15 Important Physics Laws: Definition with Examples

1. Archimedes Principle: When a body is immersed partly or wholly in a liquid, there is an apparent loss in the weight of the body (create upward thrust) which is equal to the weight of liquid displaced by the body. The body floats on liquid according to Archimedes principle.

A body floats when

·       Density of body less then liquid.

·       If density of body and liquid are equal then body floats fully submerged in liquid .

·       When body floats in neutral equilibrium equal weight displace by body.

·       Centre of gravity of the body and water displaced should in one line.

    2. Newton’s Laws of Motion: The father of physics newton give his law of motions in his book ‘Principa’ in 1687.

First law of Motion:  Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise. This law also called Law of inertia and it gives definition of force.

For example, a spaceship out in interstellar space, far from all other objects and with all its rockets turned off, has no net external force acting on it. Its acceleration, according to the first law, must be zero. If it is in motion, it must continue to move with a uniform velocity.

Second law of Motion: The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts. This law defines strength of force.

If F is Force applied , m is mass of body  and a is acceleration produced :--

                                                F= ma

Third law of Motion:  To every action, there is always an equal and opposite reaction.

Some examples of third law are

·       Motion of rocket

·       Recoil of gun

·       Earth pull the stone due to gravity

3. Avogadro’s Law: It is a gas law which states that the total number of atoms/molecules of a gas (i.e. the amount of gaseous substance) is directly proportional to the volume occupied by the gas at constant temperature and pressure. Avogadro’s law is closely related to the ideal gas equation since it links temperature, pressure, volume, and amount of substance for a given gas.

Expansion and contraction of the lung during the breathing cycle , puncture wheel of bike acquire less surface area then air filled and filling gas in hydrogen balloon are the best example of Avagadro’s law.

4. Ohm’s Law: If physical conditions like temperature, intensity of light etc. remains  unchanged then electric current flowing through a conductor is directly proportional to the potential difference across its ends. If V is the potential difference across the ends of conductors and I is the current through it, then according to Ohm,s Law

V is directly proportional to I or V=RI
is a constant called resistance of conductor
A force, such as friction, that operates opposite the direction of motion of a body and tends to prevent or slow down the body's motion.  A measure of the degree to which a substance impedes the flow of electric current induced by a voltage. Resistance is measured in ohms.
5. Coulomb's Law: According to coulombs Law the force of attraction or repulsion between two point charges at rest is directly proportional to the magnitudes of the charges and inversely proportional to square of distance between them. This force act on the line joining the two charges.

6. Pascal’s Law: The French scientist Blaise Pascal observed that the pressure in a fluid at rest is the same at all points if they are at the same height. In other words if gravitational attraction is negligible in equilibrium condition pressure is same at all the points. If external pressure is applied to an enclosed fluid it is transmitted undiminished to every direction.  The SI unit of pressure  is pascal.


o   Consider a vessel of circular shape filled with water which has 4 openings and in the entire openings 4 pistons are attached.

o   Apply force on the first piston; this piston will move inward and all other pistons will move outwards.

o   This happens because when this piston moves inwards the pressure is exerted on the water.Water transmits this pressure in all the directions.

o   The other pistons,except A, moves at the same speed which shows water has exerted pressure in all the directions.

     7. Stefan’s Law: this law is related to radiant energy of a blackbody. According to this law an object that absorbs all radiations fall on it is directly proportional to fourth power to its temperature. This is first important step towards the understanding of blackbody Radiation.

Lets take an example One end of a rod of length 20 cm is inserted in a furnace at 800 K. The sides of the rod are covered with an insulating material and the other end emits radiation like a black body. The temperature of this end is 750 K in the steady state. The temperature of the surrounding air is 300 K.

8.Hooke's Law:  Robert Hooke, an English physicist (1635 - 1703 A.D) performed experiments on springs and found that the elongation (change in the length) produced in a body is proportional to the applied force or load. In 1676, he presented his law of elasticity called Hooke’s law.

Hooke's Law is also used in other everyday objects such as trampolines, where Hooke's Law is applied when the springs that hold up the center of the trampoline all stretch when a person jump on the trampoline.

    9. Bernoulli’s principle : Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy.  The principle is named after Daniel Bernoulli who published it in his book Hydrodynamica in 1738.

       Bernoulli’s principle helps in explaining blood flow in artery. The artery may get constricted due to the accumulation of plaque on its inner walls. In order to drive the blood through this constriction a greater demand is placed on the activity of the heart. This can cause heart attack.

10. Boyle's Law: Boyle’s law is a gas law which states that the pressure exerted by a gas (of a given mass, kept at a constant temperature) is inversely proportional to the volume occupied by it. In other words, the pressure and volume of a gas are inversely proportional to each other as long as the temperature and the quantity of gas are kept constant. Boyle’s law was put forward by the Anglo-Irish chemist Robert Boyle in the year 1662.
The gas law specifically applies to the lungs. When a person breathes in, their lung volume increases and the pressure within decreases. Since air always moves from areas of high pressure to areas of low pressure, air is drawn into the lungs.

     11. Charles's Law: Charles and Gay Lussac performed several experiments on gases independently to improve upon hot air balloon technology. Their investigations showed that for a fixed mass of a gas at constant pressure, volume of a gas increases on increasing temperature and decreases on cooling. They found that for each degree rise in temperature, volume of a gas increases by 1/ 273.15 of the original volume of the gas at 0 ° C.

      Charles’ law, states that pressure remaining constant, the volume of a fixed mass of a gas is directly proportional to its absolute temperature. 

    12. Kepler’s Law:  Kepler give 3 law related to planetary motion which are-

1.Law of orbits : All planets move in elliptical orbits with the Sun situated at one of the foci of the ellipse.

Here F1 and F2 are foci , A1 and A2 are area

2. Law of areas : The line that joins any planet to the sun sweeps equal areas in equal intervals of time (Fig. 8.2). This law comes from the observations that planets appear to move slower when they are farther from the sun than when they are nearer.
3. Law of periods : The square of the time period of revolution of a planet is proportional to the cube of the semi-major axis of the ellipse traced out by the planet.

         13. Law of conservation of energy: according to this law energy can neither be created nor be destroyed. Only energy can be transformed from one form to another form. Whenever energy is utilized in one form equal amount of energy is produced in another form. Hence total energy of the universe always remains the same.

     Some examples of conservation of energy dynamo which convert mechanical energy into electrical energy and solar cell converts photons (sunlight) into electrical energy.

     14. Tyndall effect: Because of the small size of colloidal particles, we cannot see them with naked eyes. But, these particles can easily scatter a beam of visible light. This scattering of a beam of light is called the Tyndall effect after the name of the scientist who discovered this effect.

      Tyndall effect can also be observed when a fine beam of light enters a room through a small hole. This happens due to the scattering of light by the particles of dust and smoke in the air.

    15. Grahams Law: Graham's law states that the rate of diffusion or of effusion of a gas is inversely proportional to the square root of its molecular weight. Thus, if the molecular weight of one gas is four times that of another, it would diffuse through a porous plug or escape through a small pinhole in a vessel at half the rate of the other (heavier gases diffuse more slowly). A complete theoretical explanation of Graham's law was provided years later by the kinetic theory of gases.

           We can separate the isotopes of an element using Graham's law. A common example is enriching uranium from its isotope. U is desired uranium which amounts 0.72 % of natural uranium. The separation of 235U from the rest isotope 238U is achieved by passing uranium hexafluoride gas into porous membranes.