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Physics concept

physics concept

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Laws of Motion

Newton’s laws of motion

First law [law of inertia]: An object remains in the state of rest or uniform motion till a force compels it to change that state. Tendency of objects to stay at rest or keep moving is inertia.


  1. when a bus starts motion our body is pulled backwards as it is at rest and the feet and bus are in motion.
  2. When a bus applies brakes the body moves forward as it’s in motion but the feet and the bus are in rest.
  3. When a car takes a turn our body swings as it was in a straight line motion.
  4. When we make a heap of carom coins and take a striker and hit them then the lowermost coin moves, but the remaining heap falls in place due to inertia.

Inertia is present in all objects; more the mass more is the inertia. Hence mass is a measure of inertia.

Second law: The rate of change of momentum of an object is proportional to the applied unbalanced force which is in the direction of the force.


  1. A cricketer catching a ball pulls his hand back to increase the time his hand takes to reduce the speed of ball to zero.
  2. An athlete also jumps on a cushion to increase the time his body would take to reduce its speed to zero.
  3. A karate artist breaks a slab of ice with a single blow using same principle.
  4. A small bullet can kill a man if its momentum is great, a heavy truck moving a small speed can kill a man easily sue to it momentum.

Third law: Every action has an equal and opposite reaction.

e.g.: when a bullet is fired, the force by which it moves forward causes recoil of the gun. When the sailor jumps forward, his boat moves backwards.

Energy and types of Energy

Kinetic energy is possessed by an object by virtue of its motion.

Potential energy is possessed by an object by virtue of its position or configuration.

Sum of potential and kinetic energy is called Mechanical energy.

Power is the rate at which work is done. [Unit = watt] Commercial unit of energy is kilowatt hour, thus electrical energy used by an appliance in a month is in kilowatt hour this is also expressed in terms of “unit”.

e.g.: when we stretch a bow the potential energy that is stored in it gets transferred into kinetic when the bow is released. When an object is raised it has gravitational potential energy due to the work done in raising it.

                 Types of energy

                                                                                                     Fig 1: Types of energy


In 1790 French came up with a standard system of measurements. High speed winds are accompanied by reduced air pressure.

Time period [time to complete one oscillation] of a pendulum of a given length is constant. Pendulum were discovered by Galileo Galilee

Speedometer – measures vehicle speed.

Odometer- measures distance traveled.

Galileo Galilee: proposed a new design to the telescope. Designed barometer and pendulum. Discovered moons of Jupiter and the sunspots. He said all planets revolve around the sun not earth.



Light is made of seven colors. Violet [lowest wavelength] bends most when passing through a prism and red [highest wavelength] least. This phenomenon is called dispersion. It is a transverse wave not a mechanical wave.

Law of reflection says that Angle of incidence is equal to the angle of reflection. Also the angle of incidence, angle of reflection and normal lie on the same plane.

Diffusion is caused when the reflection surface is irregular so reflected rays are not parallel.

Human Eye and its Constituents

The white part of the eye is tough to protect interior from accidents.

The transparent coating is called cornea. Behind the cornea is the iris, which gives color to the eye.

The iris has an opening called pupil. Iris controls the amount of light entering the eye. Behind the pupil is a convex lens which focuses light on the retina.

The retina has two cells: rod shaped for dim light and cone shaped for bright light and colors

Sensations of these cells go to the brain via the optic nerve.

Human eye anatomy

Sight Disorders

Sometimes the crystalline lens of eye becomes milky or translucent, this is called cataract.

In myopia, shortsightedness the person can’t see far away objects. This is corrected by using a concave lens.

Hyper metropia or farsightedness is corrected using convex lens.

Presbyopia affects accommodation of eye i.e. ability to adjust its focal length to see objects nearby or far. Such a person uses bi-focal i.e. upper part concave for far sight and lower part convex for near sight.

Astigmatism: This occurs when the cornea is not spherical in shape. For example, the cornea could have a larger curvature in the vertical plane than in the horizontal plane or vice-versa. If a person with such a defect in eye-lens looks at a wire mesh or a grid of lines, focusing in either the vertical or the horizontal plane may not be as sharp as in the other plane. Astigmatism results in lines in one direction being well focused while those in a perpendicular direction may appear distorted. Astigmatism can be corrected by using a cylindrical lens of desired radius of curvature with an appropriately directed axis. This defect can occur along with myopia or hypermetropia.

Human Eye defects

                                                                              Fig 2: Eye defects


Image formed by a plane mirror is virtual, erect, laterally inverted.

Spherical mirrors are concave or convex.

