Questions

Scientific Explanation

A man with a load on his head jumps from a high building. What will be the load experienced by him? Concept: Gravitation & Gravity.

Zero, because the acceleration of his fall is equal to the acceleration due to gravity of the earth. 

Why is spring made of steel and not copper? Concept: Hook’s Law

The elasticity of steel is greater than that of copper.

Which is more elastic, rubber or steel? Concept: Hook’s Law

Steel is more elastic for the same stress produced in rubber is more than that in steel, the elasticity is 1/strain.

Why is it easier to spray water to which soap is added? Concept: Surface tension

Addition of soap decreases the surface tension of water. The energy for spray is directly proportionate to surface tension.

A piece of chalk when immersed in water, emits bubbles. Why?

Concept: Capillarity/Capillary Action

Chalk consists of pores forming capillaries. When it is immersed in water, the water begins to rise in the capillaries and air present there is expelled in the form of bubbles.

Why does a liquid remain hot or cold for a long time inside a thermo flask?

Concept: Conduction & Convection

Because of the presence of air, which is a poor conductor of heat, in between the double glass walls of a thermo flask.

Why does a ball bounce up on falling?

Concept: Elasticity & Newton’s law

When a ball falls, it is temporarily deformed. Due to elasticity the ball tends to regain its original shape, for which it presses the ground and bounces up (Newton’s third law of motion).

Why is standing in boats or double-decker buses not allowed, particularly in the upper floor of buses?

On tilting the centre of gravity of a boat or bus is lowered and it is likely to overturn.

Why is the boiling point of sea water more than that of pure water?

Sea water contains salts and other impurities with different boiling points which jointly raise its boiling point.

Why is it recommended to add salt in water while boiling grams?

By addition of salt the boiling point of water gets raised which helps in cooking.

Why is soft iron used as an electromagnet?

Concept: Controlled magnetism property.

Because it remains magnetic only till the current passes through the coil and loses its magnetism when the current is switched off (principle of electric bells).

Why is the sky blue?

Concept: Scattering of light; blue light being of shorter wavelength scattered the most.

Violet and blue light have short waves and are scattered more than red light waves. While red light goes almost straight through the atmosphere, blue and violet are scattered by particles in the atmosphere. Thus, we see a blue sky.

Why does ink leak out of a partially filled pen when taken to a higher altitude?

Concept: Low Atmospheric pressure at higher altitudes.

As we go up the pressure and the density of air go on decreasing. A partially filled pen leaks when taken to a higher altitude because the pressure of air acting on the ink inside the tube of the pen is greater from the pressure of air outside.

On the moon will the weight of a man be less or more than his weight on the earth?  Concept: Gravity on moon is 1/6th of the earth.

The gravity of the moon is one-sixth that of the earth; hence the weight of a person on the surface on the moon will be one-sixth of his actual weight on the earth.

Why do some liquids burn while others do not? Concept: Combustion

A liquid burns if its molecules can combine with oxygen of the air with the production of heat. Hence, oil burns but water does not.

Oil and water do not mix. Why?

Concept 1: Likes do not dissolve unlike.

Concept2: Specific gravity of oils (fats) is less than that of the water.

Molecules of oil are bigger than that of water and therefore do not mix easily.

Molecules of water are polar, i.e. they have opposite charges at two ends whereas oil molecules do not; as a consequence they tend to stay away from water molecules.Since oils are essentially fats which have very less specific gravity as compared to water hence, all fats float on water & are immiscible with water. 

How can we see ourselves in a mirror? Concept: Reflection of light.

We see objects when light rays from them enter our eyes. As mirrors have a shiny surface, the light rays are reflected and come back to us and enter our eyes.

Why does a solid chunk of iron sink in water but float in mercury? Concept: Laws of floatation & sinking.

Because the density of iron is more than that of water but less than that of mercury.

Why do stars twinkle?

Concept: Refraction of Light.

The light from a star reaches us after refraction as it passes through various layers of air. When the light passes through the earth’s atmosphere, it is made to flicker by the hot and cold ripples of air and it appears as if the stars are twinkling.

Why is cooking quicker in a pressure cooker? Concept: Increase in the boiling point.

As the pressure inside the cooker increases, the boiling point of water is raised, hence, the cooking process is quick.

When wood burns it crackles. Why?

Concept: Escape of the volatile gases and vapors from the burning wood.

