Everyday Physics Questions CBSE Class 9 Physics Worksheets Set 06

Questions with Answers

Question 1. Why is it difficult to walk on sand?
Answer:
Walking depends on friction: the ground pushes our foot forward only if it can resist the backward push of the foot. Loose sand grains slide and give way under the foot, so the ground cannot provide a firm opposing push and every step wastes effort.
In simple words: Loose sand gives way under your foot, so there is little grip to push against.

Teacher's Note:
1. Friction between foot and ground is what lets us move forward.
2. A firm, rough surface gives a better grip than loose sand or a smooth floor.

Question 2. Why does sugar dissolve more readily in hot water than in cold water?
Answer:
In hot water the water molecules move faster and collide with the sugar crystals more often and with more energy, pulling sugar molecules away from the crystal quickly. The solubility of sugar also increases with temperature.
In simple words: Hot water has faster-moving molecules, so it breaks up sugar faster and can hold more of it.

Teacher's Note:
1. Rate of dissolving and solubility both rise with temperature for most solids.
2. Stirring also speeds up dissolving because it brings fresh water to the sugar.

Question 3. Does the volume of water increase if salt is added to it?
Answer:
Practically no. Salt particles fit into the gaps between water molecules, so a small amount of salt dissolves without a noticeable rise in the volume of the liquid. (The mass, however, increases.)
In simple words: Dissolved salt hides in the spaces between water molecules, so the level hardly changes.

Teacher's Note:
1. This supports the idea that matter is made of particles with space between them.
2. Mass always increases on dissolving, but the volume change is very small.

Question 4. How is a mirage formed?
Answer:
On a hot day, especially in deserts or on a tarred road, the layers of air close to the ground become hotter and less dense than the layers above. Light from a distant object (such as the sky or a tree) travelling downwards passes from denser to rarer air, bends away from the normal and undergoes total internal reflection. The observer sees an inverted image that looks like a pool of water.
In simple words: Light from the sky bends upward through layers of hot air, so the sky appears as a shimmering water surface on the ground.

Teacher's Note:
1. A mirage is caused by refraction with total internal reflection, not by water.
2. Optical density of air falls as its temperature rises.

Question 5. What is a burning glass?
Answer:
A burning glass is a convex lens. It converges parallel rays of the Sun to a bright spot at its focus. Paper kept at this point gets heated and catches fire.
In simple words: A convex lens that focuses sunlight on a point hot enough to burn paper.

Teacher's Note:
1. Rays parallel to the principal axis meet at the principal focus of a convex lens.
2. Never look at the Sun through a lens.

Question 6. What does radar stand for and what is its use?
Answer:
Radar stands for Radio Detection And Ranging. It sends out short pulses of radio waves (microwaves) and receives the echo from an object. The time taken for the echo to return gives the distance, so radar is used to locate aircraft, ships and storm clouds.
In simple words: Radar uses reflected radio waves to find where distant objects are.

Teacher's Note:
1. Distance = (speed of wave x time for echo) / 2.
2. Radar and bat echolocation work on the same reflection principle.

Question 7. Why cannot jet engines be used to propel vehicles in outer space?
Answer:
A jet engine burns its fuel with oxygen taken from the surrounding air. There is no air in outer space, so the fuel cannot burn. Rockets carry their own oxidiser along with the fuel and therefore work in space.
In simple words: Jet engines need air to burn fuel; space has none, so rockets which carry their own oxygen supply are used.

Teacher's Note:
1. Rockets work on Newton's third law: the exhaust gases are pushed back and the rocket moves forward.
2. A rocket does not need air to push against.

Question 8. A passenger in a fast moving bus falls forward when the bus stops suddenly. Why?
Answer:
The passenger moves with the bus. When the bus stops suddenly, the feet, which are in contact with the floor, stop with it, but the upper part of the body tends to continue in its state of motion because of inertia, so the person falls forward.
In simple words: The body wants to keep moving even though the bus has stopped: this is inertia.

Teacher's Note:
1. This is Newton's first law, the law of inertia.
2. The same reason makes a passenger lean backward when a bus suddenly starts.