Concave Mirrors

Concave mirrors are used in dental equipment's, by doctors to check ear, nose, and throats. Reflectors of torches, headlights of vehicles, shaving mirrors to get enlarged image of face.

Image of concave mirrors can be real or virtual, inverted or erect and same size, larger or smaller, depending on position of object with respect to mirror.

Concave mirrors

                                                        Fig 1: Concave mirror

Ref: N.C.E.R.T Class XI Science

P = center of the reflecting surface. Pole

C= Center of curvature. Lies outside in case of concave, inside in case of convex.

F= Principal focus. Distance between P and F is focal length f.

Convex Mirror

Convex mirror form image of objects spread over a large area. They are used in rear view or side mirrors of vehicles. Images are virtual, erect and diminished.

convex mirror

                                                       Fig 2: Convex Mirror

Ref: N.C.E.R.T Class XI Science


Lenses are transparent. They form images by refraction of light.

Convex lens

Convex lenses [magnifying glass] are thicker at middle and thinner at edges.

Image of convex lens can be real or virtual, inverted or erect and same size, larger or smaller, depending on position of object with respect to mirror [same as concave mirrors].

Concave lens

Image of concave lens is always virtual, erect and diminished [same as convex mirror].

Convex lens is used in microscope, refracting telescopes, reflecting telescopes [also have concave lens].

Concave lenses are other way round. Convex lens are converging and concave lenses are diverging. Power of convexlens is positive and concave lens is negative.


Refraction of light: Bending of light when it enters a medium is called refraction. Due to refraction pencil appears to bend in water, coin in water appears raised. The object inside water appears enlarged. Twinkling of stars, advanced sunrise or delayed sunset and apparent flattening of sun at sunrise and sunset into oval shape is also caused due to atmospheric refraction.

Dawn and twilight are also due to refraction. they are maximum at the poles and decrease towards the equator.

Total internal reflection occurs when a ray of light traveling from denser to rarer medium is incident at an angle more than the critical angle, it is reflected inside and no refraction takes place.

e.g.: Mirage, diamonds can be cut in such a manner to facilitate total internal reflections to make it shiny, Optical fibers, prisms.

When light ray entered from rarer [optically rarer] to denser [optically denser] medium it bends towards the normal [also speed decreases] and when it enters rarer medium from denser it bends away from the normal [speed increases]. Optical density is different from mass density.

Light travels fastest in vacuum, then air, then liquids and finally solids.

Tydall effect - Scattering of light causes blue sky phenomenon and reddening of sun at sunrise and sunset. Blue wavelength is easily scattered by dust particles, hence sky appears blue. Violet is scattered more easily than blue but our eyes are more sensitive to blue color. However at a great height effect of scattering are negligible so sky appears dark.

Alert lights are Red as it has higher wavelength and won’t be scattered by dust or fog or smoke.

Similarly at sunrise or sunset, the sunlight travels greater distance to reach us; the smaller wavelengths are already scattered so only longer wavelength i.e. Red reaches us.

Day birds have more cones than rod cells and in night birds reverse is the case.

Braille pattern founded in 1821 has 63 characters.

Persistence of vision: Image on the retina persists for one sixteenth of a second after it’s removed. Hence in cinemas the images are flashed at a rate faster than 16 per second.

Objects that give out light are luminous objects.

Shadows need a source of light and an opaque object.


Conductors and Insulators

Conductors –Allow electricity to pass through them. Due to free electrons present on their surface. [Metals, Human body]

Insulators – Don’t allow electricity to pass through them. [Air]

Pure water doesn’t conduct electricity. Most of the acids, bases and salts are conductors. When electrodes are dipped in water and an electric current is passed through it, Oxygen bubbles formed on the electrode connected to the positive terminal of the battery and hydrogen bubbles formed on the other electrode.

Effects of Electric current

Heating effect of electric current is used in geysers, heaters, irons etc. The heating produced in a wire depends on its material, thickness, length. Bulbs have filament of tungsten that emits light when heated at high temperatures, it is also filled with inert gas like nitrogen or argon.

Magnetic effect of current was discovered by Hans Oersted. This principle is used in electromagnets, electric bells.

Uses of Electric current

Electroplating is a common application of chemical effects of electric current.

e.g.: Chromium doesn’t rust or scratch so a metal is coated with chromium for utensils, tin is coated on iron in tin cans so food doesn’t come in contact with iron. Iron objects are coated with zinc to prevent corrosion "Galvanization" .

Corrosion is an oxidation reaction.

Oily foods get oxidized if kept in the open and their taste changes [rancidity], due to this they are kept in air tight containers. Hence, Chips bags are flushed with nitrogen to prevent oxidation.