Wood contains a complex mixture of gases and tar-farming vapours trapped near the surface. These gases and tar vapours escape making a crackling sound.

If a feather, a wooden ball and a steel ball fall simultaneously in a vacuum, which one of these would fall faster?

Concept: Laws of Gravity & Gravitation.

All will fall at the same speed in a vacuum because there will be no air resistance and the earth’s gravity will exert a similar gravitational pull on all.

When a man fires a gun, he is pushed back slightly. Why?

Concept: Newton’s third law of motion.

As the bullet leaves the nozzle of the gun’s barrel with momentum in a forward direction as per Newton’s third law of motion the ejection of the bullet imparts to the gun an equal momentum in a backward direction.

Why does a body weigh slightly more at the poles than at the equator?

Concept: Maximum magnetism of the earth and hence, high gravitational pull

The gravitational pull of the earth is more at the poles because the poles being nearer to the centre of the Earth & thus, the weight of a body is greater at this point.

Why is it easier to roll a barrel than to pull it? Concept: Force of Friction.

Because the rolling force of friction is less than the dynamic force of sliding friction.

Ice wrapped in a blanket of saw dust does not melt quickly. Why? Concept: Heat transmission by conduction.

Both wool and wood are bad conductors of heat. They do not permit heat rays friction.

Why do we perspire on a hot day?

Concept: Evaporation has a cooling effect

When the body temperature rises, the sweat glands of the body are stimulated to secrete perspiration. It is nature’s phenomenon to keep the body cool. During the process of evaporation of sweat some body heat is taken away thus giving a sense of coolness.

Why do we perspire before rains?

Concept: Vapor pressure

Before the rain falls, the atmosphere gets saturated with water vapours as a result the process of evaporation of sweat is delayed.

Why does ice float on water but sink in alcohol? Concept: Water expands at zero degree & ice formed is less dense as compared to water.

Because ice is lighter than water it floats on it. However, ice is heavier than alcohol and therefore it sinks in alcohol.

Why does a thermometer kept in boiling water show no change in reading?

The boiling point of water is 1000C. Once water starts boiling at this temperature, thermometer records no change in temperature. The quantity of heat supplied is being utilized as latent heat of evaporation to convert the water at boiling point into vapour.

Why do we bring our hands close to the mouth while shouting across to someone far away?

By keeping hands close to mouth, the sound is not allowed to spread (phenomenon of diffraction of sound) in all directions but is directed to a particular direction and becomes louder.

Why does a corked bottle filled with water burst if left out on a frosty night?

Due to low temperature the water inside the bottle freezes. On freezing it expands, thereby its volume, increases and pressure is exerted on the walls.

Why is small gap left at the joint between two rails?

To permit expansion of rails due to heat generated by friction of the moving train.

A cyclist has to use more force at the start than when the cycle is in motion. Explain. 

Momentum has to be produced to set a cycle in motion requiring more force but once momentum has been gained and set in smaller force is required to maintain it.

Why cannot a copper wire be used to make elements in electric heaters?

Copper melts at 1,0830C, and forms a black powder on reacting with atmospheric oxygen. For heater element a metal should have more resistance to produce heat. 

Why is water or mercury always round when dropped on a clean glass?

The surface of a liquid is the seat of a special force as a result of which molecules on the surface are bound together to form something like a stretched membrane. They tend to compress the molecules below the smallest possible volume which causes the drop to take a round shape as for a given mass the sphere has minimum volume. This entire is attributed to surface tension.

Why does a balloon filled with hydrogen rise in the air?

Normally, in balloons hydrogen is filled which is lighter than air. Its weight is less than the weight of air displaced by it.

Why does smoke curl up in the air?

Smoke contains hot gases which, being lighter in weight, follow a curved path because of the eddy currents that are set up in the air. 

Why do we lean forward while climbing a hill?

In order to keep the vertical lines passing through our centre of gravity always between our feet to attain balance or stability. 

Why does an electric bulb explode when it is broken?

The bulb is a partial vacuum and as it breaks air rushes in causing a small explosion.

Why does a man fall forward when he jumps out of a running train? Or so when a sudden brake is applied in a running vehicle.

He is in motion while in the train. When he jumps out, his feet come to rest while touching the ground but his upper portion which is still in motion takes him forward.

Why does an ordinary glass tumbler crack when very hot tea is poured in it?