Question 9. What is the meaning of 'Eureka'?
Answer:
'Eureka' is a Greek word meaning 'I have found it'. Archimedes is said to have shouted it when he found, in his bath tub, that a body immersed in a liquid loses weight equal to the weight of liquid it displaces.
In simple words: It means 'I have found it', shouted by Archimedes on discovering buoyancy.

Teacher's Note:
1. Archimedes' principle explains the floating and sinking of bodies.
2. The word is used today for any sudden discovery.

Question 10. Why does a car consume more fuel in travelling the same distance on a crowded road than on a free road?
Answer:
On a crowded road the car has to stop and start again and again. Each time it speeds up, the engine must supply extra force and energy to accelerate the car, which comes from fuel. On a free road the car moves at nearly uniform speed and needs much less fuel for the same distance.
In simple words: Repeated starting and stopping needs repeated acceleration, which burns more fuel.

Teacher's Note:
1. A steady speed needs force only to overcome friction and air resistance.
2. Smooth driving saves fuel.

Question 11. How can we test the purity of milk?
Answer:
A lactometer is used. It is a hydrometer that floats in milk. Pure milk has a definite density, so it floats up to a particular mark. If water has been added, the density decreases and the lactometer sinks deeper.
In simple words: A floating instrument (lactometer) sinks deeper in diluted milk than in pure milk.

Teacher's Note:
1. A hydrometer measures density from the depth to which it sinks.
2. Adding water lowers the density of milk.

Question 12. What is the Plimsoll line?
Answer:
It is a mark painted on the hull of a ship showing the maximum safe level up to which the ship may be loaded. The density of water differs in rivers, seas and at different temperatures, so the marks help to see that the ship does not sink too deep.
In simple words: A load-line mark on a ship's side that shows how deep it may safely sink.

Teacher's Note:
1. Sea water is denser than river water, so a ship floats higher in sea water.
2. Overloading beyond this mark is unsafe.

Question 13. A person carrying a weight in his left hand bends to the right. Why?
Answer:
The weight in the left hand shifts the combined centre of gravity of the man and the load to the left. To bring the vertical line through the centre of gravity back within the base (feet) and avoid toppling, he bends to the right.
In simple words: He bends so that the combined centre of gravity stays over his feet.

Teacher's Note:
1. A body is stable if the vertical line from its centre of gravity falls inside its base.
2. A person carrying a load on the back bends forward for the same reason.

Question 14. What is the apparatus used for measuring air pressure?
Answer:
A barometer. The mercury barometer (Fortin's barometer) is the standard one. The atmosphere supports a column of mercury about 76 cm high at sea level.
In simple words: The instrument is called a barometer.

Teacher's Note:
1. Normal atmospheric pressure is 76 cm of mercury.
2. Barometer readings are used in weather forecasting.

Question 15. What is the instrument that measures atmospheric pressure without using any liquid?
Answer:
The aneroid barometer. 'Aneroid' means 'without liquid'. It has a sealed, partly evacuated metal box whose lid moves in and out with changes in air pressure, and this movement turns a pointer on a dial.
In simple words: It is the aneroid barometer, which uses a metal box instead of mercury.

Teacher's Note:
1. It is portable and is used in aeroplanes after being marked in heights.
2. It is less accurate than a mercury barometer.

Question 16. Why does ice float in water?
Answer:
When water freezes, its volume increases, so ice has a lower density (about 0.9 g per cm3) than water (1 g per cm3). A body whose density is less than that of the liquid floats, with about nine-tenths of the ice below the surface.
In simple words: Ice is lighter than the same volume of water, so it floats.

Teacher's Note:
1. A body floats if its density is less than the density of the liquid.
2. Floating of ice protects aquatic life in cold regions.

Question 17. Why does a satellite orbiting the earth sometimes appear stationary?
Answer:
A satellite which revolves above the equator at a height of about 36,000 km takes exactly 24 hours for one revolution, the same time as the Earth takes to rotate once. Seen from the ground it stays over the same spot. Such a satellite is called geostationary.
In simple words: Its orbital period equals the Earth's rotation period, so it stays above one point.

