Laws of Motion | Newton Motion| Newton’s 3rd law definition

Newton’s gives three following laws of motion:-

First Law’s:- In an inertial frame of reference an object or body does not change its state of motion or rest unless an external force does not act on the body.


Everybody continues its state of rest and uniform motion in a straight line unless an external force does not disturb its motion.
Second Law’s:- Acceleration of a body is inversely proportional to its mass and directly proportional to the external force acting on it.

\(\sum \overrightarrow {F}=\left( \overrightarrow {F}\right) _{net}=m\overrightarrow {a}\)

Problems On Second Law’s

Third Law’s:- If a body ‘A’ exerts a force on body ‘B’ an equal but opposite force is exerted by body ‘B’ on the body ‘A’ this is called the third law of motion.

How to solve a problem with Newton’s first and second law of motion 

  • First of all, we decide the system on which the law of motion is to be applied. The system may be a single particle a block or a combination of two or more blocks, two blocks connected by a string, etc.
  • The only restriction is that all parts of the system should have the same acceleration.
  • Once the system is decided we make the list of all the forces acting on the system. Any force applied by the system on another block or body is not included in the list of Forces.
  • After that make a free body diagram of the system and indicate the magnitude and direction of all the forces listed in steps.
  • In the final step, we have to choose any two mutually perpendicular axes and after that apply Newton’s second law of motion in each individual axes.

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