Home Forums General Knowledge for Law Entrance Exams Science Part III Physics Short notes for Law Entrance Exams

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    Law Giri


    Matter : Whatever occupies space, possesses Weight and can affect one or more of our senses is called Matter. Matter is made up of atoms, molecules.

    Mass : It is the quantity of matter contained in a body.

    A molecule is the smallest paricle of a substance which can exist in a free state, and has the properties of the substance.

    An atom is the smallest particle of an element, which can take part in a chemical reaction.

    Weight is the downward force acting on a body due to the attraction of the earth.


    Note that the weight of a body differs at different places of the earth, on account of the variation of gravity, but its mass remains the same. Similarly, a body will have the same mass on the moon as on Earth whereas its weight on the Moon will he a sixth of its weight on Earth.

    Mass is measured in kilograms, grams or pounds, whereas weight is measured in Newtons, dynes or poundals.


    There are two units :

    • British unit. It is called a pound.

    16 oz. = 1 lb,; hundred weight (cwt)

    1 ton = 2240 lbs.

    • International unit. It is a kilogram.

    1 kg = 1000 g, 1 kg = 2.2 lbs approx.

    1 g = 1000mg

    Density : Mass per unit volume of a body is called its density. Density of water is 1 gm. Per c.c.

    In the F.P.S. (Foot-Pound-Second) system, the density of water is 62.4 lbs (per cubic ft. (cft).

    Specific Gravity is the ratio of the mass of a given volume of the substance to the mass of an equal volume of water.

    Submarine is a ship so designed that ordinarily it floats on the surface of the sea but can be submerged in water when desired.

    To raise it to the surface the water in the tanks is forced out by compressed air, and it becomes lighter again.

    Friction is the force of resistance which comes into play when a body slides over a rough surface.

    Motion of Object : Motion is the change of position of a body with respect to its surroundings.

    Speed is the rate of motion. It is motion. It is scalar quantity, i.e. it possesses magnitude only.

    Velocity is a rate of motion in a particular direction. It is a vector quantity, i.e. It possesses both magnitude and direction.

    Uniform Speed : When a body travel equal distances in equal intervals of time, however, small the interval of time may be, it is said to have a uniform speed.

    Acceleration is the rate of increase of velocity, its units are m/s2 .

    Retardation or Deceleration is the rate of decrease of velocity.

    Force is that which changes or tends to change the state of rest or of uniform motion of a body in a straight line.

    Newton’s First Law of Motion states that a body continues in its state of rest or of uniform motion in a straight line unless indicates the presence of an external force.’’ Prof. S.N. Das Gupta.

    Illustrations :

    • A man jumping out of the moving train falls forward because his feet suddenly come to rest, while his body is in motion with the train.
    • A running person falls down when his foot stumbles against a stone. This is because the efoot is brought to rest while the upper part remains moving.

    Centripetal Force : When a small piece of stone tied to a string is whirled, one has to pull the string inwards This pull on the stone is called the centripetal force. An equal and opposite force is exerted by the stone on the hand. This force on the hand is called centrifugal force. Thus, centripetal force is directed towards the center while centrifugal force is directed away from the centre.

    Illustrations :

    • Mud sticking to a bicycle tyre is pulled in radially by the force of adhesion ; when the latter force is less than the centripetal – force required to move the mud in a circle, the mud flies off tangentially.
    • The spark, which fly off from the grinding stone of a blacksmith, are also due to the same phenomenon.

    Newton’s Second Law of Motion states that the rate of change of momentum is in proportion to the impressed force and takes

    Newton’s Third Law of Motion states that the rate of change of momentum is in proportion to the impressed force and takes

    Newton’s Third Law of Motion States that to every action there is an equal and opposite reaction.

    Illustrations :

    • If we slop out of a boat, we go in one direction and the boat goes in the other direction.
    • When a bullet is fired from a gun, the bullet goes forward and the gun kicks backward. This is called recoil of the gun.

    Unless there is action we cannot have reaction.

    • We cannot drive a nail into a wooden block unless it is supported against something to offer reaction.
    • We cannot cut a piece of paper with one blade of a pair of scissors, since there will be no reaction.

    Relative Velocity is the velocity of one body in relation to another body. For example, a motor boat in a river is moving amidst a river current – water which is moving with respect to an observer on dry land. That is to say, the speedmeter on the motor boat might read 20 mi/hr; yet the motor boat might be moving relative to the observer on shore at a speed of 25 mi/hr.

    Gravity is the pull of the earth with which it attracts bodies.

    Law of Gravitation states that “every particle in this universe attracts every other particles with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

    Momentum literally means the quantity of motion in a body. Mathematically, Momentum  = Mass * Velocity of the body, Thus force is equivalent to mass * acceleration of the body.

