Vinod Singh

teacher | Chemistry


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7+ Years

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Harischandra Mahavidhyalaya

Important Laws of Physics

Physical laws are the conclusions drawn on the basis of long year scientific observations and experiments which are repeated again and again under different conditions to reach the assumptions which can be accepted worldwide. Let us discuss some important laws of physics which will enhance knowledge as well as help in the preparation of various entrance examinations.

  1. Archimedes Principle:  It states that when a body is partially or totally dip in a fluid, it experiences an upward thrust equal to the weight of the fluid displaced by it that i.e. its apparent loss of weight is equal to the weight of liquid displaced. 

  2. Avogadro’s Law: This law states that equal volume of all gases under the same conditions of temperature and pressure contain equal number of molecules.

  3. Ohm's Law: It states that the current passing through a conductor between two points is directly proportional to the potential difference across the two points provided the physical state and temperature etc. of the conductor does not change.

  4.  Newton’s Laws: Law of Gravitation: Objects attract each other with a force directly proportional to the product of the masses of the objects and inversely proportional to the square of the distance between them. Hence, for objects on or near the earth, the mass of the earth is very much greater than the object, and so the gravitational force between them makes objects fall towards the earth. That is why lead and feather fall at the same rate in a vacuum.

Newton’s First law of Motion: A body continues in its state of rest, or of uniform motion in a straight line, except in so far as it is compelled by external impressed forces to change that state. It is also called Law of Inertia.

Newton’s Second Law of Motion: The rate of change of momentum is proportional to the impressed force and takes place in the direction of the straight line in which the force acts. In other words, “Force is equal to mass multiplied by acceleration”.

Newton’s Third Law of Motion: To every action there is equal and opposite reaction. This is the principle behind the recoil felt on pulling the trigger of a gun.

Newton’s Law of cooling: The rate at which a body cools or loses its heat to its surroundings is proportional to the excess of mean temperature of the body over that of the surroundings, provided this temperature excess is not too large.


  1. Coulomb’s Law: The force between the two electric charges reduces to a quarter of its former value when the distance between them is doubled. The SI unit of electric charge, coulomb, is named after Charles Augustin de Coulomb who established the law.

  2.  Stefan’s Law: The total energy radiated from a black body is equal to the fourth power of its absolute temperature.

  3. Pascal’s Law: - When pressure is applied to a fluid, the pressure change is transmitted to every part of the fluid without loss. Hydraulic machines like the hydraulic press work on this principle.
    - Atmospheric pressure decreases with increase in height. The SI unit of pressure is Pascal which is named after Pascal who established this law.

  4. Hooke’s Law: This law states that the extension of a spring is proportional to the tension stretching it. Doubling of the tension results in the doubling of the amount of stretch.

  5. Bernoulli's Principle: It states that as the speed of a moving fluid, liquid or gas, increases, the pressure within the fluid decreases. The aerodynamic lift on the wing of an aero plane is also explained in part by this principle.

  6.  Boyles's Law: It states that temperature remaining constant, volume of a given mass of a gas varies inversely with the pressure of the gas.

  7. Charles's Law: It states that pressure remaining constant, the volume of a given mass of gas increases or decreases by 1/273 part of its volume at 0 degree Celsius for each degree Celsius rise or fall of its temperature.

  8. Kepler's Law: Each planet revolves round the Sun in an elliptical orbit with the Sun at one focus. The straight line joining the Sun and the planet sweeps out equal areas in equal intervals. The squares of the orbital periods of planets are proportional to the cubes of their mean distance from the Sun.

  9. Law of conservation of energy: It states that energy can neither be created nor destroyed but it can be transformed from one form to another. Since energy cannot be created or destroyed, the amount of energy present in the universe is always remaining constant.

  10. Graham’s Law: It states that the rates of diffusion of gases are inversely proportional to the square roots of their densities under similar conditions of temperature and pressure.

