please explain me the differnce between external and internal force

 

 

friend,as sg said  first you decide a system,and your system consists of particles.if these particles exert forces on each other,then it is a internal force .we know,force is always exerted by a body on other body.here body means particle or collection of particles.if  that force exerting agent is not the part of your system,then force exerted by it is called external force.and if it is included in your system i.e. if it is one of the particles out of the particles of your system,then force exerted by it is called internal force.for eg.you take book + table as your system.then normal force by book on table is an internal force,bcoz book is the part of your system,where as gravitational force exerted by earth on bookis an external force.

Dear

 

  Sppose two balls comming towards each other collides

 

 

then at there collision point there is pair of newtonian third law impulsive force pair Right ????

 

 

Now if you will chose both the balls as your system (which we generally do to apply conservation of momentum) then these forces will be internal to the system

 

Now suppose take one of the ball as your sdystem then on it there is an impulsive force at the contact point and in this case this will be external to the system note you cant apply conservation of liner momentum taking a sinlge ball as your system can you ????