AC current can be transmitted farther than DC without much loss of electrical energy.

Household devices are connected in parallel.

Fuel Cells

Fuel cells use reaction of oxygen and hydrogen, porous carbon electrodes and aqueous solution of NaOH. Platinum or palladium electrodes are used as catalysts.
        Fuel cell structure


                                                                Fig 1: Fuel cell



All magnets have two poles, north seeking end or North Pole and south seeking end or South Pole.

Magnetic – Materials attracted towards a magnet [iron, nickel, cobalt]

Non magnetic – Materials that are not attracted towards a magnet.

A freely suspended bar magnet always rests in the North South direction.

Take a rectangular piece of iron. Place it on the table. Now take a bar magnet and place one ofits poles near one edge of the bar of iron. Without lifting the bar magnet, move it along the length of the iron bar till you reach the other end. Now,lift the magnet and bring the pole (the same pole you started with) to the same point of the iron bar from which you began. Move the magnet again along theiron bar in the same direction as you did before.  Repeating these several caused the iron bar to become a magnet.

In magnetism, similar poles repel and opposite poles attract each other.

Magnetism can be lost by heating, hammering or dropping magnet from a height.

Electro-Magnetic Forces

Electromagnetic force can be attractive or repulsive but gravitational force is always attractive.

EM force is stronger than gravitational force.

Nuclear force is 100 times stronger than EM force but range is less.

Gravitational force doesn’t need an intervening medium.

Magnetic field produced by a current in a conductor decreases as the distance from it increases.

Right hand thumb rule gives the direction of magnetic field due to current in a conductor. Thumb points in the direction of current and curled fingers point in direction of magnetic field.


Magnetic field is produced even by the weak electrical impulses of our nerve cells. Significant magnetic field is present in brain and heart. The magnetic field produced by the body is the basis of MRI scans that get images of body parts.



When a solid is heated the kinetic energy of the molecules is increased and it overcomes the force of attraction of molecules and the solid melts. When a solid melts the temperature doesn’t increase, the heat energy is used up and is hidden inside the solid and is used for changing its state so it’s called latent heat of fusion.

Open system is where there is an exchange of matter and energy between system and surroundings. Closed system has only exchange of energy. In isolated systems both are absent.

Law of Thermodynamics

First law of thermodynamics is the law of conservation of energy

“Energy can neither be created nor destroyed”.

Specific heat capacity is the amount of heat absorbed or reject by unit mass of a substance to raiseits temperature by 1°C. Water has higher specific heat than land and so it heats and cools slower. Skating is possible on snow as water is formed belowthe skates due to low temperature. This acts as a lubricant.

                        F = 9C/5 + 32; F= temp in Fahrenheit, C= temp in Celsius

Effect of Heat on substances

Solids like camphor, ammonium chloride, naphthalene, anthracene turn from solid to gas on heating. This is called sublimation.

Even on application of pressure the state of matter changes. Pressure exerted by gases is measures using unit atmosphere. 1 atm is pressure at sea level.It decreases with height.

Evaporation occurs as a few particles of a liquid on its surface have higher kinetic energy and they break away from the rest and turn into vapor.

In solids, heat is transferred from higher temperature to lower temperature by process calledconduction [Copper is coated on the bottom of the utensils as it’s a good conductor and helps in spreading out the heat evenly].

Water and air are poor conductors of heat. In water, when its heated hot water at the bottom rises up and cold water moves down. This continues till the whole water is heated. This process is called convection.

Radiation is the process by which sun’s heat reaches us. Radiation doesn’t need a medium. All objects emit radiation when heated; the frequency of radiation goes from lower to higher as the temperature increases.

States of Matters

Plasma – fourth state of matter. This consists of super energized and excited ionized gaseous matter. The neon bulb has neon gas and CFL has argon or helium, this gets energized when electricity is passed through them. This creates plasma that glows depending on the nature of the gas. In sun and stars plasma is created due to high temperatures and produces light.

Bose-Einstein condensate – fifth state of matter. BEC is formed when a gas at very low density is cooled to super low temperatures.

Density measured as kilogram per meter cube.

Natural gas contains methane [50-90%], ethane, propane, butane and hydrogen sulphide [waste]. LPG uses propane and butane. when natural gas is heated we get carbon black used in tyre industries.

Nuclear Energy

Nuclear reactors work on nuclear fission [fission was discovered by Fermi]. Atom bombs also work on the same principle. In reactors, neutrons need to be slowed to ensure interaction with the uranium isotope this is done using heavy water [D2O] or graphite.

Cadmium rods are used to control the reaction. Energy produced by the sun and the working of the hydrogen bomb is based on nuclear fusion.