When the inner layer of the tumbler gets heated it expands before the outer layer and an unequal or mild expansion of both layers causes the tumbler to crack.

Why is a compass used as an indicator of direction?

The magnetic needles of a compass under the influence of the earth’s magnetic filed lie in a nearly north-south direction. Hence we can identify direction.

Why is water from a hand pump warm in winter and cold in summer?

In winter the outside temperature is lower than that of water flowing out of the pump and, therefore, it feels warm. Whereas in summer the outside temperature is higher than the water of the pump and, therefore, it feels cold.

Why is a rainbow seen after a shower? Concept: Dispersion of light.

After a shower the clouds containing water droplets act like a prism through which the white light is dispersed and produces a spectrum.

Why does a swimming pool appear less deep than it actually is?

Concept: Refraction of light.

The rays of light coming from the bottom of the pool pass from a denser medium (water), to a rarer medium (air) and are refracted (bend away from the normal). When the rays return to the surface, they form an image of the bottom of the pool at a point which is little above the real position.

Why does kerosene oil float on water?

Concept: Density of water.

Because the density of kerosene oil is less than that of water. For the same reason cream rises in milk and floats on the top.

Why is one’s breath visible in winter but not in summer? Concept: Conduction of heat.

In winters, the outside temperature is far below the constant body temperature and thus, conduction of heat occurs.

Why does not the electric filament in an electric bulb burn up?

Concept: Presence of vacuum or neon gas instead of oxygen that is essential for combustion of the filament.

First because it is made of tungsten which has a very high melting point (34100C) whereas the temperature of the filament required to glow is only 27000C. Second, oxygen is absent since the bulb is filled with an inert gas (Neon) which does not help in burning.

Why does blotting paper absorb ink?

Concept: Capillarity

Blotting paper has fine pores which act like capillaries. When a portion of blotting paper is brought is contact with ink, due to surface tension (capillary action of liquids) ink enters the pores and is absorbed.

Why does a small ball of iron sink in water but a ship float?

The weight of water displaced by an iron ball is less than its own weight, whereas water displaced by the immersed portion of a ship is equal to its weight (Archimedes Principle).  

Why does ice float on water?

The weight of the ice block is equal to the weight of the liquid displaced by the immersed portion of the ice.

A tumbler is filled to the brim and a piece of ice is placed in it. When the ice melts will the tumbler containing cold water?

The water vapour of the air condenses on cooling and appears as droplets of water.

Why is the water in an open pond cool even on a hot summer day?

As the water evaporates from the open surface of a pond, heat is taken away in the process, leaving the surface cool.

Why is it difficult to cook rice or potatoes at higher altitudes?

Atmospheric pressure at higher altitudes is low and boils water below 1000C; because the boiling point of water is directly proportional to the pressure on its surface.

Why is difficult to breathe at higher altitude?

Due to low air pressure at higher altitudes the quantity of air is less and so also that of oxygen.

Why are winter nights warmer in a cloudy weather and summer nights hotter in a cloudy weather than when the sky is clear?

Clouds being bad conductors of heat do not permit radiation of heat from land to escape into sky. As this heat remains in the atmosphere, the cloudy nights are warmer.

Why are cloudy days cooler?

Clouds do not permit the radiation of the sun to reach the earth.

Why is a metal tyre heated before it is fixed on wooden wheels?

On heating, the metal tyre expands by which its circumference also increases. This making fixing the wheel easier and thereafter cooling down shrinks it. Thus, securing it tightly.

Why is it easier to swim in the sea than in a river?

The density of sea water is higher; hence the upthrust is more than that of river water.

Who will possibly learn swimming faster – a fat person or a thin person? 

The fat person displaces more water which will help him float mere freely compared to a thin person.

Why is a flash of lightning seen before thunder?

Because light travels faster than sound; it reaches the earth before the sound of thunder.

Why cannot petrol fire be extinguished by water?

Water, which is heavier than petrol, slips down permitting the petrol to rise to the surface and continue to burn. Besides, the existing temperature is so high that the water poured on the fire evaporates even before it can extinguish the fire. The latter is true if a small quantity of water is poured.

Why does water remain cold in an earthen pot?

There are pores in the earthen pot which allow water to percolate to the outer surface. Here evaporation of water takes place thereby producing a cooling effect.

Why does an ordinary pendulum clock lose time in summer?

