General Science (Physics)

General Physics

  • A fourth state of matter, the ionized state is called plasma.
  • Mass remains constant while the weight varies from place to place and is zero at the center of the earth.
  • On the moon a body will weigh much less than on the earth because the moon’s surface gravity is only 1/6th that of the earth.
  • A ship floats on the surface of water because the weight of water displaced by the immersed portion of the ship is equal to its own weight.
  • Ice floats on water because its weight is less than the weight of an equal volume of water.
  • The Lactometer, used to test purity (density) of milk is based on the principle that the greater the density of a liquid, the lesser will be the immersion of an object.
  • When a piece of stone tied to a thread is whirled you have to pull the thread inwards. This pull on the stone is called centripetal force and an equal and opposite force is exerted by the, stone on the hand which is called centrifugal force.
  • Curved rail tracks or curved roads are banked or raised on one side so that a fast moving train or vehicle leans inwards, thus providing required centripetal force to enable it to move round the curve.
  • A cyclist while turning around a curved road leans inwards so as to provide himself with the required centripetal force which enables him to take the turn.
  • Friction is very necessary because without it brakes would not work; belts would not drive the wheels of a machinery, etc.
  • The energy possessed by a body owing to its position is called potential energy, e.g., a wound up spring, a stretched rubber band, a bent bow, or a reservoir of water. The energy possessed by a body due to its motion is called kinetic energy, e.g., a bullet shot from a gun or water flowing in downstream.
  • It is surface tension which causes water to climb up a narrow capillary (phenomenon of capillary action). Rain drops are spherical due to surface tension. This phenomenon is applicable to the capillary action of a blotting paper absorbing ink.
  • The more rigid a body, the more elastic it is said to be. That is why steel is more elastic than rubber.
  • When alcohol is applied on the forehead of a person having high temperature, the temperature reduces because alcohol while evaporating draws the heat out of the body.
  • Metals are good conductors of heat because in metals there are a large number of free electrons, which transport major amount of heat and move from hotter part of the solid to the colder part transporting heat energy. Silver is the best conductor followed by copper.
  • In winter a steel chair appears colder than a wooden chair because steel being a good conductor of heat. When touched absorbs heat from the hand rapidly giving the feeling of coldness.
  • Metals are good conductors while gas and air are poor conductors. Hence: o Two thin blankets are warmer as they enclose air in between

o  A new quilt is warmer than an old one because it encloses more air

o Eskimos make double-walled ice houses and the air in between two ice walls does not allow heat to pass

o  A thermos flask is made of double-walled glass bottles.