    Impulse is the change of momentum produced in body and is equal to the product of force and time.

    If a person were to catch a cricket ball by keeping his hands still he would require a much greater force to destroy the momentum of the ball. The force required may be enough to hurt the hands.

    For the same reason a man who falls from a height on a pucca floor receives far more serious injuries than a man who falls on a heap of sand.

    It is to reduce the force of blows received in carriage when moving over uneven roads that they are fitted with springs.

    Work : In science, work does not mean an exertion or fatigue. Work is said to have been done only if the force move-, the body on which it acts. If acts. If we support a heavy beam, or try to lift it from the floor a heavy stone slab, which we cannot move, no work is done. But if a laborer carries 8 bricks at a time up a ladder to the top of the first storey 15 ft. high, he is doing work. Work does not depend on the time taken to move the body. For instance, if two laborers carry the same number of bricks to the same height A doing it quickly, and B doing it of force and distance moved by the body in the direction of the applied force.

    Erg is amount of work done when a force of a 1 dyne moves a body through 1 cm, in the direction of the force. It is the C.G.S. until of work. Joule is the S.I. unit of work and is equal to erg. Rate of doing work is called power. Horse power is unit of power, i.e. raising 33,000 lbs. of weight through one ft. high in one minutes. H.P. = 550 foot pounds of work done in 1 second, 1 H.P. = 746 watts. 1 watt (=erg/sec) is S.I. unit of power

    The capacity for doing work is called energy. This is of two types : Potential Energy is the energy which a body possesses owing to its position and Kinetic Energy is the energy possessed by a body due to its motion.

    In the case of stretched or wound up string our bent bow, the energy is potential. The ball lying on the roof of a house is capable of driving a nail-into the ground, if allowed to fall on it. A watch spring when wound up possesses energy to keep the wheels of the watch in motion for a day. Each one provides and example of potential energy.

    On the other hand, the water flowing downstream on account of gravity is, by virtue of its motion, capable of setting a water mill in motion. Similarly, a bullet shot from a gun pierces through objects and overcomes resistance offered by them. Each one here provides an example of kinetic energy.

    Principle of work : Input = Output.

    Work done on a machine = Work done by the machine.

    What we gain in power, we lose in speed or distance. A screw jack lifts a huge car through small distance by the application of small force through a large distance. The various kinds of simple machines are : lever, wheels and axle, pulley, inclined plane, wedge and screw.

    A lever is a rigid bar which is capable of rotation about a fixed point called the fulcrum.

    Levers are of three kinds :

    First order. When the weight is in the middle of power an Fulcrum – as a wheel barrow the oaf of a boat (when the end of the oar is at rest in water).

    Second order. When the weight is in the middle of power an Fulcrum – as a wheel barrow the oaf of a boat (when the end of the oar is at rest in water).

    Third Order. In this case the point of application of effort, i.e…, P is between the Fulcrum and the point of application of the Resistance, i.e. , Weight – as human force arm (when raising a load placed on the palm) the treadle of the sewing machine.

    Aid to memory : To remember which order of lever has which (out of F, W, P) in the centre, remember’ FWP in order in centre must be.

    Equilibrium : When all the forces acting on a body produce no change in its state of rest or motion, it is said to be in equilibrium. The equilibrium is called stable when the body after being disturbed is let free, it returns to its former state forest e.g., a tub lying on a smooth table. The centre of gravity of a stable body is normally in the lowest position, and it is raised when disturbed.

    The equilibrium is called unstable, when the body on slight displacement moves further off from its original position.

    A body is in stable equilibrium, when its centre of gravity is as low as possible.

    Illustration. A boat would capsize and it would not move easily, if the men were standing in it than it would, if they were all sitting.

    The stability of an object may be increased by enlarging the base, and by having the centre of gravity as low as possible.

    Surface Tension : The force of tension on account of the inter-molecular forces in the surface of a liquid is called surface tension. The effect of the surface tension is to make the area of the free furface as small as possible.

    It is on account of this fact that main drops are spherical, and the free surface of water is plain like a stretched membrane.

    Another effect of surface tension is to raise liquids in capiliary tubes.

    Pascal’s Law : The liquids transmit pressure equally in all directions. If a vessel has two pistongs A and B, Cross-section area is employed in Brahma Press or Hydraulic Press.

    Principle of Archimedes says when a body is immersed in a fluid it experiences an upward thrust.


    Effects of Heat : 1.  Raises the temperature 2. Increases volumes. 3. Changes state 4. Brings about chemical action. 5. Changes physical properties.

