Newton’s Laws of Motion

     Newton has published three laws, which describe how forces affect motion of a body on which they act. These laws are fundamental in nature in the sense that the first law gives concept of force, inertia and the inertial frames; the second law defines force and the third law, action and reaction as two aspects of mutual interaction between two bodies.

 

     The First Law  

     Every material body has tendency to preserve its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by external forces impressed on it. This property is called Inertia.

• Inertia

   The tendency of a material body to preserve its present state of uniform motion or of rest is known as inertia of the body. It was first conceived by Galileo.

   Inertia is a physical quantity and mass of a material body is measure of its inertia.

• Inertial Frame of Reference  

   The first law requires a frame of reference in which only the forces acting on a body can be responsible for any acceleration produced in the body and not the acceleration of the frame of reference. These frames of reference are known as inertial frames.

 

 

The Second Law

     The rate of change in momentum of a body is equal to, and occurs in the direction of the net applied force.

      A body of mass m in translational motion with velocity , if acted upon with a net external force , the second law suggests:

 

     If mass of the body is constant, the above equation relates the acceleration  of the body with the net force  acting on it.                     

 

     The first law provides concept of force and the second law provides the quantitative definition of force, therefore the second law is also valid only in inertial frames.

     SI unit of force is newton. It is abbreviated as N. One newton equals to one kilogram-meter per second square.

1 N = 1 kg-m/s2

     Dimensions of force are MLT–2

   

     The Third Law

     Force is always a two-body interaction. The first law describes qualitatively and the second law describes quantitatively  what happens to a body if a force acts on it, but do not reveal anything about what happens to the other body participating in the interaction responsible for the force.

      The third law accounts for this aspect of the force and states that every action on a body has equal and opposite reaction on the other body participating in the interaction.