  • Polished surfaces are the best radiators of heat. Hence:
  • The double-walled glass bottles used in a thermos flask have a silver coating-outside the inner wall and inside the outer wall.
  • White clothes are preferred in summer.
  • Radiation is greater from black surfaces which are very good absorbers of heat. Hence:
  • It is warmer on a cloudy night because the heat radiated by earth is obstructed and sent back to the atmosphere.
  • Ventilators in room are on the top portion of the walls because air after being heated rises upwards and passes through the ventilators and cold-air enters the room through doors and windows which are at lower levels.
  • Light a form of energy which travels in straight lines and causes the sensation of vision. If we interpose a small obstacle between the object and our eyes we are unable to see it. Formation of shadows is another proof of the fact that light travels in straight line.
  • When a rose is viewed in white light, it appears red and the leaves appear green, because the petals reflect the red part and the leaves green part. The remaining colours are absorbed. When the same rose is viewed in green light, the petals will appear black and the leaves green. In blue or yellow light both petals and leaves will appear black.
  • A white paper appears white because it reflects all the colours of light, whereas a print on it appears black because it absorbs all the colours.
  • In red light green grass will appear black because it absorbs all colours except green and it would absorb the red rays of light falling on it.
  • A ray of light passing from air into water bends towards the perpendicular.
    • A stick immersed in water appears bent.
  • A pond looks shallower than it really is as the rays start from rarer to denser medium, the rays bend towards the normal.
  • A diamond sparkles because when light enters a piece of diamond and the cut of its faces the light is totally refracted to various faces of the diamond, as diamond has a high refractive index.
  • A person looks shorter when he is standing in water and if we look at him from a side.
  • A stone lying at the bottom of pond appears to be at a higher point than it actually is.
  • Diffraction is the spreading out of waves of light as it passes through a narrow aperture, because light is a wave of motion. Hence, light rays emerging from a cinema machine spread wide on the screen.
  • When the sun shines after a shower, a rainbow is seen in the sky opposite the sun. The colours of the rainbow are due to the dispersion of sunlight by water droplets suspended in the air.
  • The sky is blue in colour because violet and blue light have short wavelength and are scattered by the atmosphere ten times more than red light waves. While the red light, with long wave length, goes almost straight through the atmosphere, blue and violet are scattered by particles in the atmosphere. Thus we see a blue sky. If light of all colours is absorbed, the eyes see black. A black surface absorbs more light than a white surface.
  • When a man stands in front of mirror, light from all parts of the body (light which is reflected from the body in the presence of a source of light) is reflected from the mirror back to the eyes and a virtual image appears to be formed behind the mirror.
  • A concave mirror can be used as a burning glass and is used in solar cookers.
  • The convex mirror is used as a rear-view mirror in vehicles as it has the advantage of a wide field of view.
  • Batteries supply direct current (d.c.) and the mains supply alternating current (a.c.).
  • The radio and T.V. require d.c. and this is obtained from a.c. mains by means of a rectifier which converts a.c. into d.c.
  • The filament of electric bulbs and heater elements is made of platinum or tungsten because they have very high resistance and becomes white, hot and glow and light is produced by the electric current.
  • Fuse wire is made of material having low melting point and high resistance because if it is inserted in an electric circuit, it will not allow excess electricity to flow through it. When current exceeds its limit, the fuse wire gets heated, melts and breaks the circuit.
  • When electricity is passed through a coil it becomes magnet due to the magnetic field produced by electricity.
  • When a glass rod is rubbed with a silk cloth, some electrons from the rod are transferred to the silk cloth. Thus by losing electrons the glass rod becomes positively charged and by gaining the number of electrons the silk acquires an equal negative charge. Similarly by rubbing an ebonite comb on hair a magnetic property is produced which can attract small pieces of paper.
  • Loudness is measured in decibels (db), a unit based on the weakest sound that can be detected by the human ear.
  • Sound can he heard round the corners of a corridor due to diffraction of sound.
  • The human ear is normally sensitive to sounds whose frequencies are between 16 Hz and 20,000 Hz.
  • Bats can fly in the dark because ultrasonic waves produced by them during flying are reflected back to them from the obstacles in their way and hence they can fly without difficulty.
  • In dry air at 0° C the speed of sound is about 331 m/sec or 750 miles/h. The presence of water vapours in the air increases the speed of sound slightly.
  • The electric bell works on the principle of magnetic induction.
  • Lead and feather fall at the same rate in a vacuum.
  • Hydraulic machines like the hydraulic press work on principles of Pascal’s law.




Absolute Temperature and Absolute Zero: A temperature scale based on Charles Law of expansion of gases. It measures temperature from absolute zero (i.e., #273.15° C, below which matter cannot be cooled) in kelvin, (the Kelvin degrees being of the same magnitude as the degree centigrade). To convert an absolute temperature to centigrade simply adds 273.15.


Armature: The coils of an electric motor or dynamo.


Calorie: It is the unit of heat, which is equal to the amount of heat required to raise the temperature of 1gm of water by 1° C.


Curie point: Temperature above which a strongly magnetizable or ferromagnetic material looses its magnetic properties and acquires paramagnetic properties. The curie point of iron is 870° C.


Depression in Freezing Point: The lowering of the temperature at which a liquid freezes as is done by adding common salt and saltpeter to ice to make ice cream.


Electrode: The name given to the part by means of which electricity is led into or away from a gas or a liquid. The negative electrode is cathode and the positive electrode is known as anode.


Electromagnetic Waves: Waves that consist of varying electrical and magnetic quantities traveling along at the speed of light. Light, radio waves, X-rays, Gamma rays, etc., are covered under this category.

Electromotive Force: The electrical pressure developed by a cell, battery or generator which enables it to produce an electrical current in a circuit. It is measured in volts.


Fluorescence: The property of some substances to absorb light of one wavelength and emit light of a longer wavelength.


Insulator: Material such as glass, rubber, porcelain, plastic, etc., which have no free electrons and therefore, will not allow electric current to pass through even when an e.m.f. is applied.


Mole: The amount of substance that contains as many particles as there are atoms in 12 g of the isotope carbon 12.


Pressure: The force or weight per unit of area acting upon a surface. It is measured in newtons/m2 (S.I. units) or kg/m2.


Super-conductivity: A property of some metals and alloys to lose electrical resistance below a certain temperature (usually within 20° of Absolute temperature). This means that an electrical current can flow in a loop of the metal indefinitely without generating heat or decreasing in strength.