    • The rails on a railway line are laid with a small gap between them so that with a rise in the temperature in summer the gap would provide room for expansion.
    • The iron tyre of a cart wheel is always made a bit smaller in diameter than the wooden wheel. After making the tyre redhot, it is slipped on the wheel, and water is poured on it. On cooling the iron tyre contracts, and holds the parts firmly together.

    Temperature : It is the degree of heat, and it can be measured in three scales : Centigrade, Fahrenheit, Reaumur, and Absolute or Kelvin scales.

    The freezing and boiling points of water in these scales are as follows :

    Freezing Point                          Boiling Point

    Centigrade                                                              0°                                             100°

    Fahrenheit                                                              32°                                           212°

    Reaumur                                                                0°                                             80°

    Kelvin or Absolute                                                 273.15°                                     373.15°

    They are related to each other as :

    = =

    Unit of Heat is a calorie. It is equal to the heat given to one gm of water to raise its temperature by 1°C.

    Latent heat is the heat which is used up in changing the state of a body without raising its temperature. One gram of ice at 0°C takes in needs to be given 537 calories of heat of change it to 1 gm of steam of 100°C 80 the latent heat of fusion of ice and latent heat of vapourisation of water respectively.

    Evaporation : It is the change of state from liquid to vapour. Cooling is caused by evaporation.

    Illustrations :

    • If is pour some ether on our hand, ether evaporates leaving our hand cold.
    • The refreshing effect produced on a hot day by watering a road is due to the cooling effect produced by evaporation of water.

    Transfer of Heat : Heat can be transferred in three ways :

    • Conduction : It is the process in which heat is transferred from particle to particle.

    Gases are very poor Conductors of Heat


    • If we pour some ether on our hand, ether evaporates leaving our hand cold.
    • The refreshing effect produced on a hot day by watering a road is due to the cooling effect produced by evaporation of water

    Transfer of Heat : Heat can be transfered in three ways :

    • Conduction : It is the process in which heat is transferred from particle to particle.

    Gases are very poor Conductors of Heat

    Illustrations : The warmth of woolen clothing is mostly due to the presence of air in the minute spaces in the cloth.

    • Convection is the transmission of heat from one part of the body to another by the actual motion of the heated particles of liquid or gas.

    Illustrations: Trade winds, land and sea breeze are all convection currents in the atmosphere.

    • Radiation: It is the process by which heat is transmitted from one point to another without heating the medium.

    Radiation is the greatest form a black surface and the least from a polished bright surface. A black body is a very good absorber of heat radiation, and a polished bright surface is a poor absorber of heat radiation.

    Illustrations : Thermos flask is a glass vessel with double walls, the space between the walls being vacuum. The outer surface of the inner walls and the inner surface of the outer walls are silvered. The vacuum does not allow the conduction and convection to take place and the polished surface minimizes radiation of heat.

    The melting point of a substance which expands on freezing (e.g. transformation of water to ice) is lowered by the increase of pressure; whereas it is raised in the case of substance which contracts on freezing (e.g., wax).

    Latent heat of fusion of ice is very high (80 calories). If it were low, ice would melt very soon and disastrous floods would result.

    Moreover, the ponds and lakes would freeze very much sooner man they do at present.

     Freezing mixtures are those means of which we can produce low temperature, e.g. common salt mixed with ice. They are based on the general principle that soluble substance (solutes) rise the boiling point and lower the freezing point of the solvent.

    Air conditioning is the process of regulation the humidity, temperature and circulation of air in a factory, a public building or private house.


    Air has a weight and therefore it exerts pressure on the surface of the earth. Pressure is defined as a force per unit area, CGS system it is not expressed in dynes\cm²  and in S.I. unit it is Newtons\m².

    Atmospheric pressure = weight of 76 cm of mercury.

    = 1.013 * N\ (where g = 980.665 cm\)

    = 1.013 *  dynes\c =  1.013 millibars

    Considering the average area of a human body as 1.3sq. metre the total pressure ( or force) acting on a human being is about 13.4 metric tons, which is enormous but our bodies have been adjusted to this in its long evolutionary history.

    The instrument used to measure pressure is called barometer. There are various types of barometers like Fortin’s barometer, which is extremely accurate but bulky instrument. Anroid’s barometer does not have the accuracy of Forth’s barometer but it is very portable and can be easily carried around.


    Electricity : When on a hot dry evening we comb our hair, the hair stand on ends and try to stick to the comb; a cracking sound force of attraction between oppositely charged bodies.

    There are two kinds of charges. Positive produced on a glass rod when it is rubbed with a piece of silk cloth and Negative as produced on an ebonite rod when it is rubbed with flannel.