Teacher's Note:
1. Communication satellites are geostationary.
2. The satellite moves, but the observer on Earth moves with the same angular speed.

Question 18. Why has a drawing pin a broad head and a pointed tip?
Answer:
Pressure = force / area. The broad head spreads the thumb force over a large area so it does not hurt. The sharp tip has a very small area, so the same force produces a very large pressure that easily pierces the board.
In simple words: Large area at the head means low pressure on the thumb; tiny area at the tip means high pressure on the board.

Teacher's Note:
1. Pressure is inversely proportional to the area on which a force acts.
2. The same idea is used in nails, knives and needles.

Question 19. Can a thermometer be used to measure the quantity of heat?
Answer:
No. A thermometer measures temperature, which tells how hot or cold a body is. The quantity of heat depends on mass, specific heat and temperature change and is measured with a calorimeter in joules or calories.
In simple words: A thermometer gives temperature, not the amount of heat.

Teacher's Note:
1. Heat is energy; temperature is the degree of hotness.
2. A bucket of warm water has more heat than a cup of the same water at the same temperature.

Question 20. Air exerts pressure not only on the earth's surface but on all objects on the earth. But we do not feel the atmospheric pressure. Why?
Answer:
The fluids and tissues inside our body are also at nearly the same pressure as the atmosphere, so the pressure from inside balances the pressure from outside. The net force is zero and we do not feel it.
In simple words: Pressure inside the body equals the pressure outside, so the two cancel.

Teacher's Note:
1. Atmospheric pressure at sea level is about 10 to the power 5 pascals.
2. At great heights the outside pressure falls and people may feel discomfort in the ears.

Question 21. Why is it easier to cut a fruit with a sharp knife than with a blunt one?
Answer:
A sharp knife has a very thin edge, so its area of contact with the fruit is small. For the same force the pressure is very large and the knife cuts easily. A blunt knife has a larger contact area and gives less pressure.
In simple words: Smaller area means greater pressure for the same force.

Teacher's Note:
1. Pressure = force / area.
2. Knives are sharpened to reduce the edge area.

Question 22. At what temperature do the Fahrenheit and Celsius scales give the same reading?
Answer:
Let the common reading be x. Then F = (9/5)C + 32 gives x = (9/5)x + 32.
So -(4/5)x = 32 and x = -40.
At -40 degrees (that is, -40 °C = -40 °F).
In simple words: Both scales read -40 at the same temperature.

Teacher's Note:
1. Use the relation (C/5) = (F - 32)/9.
2. Set C = F and solve the linear equation.

Question 23. A solid ball of metal has a spherical cavity inside it. When the ball is heated, will the volume of the cavity increase?
Answer:
Yes. On heating, every part of a solid expands as if it were made of the same material, including the empty space. The cavity expands exactly as a solid sphere of the same metal of that size would, so its volume increases.
In simple words: The cavity expands just like the metal around it.

Teacher's Note:
1. A hole in a metal plate gets larger when the plate is heated.
2. Expansion depends on the material, not on whether a space is filled.

Question 24. A thick glass tumbler is liable to crack when hot water is poured into it. Why?
Answer:
Glass is a poor conductor of heat. The inner surface becomes hot and expands, while the outer surface is still cold and does not expand. This unequal expansion produces stress that can crack the thick glass.
In simple words: The inside expands before the outside, so the glass breaks.

Teacher's Note:
1. Thin glass heats through quickly, so it is less likely to crack.
2. Borosilicate glass expands very little and is used for laboratory vessels.

Question 25. Why are tea pots highly polished?
Answer:
A polished (shiny) surface is a poor radiator and a good reflector of heat. It loses little heat by radiation, so the tea inside stays hot for a longer time.
In simple words: Shiny surfaces lose heat slowly by radiation.

Teacher's Note:
1. Dull black surfaces are good absorbers and radiators of heat.
2. That is why a kettle meant to keep water hot is polished.