In summer, due to heat, the length of the pendulum increases. There in turn results in the increase of duration of each oscillation of the pendulum. Therefore, the clock loses time in summer.

Why are mercury thermometers not used to measure very low temperatures?

The freezing point of mercury is only 390C and it also has non-uniform expansion at low temperatures.

Why do we place a wet cloth on the forehead of a patient suffering from high temperature?

Due to body’s temperature, water evaporating from the wet cloth produces cooling and brings the temperature down.

Why do we apply Eau-de-Cologne to a person having high temperature?

Eau-de-Cologne contains alcohol which is quickly evaporated and takes away much of the local heat from the body of the person.

When a needle is placed on a small piece of blotting paper which is placed on the surface of clean water, the blotting paper sinks after a few minutes but the needle floats. However, in a soap solution the needle sinks. Why? Concept: Decrease in S. tension

The surface tension of clean water being higher than that of soap solution, it can support the weight of needle due to its surface tension. By addition of soap, the surface tension of water reduces, thereby sinking the needle.  

To prevent growth of mosquitoes, it is recommended to sprinkle oil in the ponds with stagnant water. Why?

Concept: Addition of kerosene reduces the surface tension of the water

Mosquitoes breed in stagnant water. The larvae of mosquitoes keep floating on the surface of water due to surface tension. However, when oil is sprinkled, the surface tension is lowered resulting in drowning and death of the larvae.

Why does oil rise on a cloth tape of an oil lamp? Concept: Capillarity

The pores in the cloth tape suck oil due to the capillary action of the oil.

Why are ventilators in a room always made near the roof?

Concept: Convection current

The hot air being lighter in weight tends to rise above and escape from the ventilators on the top.

Why are chimneys of factories using boilers high?

The gases produces in boilers are hot and being lighter in weight tend to go up. The Chimneys exhaust these gases above in the atmosphere without polluting the lower layer of atmosphere.

How does ink get filled in a fountain pen?

Concept: Bournille’s principle

When the rubber tune of a fountain pen immersed in ink is pressed, the air inside the tube comes out and when the pressure is released the ink rushes into fill the air space in the tube.

Why are air coolers less effective during the rainy reason?

Concept: Slow or even negligible rate of evaporation

During the rainy season the atmospheric air is saturated with humidity. Therefore, the process of evaporation of water from the moist pads of the cooler slows down, thereby not cooling the air blown out from the cooler. 

Why does grass gather more dew in nights than metallic objects like stones?

Concept: Transpiration and Root pressure

Grass being a good radiator enables water vapour in the air to condense on it. Moreover, grass gives out water constantly (transpiration) which appears in the form of dew because the air near grass is saturated with water vapour and slows evaporation. Dew is forced on objects which are good radiators and bad conductors.  

Why powdered sugar dissolves faster than the crystalline sugar?

Concept: Larger surface area; smaller the size, larger surface area.

Because it has more surface area. The greater the surface area of the solid, the faster is the rate of reaction. Note that smaller the particle size, greater shall be the surface area.

Eggs and rice do not boil easily at high altitudes. Why?

Concept: Low atmospheric pressure and hence, low boiling point.

Atmospheric pressure is lesser at higher altitudes. Therefore, water starts boiling at lower temperature. Thus, it takes more time to boil the rice and/or eggs at higher altitudes.

A lump of coal burns at moderately in air while coal dust burns explosively. Why? 

The surface area of a lump of coal is much lesser than the dust of coal. As such, much area of contact of coal dust provides excessive access to air. As a result, dust burns explosively while a lump of coal burns moderately.

Why does the temperature of a metal wire rises when an electric current is passed through it?

Concept: A good conductor of electricity.

The electric current in a metal wire is due to the motion of electrons. During their motion, electrons collide with the oscillating positive ions in the conductor and impart part of their energy to them. As ions oscillate faster, their increased energy is manifested as heat.

Artificial satellites are always launched from the earth in the eastward direction. Why?

Concept: Initial upthrust.

Launching in the direction of the Earth’s rotation (namely, eastward) does provide an initial velocity equal to the surface velocity of rotation. If the Earth did not rotate, we would have to furnish that initial velocity equal to the surface velocity of rotation which is about 8 km/sec, but because it does, rockets launched eastward get a head start. However, satellites launched into a polar orbit back the advantage. Israel’s satellites have to be launched westward over the Mediterranean sea and acquire a boost larger than 8 km/sec.