Thermocouple: A kind of thermometer made by joining a wire from both ends to another wire of a different material. If one junction is kept cool and the other heated, and electric current flows and can be used as a measure of the temperature of the hot junction.


Flux: Amount of magnetic or electrical field or radiation passing through a given area.


Heat Capacity: Heat energy in Joules required to raise the temperature of a material by 1 kelvin.


Induction: Temporary magnetization or electrification of material that is placed in a strong magnetic or electrical field.


Inertia: A body’s resistance to motion, if it is at rest, or to a change in its motion if it is moving. In other words inertia is the property of a body to continue to remain in its position of rest or motion.


Insulation: Material that will not convey electrical current or flow of heat, such as glass, rubber, porcelain, plastics, etc.


Momentum: The quantity of motion of a moving body, a measure of the time it would take that body to stop when force is applied to it.


Polarization: Separation of positive and negative charges to different regions of a molecule, thus causing it to be an electric dipole.


Raman Effect: The change in wavelength that occurs when light is scattered in a transparent medium.

Resistance: Property of an electric circuit or its part that impedes the flow of current.


Rheostat: Resistor whose resistance can be varied.


Spectrum: The band of colours produced when light is passed through a prism and is split up into its component wavelengths.


Supersonic: Description of motion through air or some other medium at a speed greater than the speed of sound through that medium.


Thermal: Connected with or produced by heat or temperature.


Thermal Radiation: Emission of electromagnetic radiation by a body due to the thermal agitation of atoms and electrons in the body.


Thermionic Effect: Emission of electrons from a hot body.


Torque: The turning effect of a force on a body that can rotate freely.


Vacuum:  Space in which there is no matter.


Viscosity: Friction or drag within a liquid.