Question 26. A stone floor feels cold to bare feet but a carpet on the same floor feels warm. Why?
Answer:
Both are at the same temperature. Stone is a good conductor of heat, so it takes heat quickly from our feet and feels cold. A carpet is a poor conductor, takes very little heat from the feet and feels warm.
In simple words: Stone conducts heat away from the feet faster than a carpet does.

Teacher's Note:
1. Our sense of hot and cold depends on how fast heat flows, not only on temperature.
2. Air trapped in a carpet makes it a poor conductor.

Question 27. When a little ether is spilt on the hand, it feels cold. Why?
Answer:
Ether evaporates very quickly. The heat needed for evaporation (latent heat) is taken from the hand, so the skin cools and the hand feels cold.
In simple words: Fast evaporation of ether takes heat from your skin.

Teacher's Note:
1. Evaporation causes cooling.
2. Liquids that evaporate quickly give a stronger cooling effect.

Question 28. When we are sweating, why do we feel comfortable under a fan?
Answer:
Moving air from the fan carries away water vapour above the skin, which speeds up the evaporation of sweat. Evaporation takes latent heat from the body, so we feel cool.
In simple words: A fan speeds up the evaporation of sweat, which cools the body.

Teacher's Note:
1. Rate of evaporation increases with air movement.
2. A fan does not lower the room temperature; it helps the body lose heat.

Question 29. Why cannot television stations transmit their regional programmes to far away places?
Answer:
Television signals have very high frequency and travel in straight lines. They pass through the ionosphere instead of being reflected back, and the curvature of the earth blocks the signal at larger distances. So the range is limited to the line of sight. Relay towers and communication satellites are used to cover far-off places.
In simple words: TV waves go straight and are not bent back by the atmosphere, so they cannot reach beyond the horizon.

Teacher's Note:
1. Shortwave radio waves are reflected by the ionosphere; TV waves are not.
2. Communication satellites act as relay stations.

Question 30. Name the instrument used to measure heights above sea level.
Answer:
An altimeter. It is an aneroid barometer whose dial is marked in height instead of pressure, because air pressure decreases with height.
In simple words: The instrument is the altimeter.

Teacher's Note:
1. Altimeters are used in aircraft and by mountaineers.
2. Pressure falls as we go higher.

Question 31. Why does a drop of oil placed on the surface of water spread over the entire surface?
Answer:
The surface tension of water is greater than that of oil, so water pulls the oil outward in all directions and the drop spreads into a thin film.
In simple words: Water's surface tension is larger, so it stretches the oil into a thin layer.

Teacher's Note:
1. Surface tension is the force that makes a liquid surface behave like a stretched skin.
2. Oil films on water show colours because they are very thin.

Question 32. Pieces of camphor placed on water move rapidly. Why?
Answer:
Camphor dissolves unevenly and lowers the surface tension of water where it dissolves. The surrounding water, with greater surface tension, pulls the camphor piece towards it, so the pieces dart about on the surface.
In simple words: Uneven lowering of surface tension pulls the camphor around.

Teacher's Note:
1. Surface tension decreases when impurities dissolve in water.
2. Soap does the same thing, which is why detergents help cleaning.

Question 33. In a cricket match a fielder has to catch a fast moving ball by moving his hands backward along with the ball. Why?
Answer:
By moving his hands back he increases the time during which the ball is brought to rest. The change in momentum is the same, but since force = change in momentum / time, a longer time gives a smaller force, and the hands are not hurt.
In simple words: A longer stopping time means a smaller force on the hands.

Teacher's Note:
1. Force = rate of change of momentum.
2. High jumpers also land on soft sand or mats to increase the stopping time.

Question 34. A karate expert breaks a slab with one blow of his bare hand. What is the principle behind this?
Answer:
His hand moves with a large speed and has a large momentum. On hitting the slab, the hand stops in a very short time, so the rate of change of momentum, which is the force, is huge, and the slab breaks.
In simple words: A very short contact time produces a very large force.

Teacher's Note:
1. Force = change in momentum / time.
2. Speed of the hand is the key factor, not only its mass.