    Similarly charged bodies repel each other and dissimilarly charged bodies attract each other.

    Substances which allow electricity to How through them are called conductors and which do not allow electricity to flow through them are called insulators.

    How lightning strikes the building ? Let us consider a charges cloud hanging over the earth. By induction the earth and other break down and the discharge takes place between the cloud and some object below it. A heavy charge thus passes and building, and in its passage damages the building.

    Current Elecricity : When two vessels filled with water up to different levels are connected together, water flows from the flows from higher ‘degree of electrification’ to the lower, and this gives rise to current electricity.

    Ohm’s Law : It states that the ratio of the potential difference across the ends of a conductor to the current flowing through it is constant so long as the physicall state of the conductor, i.e. temperature etc., remains the same.

    That is  = Constant or  = R

    Where, the constant is called resistance R of the conductor. The resistance is the property of a conductor by virtue of which it S.I. units. The Ohm’s law holds good for metallic conductors. The resistance of a conductor changes with temperature and for many substances it becomes zero at some very low temperature.

    Heating effects of current : When an electric current is passed through wire of any metal it experiences (electron) resistance to

    etc. is based on the heating effects of electric current. The heat H, produced when a current I amperes flows through a conductor is given by joule’s Law.

    i.e. H = Volt Joules

    where V is the potential difference in volts, and is the time for which the current flows or H = 1²Rt.


    Sound is a form of energy which produces a sensation of hearing. Sound waves are longitudinal in character . These can propagate in solids, liquids and gases. Human ear is sensitive to frequencies from about 20 cycles / s to about 20,000 cycles /s Frequencies less than 20 cycles/s are called infrasonics and those above 20,000 cycles/s are called ultrasonics. The sound waves originate in the vibration of bodies like strings, air columns, plates and membranes etc. In human being the vocal cards vibrate to produce sound.

    The three characteristics of sound are (1) loudness (2) pitch and (3) quality.

    Loudness is determined by the intensity of sound or the degree of sensation produced in the ear drum. Pitch is directly proportional to the frequency of sound. Higher the pitch the shriller is the sound. The sound of women has higher pitch.

    Quality is that characteristic of sound which distinguishes between two sounds of the same pitch and same loudness. It is because of the quality that different musical instruments produce different sound. It is on account of the quality that we recognize a person’s voice while talking on a telephone.


    Magnet is a substance which has the property of attracting pieces of iron, cobalt and nickel, and when freely suspended, it : points towards North and South.

    Natural Magnet : A naturally occurring mineral called magnetic is found in Asia.

    Minor : It is a compound of iron and oxygen. It is also called lodestone. It attracts piece of iron, cobalt and nickel and when suspended points in the north-south direction. It is a natural magnet.

    Artificial Magnets : These are pieces of iron to which the properties of a magnet have been imparted by artificial means. Steel is mostly employed for making permanent artificial magnets.


    Rectilinear Propagation of Light : Light travels in a straight line. This is why we interpose a small obstacle between the object and our eyes, when we do not want to look at it.

    Formation of shadows is another proof of the same.

    Reflection : When a ray of light falls on a mirror, it is sent back to the first medium in a certain direction according to certain laws.

    Laws of Reflection :

    • The incident ray the reflected ray and the normal ray point line in the same plane.
    • The angle of incidence is equal to the angle of reflection; the angles being measured between the normal and the respective rays.

    When a surface is rough, the rays of a beam of light falling on it are not regularly reflected in one direction, but are irregularly reflected of scattered in all directions, and hence the surface becomes visible from all parts of the room.

    Illustration :

    • When an object is placed between two mirrors parallel to each other we see a number of images due to successive reflections. They look smaller because their distance from the eye is successively increased.
    • Construction of a toy, called kaleidoscope, is due to successive reflection at the surface of inclined mirrors.
    • When a candle is placed close in front surface and so on. Each time some light comes out showing one image.

    Reflection : It is the phenomenon by which when a ray of light is incident obliquely on a surface of separation between two media, part of it goes into the second medium along a path different from the direction of the incident beam obeying the following laws:

    1. The incident ray , the normal and the refracted ray at a point lie in the same plane.

    Illustrations :

    • A pong looks shallower than it really is because of refraction. When rays start from a denser to a rarer medium, the rays get bent always from the normal. Hence a point at the bottom of the pond appears to be raised.
    • It is due to refraction that a stick immersed in water, when looked obliquely appears bent.
    • The brilliancy of a diamond, etc., is due to the fact that when light enters a piece of diamond, on account of high refractive which it comes out throw a good deal of light and make them appear brilliantly lit.


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