Newton's Laws of Motion

(1) Introduction

  • While studying kinematics, we have already studied about the position , displacement and acceleration of a moving particle
  • Here in this chapter, we would take our understanding one step further to learn abouts origins of acceleration or force
  • Here we will specifically consider the cause behind the moving objects i.e what causes the objects to move
  • Thus we will learn the theory of motion based on the ideas of mass and force and the laws connecting these physical concepts to the kinematics quantities
  • So we will begin by stating the Newton's law's of motion which are of critical importance in classical mechanics
  • Laws of motion as stated by Newtons in his principle are
    (i) Every body continues in its state of rest or uniform motion in straight line ,unless compelled to change that state by force imposed upon it
    (ii) Change of motion of an objects is proportional to the force acting on it and is made in the direction of the straight line along the direction of force
    (iii) To every action there is always an equal and opposite reaction

(2) Force

  • Concept of force is central to all of physics whether it is classical physics,nuclear physics,quantum physics or any other form of physics
  • So what is force? when we push or pull anybody we are said to exert force on the body
  • Push or pull applied on a body does not exactly define the force in general.We can define force as an influence causing a body at rest or moving with constant velocity to undergo an accleration
  • There are many ways in which one body can exert force on another body
    Few examples are given below
    (a)Stretched springs exerts force on the bodies attached to its ends
    (b)Compressed air in a container exerts force on the walls of the container
    (c) Force can be used to deform a flexible object
    There are lots of examples you could find looking around yourself
  • Force of gravitational attraction exerted by earth is a kind of force that acts on every physical body on the earth and is called the weight of the body
  • Mechanical and gravitation forces are not the only forces present infact all the forces in Universe are based on four fundamental forces
    (i) Strong and weak forces: These are forces at very short distance (10-05 m) and are responsible for interaction between neutrons and proton in atomic nucleus
    (ii) Electromagnetic forces: EM force acts between electric charges
    (iii) Gravitational force -it acts between the masses
  • In mechanics we will only study about the mechanical and gravitational forces
  • Force is a vector quatity and it needs both the magnitude as well as direction for its complete description
  • SI unit of force is Newton (N) and CGS unit is dyne where
    1 dyne= 1005 N

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