India has a three stage nuclear program.

In first stage, the natural uranium is used as fuel, heavy water as moderator [pressurized heavy water reactors] and we get plutonium.

In second stage, plutonium and uranium oxide is used to give uranium -233 in fast breeder reactors where no moderator is needed.

In final stage, thorium and uranium-233 is used to get energy [thorium based reactors].

Heavy water reactors need frequent refueling and produce less energy compared to light water reactors that use distilled water as coolant and fissile uranium -235 as fuel.


Anomalous behavior of water

Water exhibits anomalous behavior below 4°C, its volume increases and density decreases. Due to this the ponds and lakes freeze at the top during winters as at 4°C water becomes less dense and remains on top and solidifies. This leaves animal and plant life intact at bottom.

Effect of Wool and Cotton on Heat

Dark clothes or surfaces absorb more heat and hence dark clothes are preferred in winter. Light clothes reflect more heat and so such clothes are preferred in summer.

Wool is a bad conductor of heat, woolen fibers trap air and this prevents body heat from escaping. So woolen clothes are preferred in winters. We should use two blankets instead of one in winters as the trapped layer of air between the blankets prevents heat from escaping.

Cotton clothes are sweat absorbent, they expose the sweat to the atmosphere and when it evaporates it absorbs latent heat from the body, this heat is removed and the body cools down.

Effect of Air parcel on heat

Construction of houses is done using hollow bricks that have air trapped in them. This prevents heating of houses during summer and keeps flats cool. During winter the heat from flats doesn’t escape out keeping them warm.



This is either contact [friction, muscular] or non contact [magnetic, electrostatic, gravitational, pressure]


Pressure is inversely related to area. So smaller the area, larger the pressure for the same force.

Pressure and its real applications

E.g. Nail is easily pushed into wood from its pointed end than its head, tools for cutting have a sharp edge, shoulder bags have broad straps, and porters carrying heavy loads wrap a cloth on their head.

A large wooden plank placed on top of the circus performer keeps his ribs from breaking when an elephant steps on him.

All above activities regulate pressure by changing the surface area.

Suits of astronauts are filed with air to counter the pressure exerted by the body otherwise their blood vessels would burst as there no pressure on the moon.

At high altitudes, pressure is low so liquids boil at low temperature. Hence we use pressure cookers to cook.

In autoclaves to sterilize medical instruments, waters boiling point is increased by putting higher pressure on it.

Pressure by liquids

Liquids also exert equal pressure at the same depth. Air also exerts a great pressure on organisms but our body counters the pressure so we don’t feel anything.

Liquids are treated as in-compressible, whenever pressure is applied on any part of a liquid it is transmitted in all directions equally and undiminished. This principle is used in hydraulic lifts and brakes.



What is Sound?

Sound is produced by vibrating objects. Vibrations cause compressions [high pressure] and rarefactions [low pressure].

Sound waves travel through a medium like this. Sound waves are longitudinal i.e. individual particles of a medium move in a direction parallel to the direction of propagation of the disturbance. The particles don’t move physically but oscillate back and forth.

In humans, the voice box or larynx has two vocal cords that produce sound through vibration when lungs force air through them.

Vocal cords in males are 20mm, women 15mm and children even shorter.

Larynx and Physiology of ear

Doppler effect


Fig 1: Larynx and Physiology of ear

Eardrum vibrates when sound waves hit it; these vibrations are sent to the brain from the inner ear.

The sound waves are collected by the pinna [outer ear] these then pass through the auditory canal to the ear drum which amplifies it. The middle ear [hammer, anvil, and stirrup] then amplifies it and sends it to the inner ear. In the inner ear [the cochlea] converts it to electrical signals which are taken to the brain by the auditory nerve. The brain interprets them as sound.

Properties of Sound

  • Loudness of sound is proportional to square of its amplitude.
  • Loudness is measured in decibels.
  • Amplitude decides the loudness of the sound.
  • Higher sound has higher energy so travels a longer distance.
  • However as it moves away from the source its amplitude keeps on decreasing.
  • Pitch or shrillness is determined by its frequency.
  • Quality or timber of sound decides the pleasantness of sound.
  • Sound of single frequency is a tone; sound which is a mixture of multiple frequencies is anote.

Speed of Sound

Speed of sound remains almost same for all frequencies in a given medium under same physical conditions. Intensity is the amount of sound energy passing through a unit area it’s different than loudness. Two sounds of equal intensity can be of different loudness.

Speed of sounds is highest in solids and least in gases.

It increases with temperature of the medium. Sound travels through solids, liquids, gases but not through vacuum.

