Q. A man with a load jumps from a high building. What will be the load experienced by him?
A. Zero, because the acceleration of his fall is equal to the acceleration due to gravity of the earth.
Q. A piece of chalk when immersed in water, emits bubbles. Why ?
A. 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.
Q. Why does a liquid remain hot or cold for a long time inside a thermos flask?
A. The presence of air, a poor conductor of heat, between the double glass wall of a thermos flask, keeps the liquid hot or cold inside a flask for a long time.
Q. Why does a ball bounce upon falling?
A. When a ball falls, it is temporarily deformed. Because of elasticity, the ball tends to regain its original shape for which it presses the ground and bounces up (Newton's Third Law of Motion).
Q. Why is standing in boats or double decker buses not allowed, particularly in the upper deck of buses?
A. On tilting the centre of gravity of the boat or bus is lowered and it is likely to overturn.
Q. Why is it recommended to add salt to water while boiling dal?
A. By addition of salt the boiling point of water gets raised which helps in cooking.
Q. Why is the boiling point of sea water more than that of pure water?
A. Sea water contains salt, and other impurities with different boiling points, which jointly raise its boiling point.
Q. Why is it easier to spray water to which soap is added?
A. Addition of soap decreases the surface tension of water. The energy for spraying is directly proportional to surface tension.
Q. Which is more elastic, rubber or steel?
A. Steel is more elastic for the same stress produced compared with rubber.
Q. Why is the sky blue?
A. Violet and blue light have short waves which are scattered more than red light waves. While red light goes almost straight through the atmosphere, blue and violet light are scattered by particles in the atmosphere. Thus, we see a blue sky.
Q. Why does ink leak out of partially filled pen when taken to a higher altitude?
A. As we go up, the pressure and density of air goes 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 than the pressure of the air outside.
Q. On the moon, will the weight of a man be less or more than his weight on the earth?
A. The gravity of the moon is one-sixth that of the earth; hence the weight of a person on the surface of the moon will be one-sixth of his actual weight on earth.
Q. Why do some liquids burn while others do not?
A. A liquid burns if its molecules can combine with oxygen in the air with the production of heat. Hence, oil burns but water does not.
Q. Why can we see ourselves in a mirror?
A. We see objects when light rays from them reach our eyes. As mirrors have a shiny surface, the light rays are reflected back to us and enter our eyes.
Q. Why does a solid chunk of iron sink in water but float in mercury?
A. Because the density of iron is more than that of water but less than that of mercury.
Q. Why is cooking quicker in a pressure cooker?
A. As the pressure inside the cooker increases, the boiling point of water is raised, hence, the cooking process is quicker.
Q. When wood burns it crackles. Explain?
A. Wood contains a complex mixture of gases and tar forming vapours trapped under its surface. These gases and tar vapours escape, making a crackling sound.
Q. Why do stars twinkle?
A. 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 appears as if the stars are twinkling.
Q. Why is it easier to roll a barrel than to pull it?
A. Because the rolling force of friction is less than the dynamic force of sliding friction.
Q. If a feather, a wooden ball and a steel ball fall simultaneously in a vacuum, which one of these would fall faster?
A. All will fall at the same speed in vacuum because there will be no air resistance and the earth's gravity will exert a similar gravitational pull on all.
Q. When a man fires a gun, he is pushed back slightly. Why?
A. 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 imparts to the gun an equal momentum in a backward direction.
Q. Ice wrapped in a blanket or saw dust does not melt quickly. Why?
A. Both wood and wool are bad conductors of heat. They do not permit heat rays to reach the ice easily.
Q. Why do we perspire on a hot day?
A. When the body temperature rises, the sweat glands are stimulated to secrete perspiration. It is nature's way to keep the body cool. During the process of evaporation of sweat, body heat is taken away, thus, giving a sense of coolness.
Q. Why does ice float on water but sink in alcohol?
A. Because ice is lighter than water it floats on it. However, ice is heavier than alcohol and therefore it sinks in alcohol.
Q. Why do we perspire before rains?A. Before the rain falls, the atmosphere gets saturated with water vapour, as a result, the process of evaporation of sweat is delayed.
Q. Why does a thermometer kept in boiling water show no change in reading after 100°C?A. The boiling point of water is 100°C. Once water starts boiling at this temperature, thermometer records no change in temperature. The quantity of heat supplied is being utilised as latent heat of evaporation to convert the water at boiling point into vapour.
Q. Why do we bring our hands close to the mouth while shouting across to someone far away?
A. 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.
Q. Why does a corked bottle filled with water burst if left out on a frosty night?
A. Because of low temperature the water inside the bottle freezes. On freezing it expands, thereby its volume increases and pressure is exerted on the walls.
Q. Why is a small gap left at the joint between two rails?
A. To permit expansion of rails due to heat generated by friction of a moving train.
Q. Why cannot a copper wire be used to make elements in electric heater?
A. Copper melts at I08.3°C and forms a black powder on reacting with atmospheric oxygen. For heater elements a metal should have more resistance to produce heat.
Q. Why are water or mercury droplets always round .when dropped on a clean glass?
A. 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 to the smallest possible volume which causes the drop to take a round shape as for a given mass the sphere has minimum volume.
Q. Why does a balloon filled with hydrogen rise in the air?
A. Weight of hydrogen is less than the weight of air displaced by it. In balloons hydrogen is normally filled because it is lighter than air.
Q. Why do we lean forward while climbing a hill?
A. In order to keep the vertical line passing through our centre of gravity always between our feet, which is essential to attain equilibrium or stability.
Q. Why does smoke curl up in the air?
A. 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.
Q. Why does an electric bulb explode when it is broken?
A. The bulb encompasses partial vacuum and as it breaks, air rushes in causing a small explosion.
Q. Why does a man fall forward when he jumps out of a running train or bus?
A. He is in motion while in the train or bus. When he jumps out, his feet comes to rest while touching the ground but his upper portion which is still in motion propels him forward.
Q. Why does an ordinary glass tumbler crack when very hot tea or milk is poured in it?A. When a hot liquid is poured into a tumbler, the inner layer of the tumbler gets heated, it expands before the outer layer and an unequal expansion of both layers causes the tumbler to crack.
Q. Why is a compass used as an indicator of direction?A. The magnetic needles of a compass under the influence of the earth's magnetic field lie in a north-south direction. Hence, we can identify direction.
Q. Why is water from a hand pump warm in winter and cold in summer? A. In winter the outside temperature is lower than that of water flowing out of the pump, and therefore, the water is warm. Whereas in summer, the outside temperature is higher than the water of the pump, and therefore, it feels cold.
Q. Why is a rainbow seen after a shower?A. After a shower the clouds containing water droplets act like a prism through which the white light is dispersed producing a spectrum.
Q. Why does a swimming pool appear less deep than it actually is?A. 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.
Q. Why is one's breath visible in winter but not in summer?A. In winter, water vapour contained in the breath condenses into small droplets which become visible but in summer they are quickly evaporated and not seen.
Q. Why doesn't the electric filament in an electric bulb burn up?A. Firstly, because it is made of tungsten which has a very high melting point (341O°C) whereas the temperature of the filament required to glow is only 2700°C. Secondly, oxygen is absent since the bulb is filled with an inert gas which does not help in burning.
Q. Why does blotting paper absorb ink?A. Blotting paper has fine pores which act like capillaries. When a portion of blotting paper is brought in contact with ink, ink enters the pores due to surface tension (capillary action of liquids) and is absorbed.
