.

Teacher's Note:
1. Capillary rise happens in very narrow tubes.
2. Oil rising in a lamp wick is another example.

Question 13. A ship sails from a river into the sea. Will the ship's load line remain the same?
Answer:
No. Sea water is denser than river water. A ship floats by displacing its own weight of water, so in the denser sea water it needs to displace less volume. The ship therefore rises a little, and the load line is higher above the water level in the sea.
In simple words: The ship floats slightly higher in the sea than in the river.

Teacher's Note:
1. Weight of displaced liquid = weight of the ship.
2. That is why separate load marks are drawn for fresh and salt water.

Question 14. The surface of a sunglass is spherical in shape like the lenses in spectacles. Why does it not have any optical power?
Answer:
Both surfaces of a sunglass have the same curvature and the glass is thin, so it acts like a thin concave-convex shell of uniform thickness. The converging and diverging effects cancel, giving almost zero power.
In simple words: Equal curvature on both faces cancels the bending of light.

Teacher's Note:
1. Power of a lens depends on the difference of curvatures of its two faces.
2. Spectacle lenses have unequal curvatures.

Question 15. Cotton shirts are preferred to polyester ones in summer. Why?
Answer:
Cotton is porous, absorbs sweat and allows air to pass. The sweat then evaporates and cools the body. Polyester absorbs little sweat and allows poor circulation of air, so it feels hot.
In simple words: Cotton absorbs sweat and allows air through.

Teacher's Note:
1. Evaporation of sweat cools our body.
2. Synthetic fibres are good for rainy weather because they dry quickly.

Question 16. A nail can be driven into a piece of wood but not into glass. Why?
Answer:
Wood is soft and made of fibres that can be pushed aside, while glass is very hard and brittle. A nail hammered on glass cannot push the glass particles apart; the glass breaks instead.
In simple words: Wood gives way to the nail; hard, brittle glass breaks.

Teacher's Note:
1. Hardness of a material resists penetration.
2. A pointed nail gives a large pressure on a small area.

Question 17. For the headlight of a motor car, parabolic reflectors are used rather than spherical ones. Why?
Answer:
A bulb placed at the focus of a parabolic mirror gives a strong, parallel beam which travels far without spreading. A spherical mirror suffers from spherical aberration: rays from the bulb are not exactly parallel after reflection.
In simple words: A parabolic mirror turns light from its focus into a parallel beam.

Teacher's Note:
1. Rays parallel to the axis meet at the focus; the reverse also holds.
2. A searchlight uses the same idea.

Question 18. A submarine is able to submerge in water and come out to the surface. What helps it to do this?
Answer:
A submarine has ballast tanks. To dive, the tanks are filled with water, so the weight increases and the submarine sinks. To surface, compressed air forces the water out, so the weight decreases and it rises.
In simple words: Filling or emptying ballast tanks changes its weight.

Teacher's Note:
1. A body sinks if its weight exceeds the buoyant force.
2. The depth is also controlled with horizontal fins.

Question 19. Electric bulbs have either vacuum inside or are filled with inert gases. Why?
Answer:
The tungsten filament becomes extremely hot. If air were present, it would burn (oxidise) and the filament would quickly break. In vacuum or in an inert gas such as argon, it cannot oxidise, so the filament lasts long.
In simple words: No oxygen means the hot filament cannot burn.

Teacher's Note:
1. Tungsten has a very high melting point.
2. Inert gas also reduces evaporation of the filament.

Question 20. Does a balloon filled with hydrogen go on rising up endlessly?
Answer:
No. As the balloon rises, air becomes less dense, so the buoyant force decreases. At a certain height the weight of the balloon becomes equal to the weight of the air displaced and the balloon stops rising. The balloon can also burst when it expands in the low external pressure.
In simple words: It rises only until the buoyant force equals its weight.

Teacher's Note:
1. Air density decreases with height.
2. Balloons can burst at great heights.

Question 21. A stationary carriage is pushed from inside by the passengers. Will the carriage move?
Answer:
No. The push of the passengers on the carriage is an internal force. By Newton's third law the carriage pushes back on the passengers with an equal and opposite force. A body cannot be moved by internal forces alone; an external unbalanced force is needed.
In simple words: Internal forces cancel in pairs, so the carriage does not move.

Teacher's Note:
1. Action and reaction act on different bodies, but within the system they cancel.
2. Sitting in a car and pushing the dashboard does not move the car.

Question 22. How do bats manage to fly in the dark without hitting obstacles?
Answer:
A bat sends out ultrasonic waves (frequency above 20,000 Hz) and listens for the echoes reflected from obstacles and insects. From the echoes it finds the distance and position of objects. This method is called echolocation.
In simple words: Bats use echoes of ultrasonic sounds to sense things around them.

Teacher's Note:
1. Ultrasonic waves are reflected even from small objects.
2. Radar and sonar work on the same principle.

Question 23. What is the purpose of using a single fixed pulley, even though its mechanical advantage is only one?
Answer:
A single fixed pulley only changes the direction of the force. It is easier and more convenient to pull downwards (using our body weight) than to lift a load upwards, as when drawing water from a well.
In simple words: It changes the direction of effort to a more convenient one.

Teacher's Note:
1. Mechanical advantage of a fixed pulley is 1.
2. A movable pulley gives a mechanical advantage of 2.

Question 24. What are the effects of ultraviolet rays of the Sun on human life?
Answer:
In small amounts, ultraviolet rays help the skin make vitamin D and produce melanin (the skin pigment). In large doses they damage skin cells, cause sunburn and skin cancer and harm the eyes.
In simple words: A little UV is useful; too much is harmful.

Teacher's Note:
1. The ozone layer absorbs most of the harmful UV rays.
2. Use sunscreen and sunglasses in strong sunlight.