Empirical and Molecular formula

The empirical formula is the simplest formula for a compound. A molecular formula is the same as or a multiple of the empirical formula, and is based on the actual number of atoms of each type in the compound. For example, if the empirical formula of a compound is C3H8 , its molecular formula may be C3H8 , C6H16 , etc.


Empirical formula of a chemical compound is the simplest positive integer ratio of atoms present in a compound. A simple example of this concept is that the empirical formula of sulfur monoxide, or SO, would simply be SO, as is the empirical formula of disulfur dioxide, S2O2.


A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using a single line of chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and plus (+) and minus (−) signs.


To Write a chemical Formula

Memorize the prefixes for number of atoms.

Write the chemical symbol for the first element.

Add the number of atoms as a subscript.

Write the chemical symbol for the second element.

Add the number of atoms present as a subscript.


Brackets Meaning in Chemical Equations:

Some formulas have brackets in them. For example, sodium hydroxide is NaOH, but magnesium hydroxide is Mg(OH)2. The 2 outside the brackets tells you that you have two of each atom inside the bracket. So in Mg(OH)2 you have one magnesium atom, two oxygen atoms and two hydrogen atoms.

Balancing Chemical Equations

Balance the oxygen atoms:

Because you've added coefficients to the molecules on the right side of the equation, the number of oxygen atoms has changed.

Add a coefficient of 5 to the oxygen molecule on the left side of the equation. ...

C3H8 + 5O2 --> 4H2O + 3CO2.

The carbon, hydrogen, and oxygen atoms are balanced


How to balance a chemical equation with parentheses:

Remember with subscripts, any number to the right of parentheses multiplies each subscript within the parentheses. eg Fe2(SO4)3 contains 2 Fe atoms, 3 S atoms and 12 O atoms. 5. Finally make sure that all the coefficients are in the smallest possible whole number ratio.

Characteristics of Sound Waves

Sound wave can be described by five characteristics: Frequency, Amplitude, Wavelength, Time-Period, and Velocity.

Frequency: Number of complete waves produced in 1 sec is called frequency of the wave. Since one complete wave is produced by one full vibration of the vibrating body, so we can say that the number of vibrations per sec is called frequency. e.g. if 5 complete waves are produced in 1 sec then the frequency of the waves will be 5 Hz or 5 cycles per second.

Amplitude: If a wave passes through any medium, the particles of the medium get displaced temporarily from their original positions. The maximum displacement of the particles of the medium from their original positions, when a wave passes through the medium is called amplitude of the wave.

Wavelength: Minimum distance in which a sound wave repeats itself is called its wavelength. i.e it is the length of one complete wave. It is denoted by a Greek letter λ (lambda).  We know that in a sound wave, the combined length of a compression and an adjacent rarefaction is called its wavelength. Also, the distance between the centers of two consecutive compressions or two consecutive rarefactions is equal to its wavelength.

Time-Period: Time required to produce one complete wave or cycle  is called time-period of the wave. Now, one complete wave is produced by one full vibration of the vibrating body. So, the time taken to complete one vibration is known as time-period.

Velocity: Distance travelled by a wave in 1 sec is called velocity of the wave, represented by the letter v. The S.I unit for measuring the velocity is m/s.

Velocity = Distance travelled / Time taken

Let v = λ / T     (T = time taken by one wave.)

v = f X λ         (This formula is known as wave equation.)

v = velocity of the wave, f = frequency, λ = wavelength

Velocity of a wave = Frequency X Wavelength

Barcode and how is it made?

Barcode basically encodes alphanumeric characters and symbols using black and white stripes, also called bars. it is a machine-readable code in the form of numbers and a pattern of parallel lines of varying widths, printed on a commodity. Hence a barcode essentially is a way to encode information in a visual pattern that a machine can read. The combination of black and white bars (elements) represents different text characters which follows a set algorithm for that particular barcode. Barcode contains information about a product like; price & weight of the product, date of manufacturing and expiry, name of the manufacturer etc.