  • A man with a load on his head jumps from a high building. The load experienced by him is zero, because the acceleration of his fall is equal to the acceleration due t gravity of the earth.
  • Spring is made up of steel and not copper because the elasticity of steel is greater than that of copper.
  • Steel is more elastic for the same stress produced in rubber is more than that in steel; the elasticity is R 1/strain.
  • It is easier to spray water to which soap is added because addition of soap decreases the surface tension of water. The energy for spray is directly proportional to surface tension.
  • A piece of chalk when immersed in water emits bubbles because 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.
  • A liquid remain hot or cold for a long time inside a thermoflask because of the presence of air, which is poor conductor of heat, in between the double glass walls of a thermoflask.
  • A ball bounces up on falling because 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).
  • Standing in boats or double-decker buses is not allowed, particularly in the upper floor of buses because on tilting, the center of gravity of the boat or bus is lowered and it is likely to overturn.
  • The boiling point of sea water is more than that of pure water because sea water contains salts and other impurities with different boiling points which jointly raise its boiling point.
  • It is recommended to add salt in water while boiling grams because by addition of salt the boiling point of water gets raised which helps in cooking
  • Soft iron is used as an electromagnet 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).
  • 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 than the pressure of air outside.
  • 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.
  • 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 because
  • 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.
  • We see objects when light rays from them enter out eyes. As mirrors have a shiny surface, the light rays are reflected and come back to us and enter our eyes. Thus we can see ourselves in a mirror.
  • A solid chunk of iron sinks in water but float in mercury because the density of iron is more than that of water but less than that of mercury.
  • Stars twinkle. 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.
  • Cooking is quicker in a pressure cooker because 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 because wood contains a complex mixture of gases and tar-forming 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 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 because as the bullet leaves the nozzles 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.
  • The gravitational pull of the earth is more at the poles because the poles being nearer to the centre, the weight of a body is greater at this point. That’s why a body weighs slightly more at the poles than at the equator.
  • It is easier to roll a barrel than to pull it because the rolling force of friction is less than the dynamic force of sliding friction.
  • Ice wrapped in a blanket or saw dust does not melt quickly because both wool and wood are bad conductors of heat. They do not permit heat rays to react with ice easily.
  • We perspire on a hot day because 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.
  • We perspire before rains because before the rain falls the atmosphere gets saturated with water vapours as a result the process of evaporation of sweat is delayed.
  • 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 utilized as latent heat of evaporation to convert the water at boiling point into vapour.
  • We bring our hands close to the mouth while shouting across to someone far away because 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.
  • 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.
  • A small gap is left at the joint between two rails to permit expansion of rails due to heat generated by friction of moving train.
  • A cyclist has to use more face at the start than when the cycle is in motion. Momentum has to be produced to set a cycle in motion requiring more force but once momentum has been gained smaller force is required to maintain it.
  • A copper wire cannot be used to make elements in electric heaters because copper melts at 1,083°C and forms a black powder on reacting with atmospheric oxygen. For heater element a metal should have more resistance to produce heat.
  • 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.
  • A balloon filled with hydrogen rise in the air. Its weight is less than the weight of air displayed by it. In balloons normally hydrogen is filled which is lighter than air.
  • Smoke curl up in the air. Smoke contains hot gases which, follow a curved path because of the eddy currents that are set up in the air.
  • We lean forward while climbing a hill in order to keep the vertical line passing through our centre of gravity always between our feet to attain balance or stability.
  • An electric bulb explode when it is broken because the bulb is a partial vacuum and as it breaks air rushes in causing a small explosion.
  • A man falls forward when he jumps out of a running train. 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.
  • 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 a unequal or mild expansion of both layers causes the tumbler to crack.
  • The magnetic needles of a compass under the influence of the earth’s magnetic field lie in a nearly north-south direction. Hence we can identify direction.
  • Water from a hand pump is 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.
  • A rainbow seen after a shower because after a shower the clouds containing water droplets act like a prism through which the white light is dispersed and produces a spectrum.
  • A swimming pool appear less deep than it actually is because 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.
  • Kerosene oil float on 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.
  • One’s breath is visible in winter but not in summer because in winter water vapour contained in the breath condenses into small droplets which become visible but in summer they are quickly evaporated and thus not seen.
  • The electric filament in an electric bulb doesn’t burn up. First because it is made of tungsten which has a very high melting point (3,410°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.
  • Blotting paper has fine pores which act like capillaries. When a portion of blotting paper is brought in contact with ink, due to surface tension (capillary action of liquids) ink enters the pores and is absorbed.
  • A small ball of iron sink in water but a ship floats because the weight of water displayed 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).
  • Ice float on water because 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 level of water remains unchanged because the volume of water produced by the melting ice is exactly the same as that of the piece of ice.
  • Moisture gathers outside tumbler containing cold water because the water vapour of the air condenses on cooling and appears as droplets of water.
  • The water in an open pond is cool even on a hot summer day because as the water evaporates from the open surface of a pond, heat is taken away in the process, leaving the surface cool.
  • It is difficult to cook rice or potatoes at higher altitudes because atmospheric pressure at higher altitudes is low and boils water below 100°C; because the boiling point of water is directly proportional to the pressure on its surface.
  • Due to low air pressure at higher altitude the quantity of air is less and so also that of oxygen. So it is difficult to breathe at higher altitudes.
  • Winter nights are warmer in a cloudy weather and summer nights are hotter in a cloudy weather than when the sky is clear because cClouds 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.
  • A metal tyre heated before it is fixed on wooden wheels because on heating, the metal tyre expands by which its circumference also increases. This makes fixing the wheel easier and thereafter cooling down shrinks it; thus securing it tightly.
  • It is easier to swim in the sea than in a river because the density of sea water is higher; hence the upthrust is more than that of river water.
  • A fat person will possibly learn swimming faster than a thin person because the fat person displaces more water which will help him float more freely compared to a thin person?
  • A flash of lightening is seen before thunder because light travels faster than sound; it reaches the earth before the sound of thunder.
  • Petrol fire cannot be extinguished by water because 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.
  • Water remains cold in an earthen pot because 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.
  • An ordinary pendulum clock loses time in summer. In summer, due to heat, the length of the pendulum increases. This in turn results in the increase of duration of each oscillation of the pendulum. Therefore the clock loses time in summer.
  • Mercury thermometers are not used to measure very low temperatures because the freezing point of mercury is only 39°C and it also has non-uniform expansion at low temperatures.
  • We place a wet cloth on the forehead of a patient suffering from high temperature. This is done because due to body’s temperature water evaporating from the wet cloth produces cooling and brings the temperature down.
  • We apply Eau-de-Cologne to a person having high temperature because 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 because 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 because 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.
  • Oil rise on a cloth tape of an oil lamp because the pores in the cloth tape suck oil due to the capillary action of the larvae.
  • Ventilators in a room always made near the roof because the hot air being lighter in weight tends to rise above and escape from the ventilators on the top.
  • Chimneys of factories using boilers are high because the gases produced in boilers are hot and being lighter in weight tends to go up. The chimneys exhaust these gases above in the atmosphere without polluting the lower layer of atmosphere.
  • When the rubber tube 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.
  • Air coolers are less effective during the rainy season. 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.
  • Grass gathers more dew in nights than metallic objects like stones. 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 formed on objects which are good radiators and bad conductors.