Nature of Physical Laws | Physical world, part - 3

 

Nature of Physical Laws:

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Physical Laws are the basis of study of science.  These physical laws define various types of physical phenomenon governed by different forces. In these physical phenomena some physical quantities change with time while the other remains constant. 

Such quantities which do not change with time are said to be conserved.

In order to explain or to understand the various types of motion, conservation laws were developed. These are : 

• Laws of conservation of linear momentum 

• Laws of conservation of energy and 

• Laws of conservation of angular momentum

Law of conservation of linear momentum:

                      It states that the linear momentum of a system remains constant if no  external force is applied on it.

        Consider a system of two bodies isolated from the surroundings in which no external force is applied on the bodies. Again we consider the initial linear momentum of the first body is P1i and of the second body is P2i and these two bodies are made to collide with each other such that after collision their momentum changes to P1f and P2f . 

Then according to the law of conservation of linear momentum the total linear momentum of the system  will remain unchanged.

i.e.       P1i + P2i  = P1f + P2f

Or        P1 + P2  =  constant

Law of conservation of linear momentum can be obtained from Newton's third law of motion.

Examples: 

• Recoil of a gun, i.e. when a gun fires a bullet then it recoils in a direction opposite to the direction of the bullet.

• When a heavy nucleus breaks into two smaller nuclei then the products moves in opposite direction, hence obey the law of conservation of linear momentum

Law of conservation of energy:

it states that energy can neither be created nor be destroyed, it can be  changed only from one form of energy to another form.

Examples : 

• When a body Falls freely under the effect of gravity then its mechanical energy (i.e.   the sum of kinetic and potential energy ) remains constant throughout its motion.

• When a simple pendulum oscillates between its 2 extreme points then the mechanical energy throughout its oscillation remains the same.

• Einstein's famous mass energy equivalence relation E = mc2 is an example of a law of conservation of mass and energy.

Law of conservation of angular momentum:

                     It states that the total angular momentum of a system remains constant, if no external torque is applied on it.

If L is the angular momentum of a body or system of bodies then

i.e.    L =  Iω = Constant

where I = moment of inertia of a body 

                or system of bodies ( need to

                be learnt  about it in detail in

                next Chapters )

         ω = angular velocity of the body 

                about an axis of rotation

Examples :

• When a planet moves around the sun in its Orbit, its velocity increases when it is close to the sun and decreases when it is far away from the Sun, such that the total angular momentum  of the planet ( as depends upon the distance from the Sun at any point)  always remains constant.

• A ballet dancer increases her angular velocity by folding her arms and bringing the stretched leg close to the other leg . It shows the use of law of conservation of angular momentum.

Along with these conservative laws, you will also study law of conservation of charges and law of conservation of parity in higher classes.

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