Sonic Boom

When a sound producing object moves faster than sound, it creates shock waves that have high energy. Air pressure variations due to these shockwaves produce a large, sharp sound calledsonic boom.

Sound obeys the laws of reflection same as light waves. The sensation of sound persists in our brain for 0.1s. If the obstacle from which sound is reflected is at distance more than 17.2m then we hear echo. Sound absorbent materials absorb sound and prevent multiple reflections.

Stethoscope, horns, trumpets, shehenais, auditoriums are designed to take advantage of multiple reflections of sound.

Human Ear

Human ear can hear sounds only between 20 Hz to 20000 Hz i.e. 20 -20000 oscillations per second. Children below 5 can hear infrasonic sounds i.e. below 20Hz. During earthquakes infrasonic sound are produced which are heard by birds and animals and they are disturbed earlier.

Applications of Sound

Ultrasonic sounds above 20 kHz. They have many applications like in cleaning of instruments, detecting cracks in building, machines, ultrasound machines and echo cardiograph, SONAR. All depend on detection of reflected ultrasonic waves from obstacles, body parts etc.

Doppler Effect is observed when a moving object causes change in frequency of EM waves. In military it’s used to detect enemy aircrafts.



                                                          Fig 2: Doppler effect

In astronomy it’s used to find speed of moving stars and in sonography it’s used to study heart beats and blood flow. In echocardiography it used to study heart beats.



Surface tension explains why Bubbles are spherical.

Water doesn’t spread on some surfaces but forms droplets.

Oil and water don’t mix; water wets us but not ducks.

Hairs of brush stick together when removed out of water.

Raindrops are spherical. oil rises in wick; sap and water rise in plants.

Particles of soil remain separated under water but stick together when taken out.

When glass pieces are melted the edges take rounded shape and become smooth, this is called fire polishing.

Properties of Surface Tension

Surface tension decreases with temperature.

Angle of contact determines if liquid shall spread or form droplets. It is reduced by wetting agents so they can penetrate well; whereas it is increased by waterproofing agents so water doesn’t stick. Detergents molecules attract water on one side and grease-dirt on other so they help in washing by reducing surface tension of water-oil.

                            Surface tension angle of contact

                                                                                     Fig 1: Angle of Contact

Viscosity is due to the friction between different liquid layers as they slip past each other when the liquid flows.

Properties of Viscosity

Higher viscosity means slower is the flow. Increasing the temperature decreases the viscosity.

E.g.: Glass is an extremely viscous liquid; it’s so viscous that it resembles solids. The windowpanes of old buildings are thicker at the bottom as glass flows down due to gravity.


Rolling friction [rolling a body on ground] is less than sliding friction [pushing a body already in motion] is less than static friction [pushing a static object].

e.g.: Use of ball bearings in hubs and axels of ceiling fans and bikes to reduce contact surface between two solid parts. Thin cushion of air is maintained between solids in relative motion for same purpose. Lubricants too reduce friction.

Earthquakes occur when continental plates brush past each other or collide. Earthquakes are measured in Richter’s scale. An earthquake of intensity 6 is 1000 times more destructible than earthquake of intensity 2, so Richter scale is not linear.


The escape velocity on moon is lower so gas molecules if formed escape as they can easily overcome the gravitational pull of the moon. Thus the moon has no atmosphere.

Geostationary satellites appear to be appearing fixed from any point on the earth. To do this the satellite must be at a height of 35800km from the earth’s surface and in the equatorial plane. Since EM waves beyond a certain frequency can’t be reflected by the ionosphere [TV], for reflecting such waves we use geostationary satellites.

Polar satellites go around earth in north south direction, their time period is 100 mins approx and their altitude is 500-800km. The satellites at a higher height have a lower speed of revolution to stay in orbit as gravitational pull is lesser but this is independent of their weights. They don’t fall as earth’s gravitation provides necessary acceleration to it.

Polar and geostationary satellites

Fig 2: Types of Satellite

Satellites are launched preferably from equator and eastwards as it aids the speed due to lower acceleration due to gravity ‘g’. Speed of rotation of satellites is independent of its size and mass.

Note: centrifugal force is more at equator than poles. Gravity is higher at poles than equator.

Electromagnetic Waves

EM waves in decreasing order of wavelength

Long Radio wave AM radio > TV and FM waves > short radio > microwave > infrared > visible > UV > X-ray > gamma

In satellites, the acceleration towards the center of the earth is equal to earth’s acceleration due to gravity at that point. Thus the objects inside the satellite experience weightlessness. A body in free fall experiences no upward force on him and so feels weightless.