Question 35. Can the centre of gravity of a body lie outside the actual material of the body?
Answer:
Yes. For a ring, a hollow sphere or a bent bar, the centre of gravity lies at the geometric centre, which is empty space and not a part of the material.
In simple words: Yes; for example, the centre of gravity of a ring is at its centre, where there is no material.

Teacher's Note:
1. The centre of gravity is the point where the whole weight seems to act.
2. A bent banana has its centre of gravity outside the fruit.

Question 36. Why are racing cars built low and with wide wheel bases?
Answer:
A low centre of gravity and a wide base make a body stable. Even when the car takes a sharp turn at high speed, the vertical line through its centre of gravity stays inside the wheel base, so it does not overturn.
In simple words: Low centre of gravity and wide base give stability.

Teacher's Note:
1. Stable equilibrium needs a low centre of gravity and a large base.
2. Double-decker buses carry heavy loads on the lower deck for this reason.

Question 37. The speed of water from a ground-floor tap is much greater than that from a top-floor tap. Why?
Answer:
The pressure in a liquid increases with depth (P = h x density x g). The ground-floor tap is much lower than the water tank, so the water pressure there is greater and the water comes out faster.
In simple words: Greater depth of water above the tap means greater pressure and faster flow.

Teacher's Note:
1. Pressure in a liquid depends on the depth.
2. That is why water tanks are placed at the roof.

Question 38. A solid chunk of iron sinks in water while it floats on mercury. Why?
Answer:
Iron is denser than water (about 7.8 g per cm3 against 1 g per cm3), so it sinks in water. Mercury is denser than iron (13.6 g per cm3), so iron floats on it.
In simple words: A body floats in a liquid denser than itself and sinks in one that is less dense.

Teacher's Note:
1. Compare the density of the body with the density of the liquid.
2. A ship made of iron floats because its shape displaces a large volume of water.

Question 39. When we strike an iron nail with a hammer, the nail becomes warm. Why?
Answer:
The kinetic energy of the hammer is changed into heat energy when the hammer stops. The molecules of the nail move faster and its temperature rises.
In simple words: Work done by the hammer is changed into heat in the nail.

Teacher's Note:
1. Energy is converted from one form to another and is never lost.
2. Rubbing hands together warms them for a similar reason.

Question 40. When a clinical thermometer is removed from the mouth of a patient, the mercury stays at the same level and does not come down. Why?
Answer:
There is a constriction (narrow bend) just above the bulb. When the thermometer is taken out and cools, the mercury thread breaks at the constriction and cannot flow back into the bulb. It is brought down by giving a jerk.
In simple words: A narrow kink above the bulb stops the mercury from falling back.

Teacher's Note:
1. This lets us read the temperature after removing the thermometer.
2. Shake the thermometer well before reusing it.

Question 41. Before fitting iron tyres on cart wheels, the tyres are first heated. Why?
Answer:
The tyre is made slightly smaller than the wheel. When heated it expands, slips over the wheel easily and, on cooling, contracts and grips the wheel tightly.
In simple words: Heating enlarges the tyre so that it can be fitted; cooling gives a tight grip.

Teacher's Note:
1. Solids expand on heating and contract on cooling.
2. Gaps are left in railway tracks for the same reason.

Question 42. Which is the best conductor of heat among liquids?
Answer:
Mercury, because it is a liquid metal and has free electrons that carry heat quickly. Most other liquids are poor conductors of heat.
In simple words: Mercury is the best liquid conductor.

Teacher's Note:
1. Metals are good conductors of heat.
2. Heat travels through water mainly by convection, not conduction.

Question 43. Why do we wear woollen clothes in winter?
Answer:
Wool has many tiny pores filled with air, and both wool and trapped air are poor conductors of heat. Woollen clothes keep our body heat from escaping and also prevent cold air from reaching the body.
In simple words: Wool traps air, which stops body heat from escaping.

Teacher's Note:
1. Trapped air is an excellent insulator.
2. Birds fluff their feathers in winter to trap more air.