Structure of Barcode:

Quiet Zone – The minimum required space for barcode scan-ability, preceding the Start Character of a bar code symbol. The quiet zone should be free from any printing and be the same color and reflectance as the background of bar code symbol.

Starts Code – Indicates the start of the barcode. These are special barcode characters & they signify the start of data to the scanner/reader. Start characters are usually stripped-off and not transmitted to the host.

Data –  The actual data the barcode stores.

Check Digit – Check digit (not always present) is a mathematical sum that is used to verify the accuracy of the other elements of the barcode. It is the extra digit added at the end of a bar code to allow the scanner to confirm that it read the barcode correctly. It is typically stripped from the data and not transmitted to the host.

Stop Code – Indicates the stopping point of the barcode. These characters signify the end of data to the scanner/reader. They are also stripped-off and not transmitted to the host.

Trailing Quiet Zone – Another clear space with no printing following the Stop Character.

Uniform Circular Motion

Circular Motion: When an object moves in a circle, it is said to be in a circular motion. That is we can say that motion in a circle is a circular motion. When a body or object moves along a circular path, then its direction of motion or direction of speed keeps changing continuously. So, if an athlete moves with a constant speed along a circular path, then the velocity of the athlete will not be constant because velocity is the speed in a specified direction and here the direction of speed changes continuously. Since, the velocity changes with the continuous change in direction, therefore, the motion along a circular path is said to be accelerated.


Uniform Circular Motion: When a body moves in a circular path with uniform speed or constant speed, its motion is known as Uniform Circular motion. It is possible for a body to move in a circular path with uniform speed as long as it is travelling equal distances in equal intervals of time. But the velocity of the body moving in a circle with uniform speed is not uniform because the direction of motion is constantly changing.



Examples of Uniform Circular Motion:

1. Artificial satellites move in uniform motion around the earth. Therefore, the motion of a satellite around the earth is accelerated.

2. The moon moves in a uniform circular motion around the earth. We know that moon is a natural satellite of the earth.

3. Similarly, we can say that movement of earth around the sun is also a uniform circular motion. So, the motion of earth around the sun is accelerated.

4. The tip of a second’s hand of a watch exhibits uniform circular motion on the circular dial of the watch.

Laws of Floatation

Archimedes Principle: Principle of buoyancy which explains that when an object is immersed in a fluid, it appears to become lighter because there is an upward buoyant force exerted on the object. The buoyant force is equal to the mass of water displaced by the object.


The object will float if the buoyant force exceeds the mass of the object, and sink if the mass exceeds the buoyant force as:

•    If density of material of body is equal to density of liquid, the body floats fully submerged in liquid in neutral equilibrium.

•    When body floats in neutral equilibrium, the weight of the body is equal to the weight of displaced liquid.

•    The center of gravity of the body and center of gravity of the displaced liquid should be in one vertical line.



Centre of Buoyancy and Meta Centre

Center of Buoyancy: Center of Buoyancy is the center of gravity for the volume of water which a hull displaces. Center of Gravity is the point in a body where the gravitational force may be taken to act.

•    When the hull is upright the center of gravity and center of buoyancy are on the same vertical line, and the hull is stable.

•    For most hulls the center of buoyancy is below the center of gravity and the hull is said to be metastable.

•    When the hull tilts the center of gravity remains in the same position related to the hull. The center of buoyancy will move to fit the new center of gravity for the volume of water replaced by the hull. In the beginning the gravity force and the buoyancy force will create a righting torque that tries to move the hull back to the upright position.

•    If the hull is tilted to much the center of buoyancy will move to a position where the buoyancy and gravitational force starts to create a moment that work in the same direction and the hull will capsize.


Meta center: When a floating body is slightly tilted from equilibrium position, the center of buoyancy shifts. The point at which the vertical line passing through the new position of center of buoyancy meets with the initial line is called meta center.

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