General science: physics,Motion,Newton’s laws

General science: physics

Physics is one of the oldest academic disciplines, perhaps the oldest if one includes astronomy in it. Measurement system: Unit: the chosen standard used for measuring a physical quantity is called unit. It is a definite magnitude of a physical quantity and adopted by law. It should be: well defined, easy to reproduce easy to compare, independent of changes in physical conditions inter nationally accepted. Units are of two types: fundamental unit and derived unit. System of units, units depend on choice. Each choice of units leads to a new system (set) of units. The internationally accepted systems are CGS system, MKS system, FPS system and SI Unit.
In SI units, there are seven fundamental units given below.

Physical quantity                   SI Unit                               Symbol
Length                                  meter                                m  
Mass                                  kilogram                                kg
Time                                 second                                    s
Electric current                  ampere                                   A
Temperature                      Kelvin                                   K
Luminous intensity              Candela                              Cd
Amount of substance           mole                                 mol

Besides these seven fundamental units, two supplementary units are also defined. These are radian [rad] for plane angle and steradian (sr) for solid angle. All the units which are defined or expressed in terms of fundamental units are called derived units.
                                 Some derived units
Physical quantity CGS               Units SI Unit Relation
Force dyne Newton 1 Newton= 105 dyne
Work erg joule 1 joule= 107 erg

Motion: distance: it is the length of actual path covered by a moving object in a given time interval.
Displacement: It is the shortest distance covered by a body in a definite direction. Distance is a scalar quantity where as displacement is a vector quantity but both having the same unit (meter). Displacement may be positive, negative or zero whereas distance is always positive. Generally the magnitude of displacement is lesser than or equal to the distance.
Speed: distance travelled by any moving object in unit time interval is called speed. I.e. speed=distance/time. It is a scalar quantity and its SI unit is meter/second (m/s).
Velocity: the velocity of any moving object is defined as the displacement of the object in unit time interval. I.e. velocity= displacement/time. It is a vector quantity and its SI unit is meter/second (m/s).

Acceleration: the acceleration of an object is defined as the rate of change of velocity of the object. I.e., acceleration=change in velocity/time. It is a vector quantity and its SI unit is meter/second2 (m/s2).
Laws of motion: Newton the father of physics established the laws of motion in his book “Principia” in 1667. These Physical laws form the basis of classical mechanics.
Inertia: inertia is the property of a body by virtue of which the body opposes change in its initial state of rest or motion with a uniform speed and on a straight line. It is of two types: Inertia of rest and inertia of motion. Some examples of inertia are
a. When a car or train starts suddenly, the passengers bend backward.
b. When a running horse stops suddenly, the rider bends forward.
c. When a coat/blanket is beaten by a stick, the dust particles are removed and scattered.
This law gives the definition of force.
Force: force is that external cause which, when acts on a body, changes or tries to change initial state of the body.
Momentum: Momentum is the property of a moving body and is defined as the product of mass and velocity of the body, i.e. momentum=mass*velocity. It is a vector quantity. Its SI unit is Kg-m/s.
Impulse: when a large force acts on a body for very small time, then that force is called impulsive force. It is defined as the product of force and time. Impulse=force*time=change in momentum. It is a vector quantity and its direction is the direction of the force. Its SI unit is Newton second (Ns).

Newton’s first Law of Motion: Everybody maintains its initial state of rest or motion with uniform speed on a straight line unless an external force acts on it. This law is also called law of Galileo or the law of inertia.

Newton’s Second Law of Motion: the rate of change is momentum of a body is directly proportional to the force applied on it and takes place in the direction of the force. If F= the force applied, a= the acceleration produced and m= mass of the body, then the nF= ma. Newton’s second law gives the magnitude of force. Newton’s first law is contained in the second law.

Newton’s third law of motion: To every action, there is an equal and opposite reaction. Examples of third law are: recoil of a gun, motion of rocket, swimming, while drawing water from the well, if the strong breaks up then the man drawing water falls back.