Distinguishing real forces and fictitious/pseudo forces in Newtonian mechanics In understanding the law of inertia I had to consider the motion of bodies screened from the so called "real forces". 


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*What characterises these real forces?

*What makes us call them real? 

*Or what is separating the forces called true or real from another group of forces called fictitious/pseudo forces?
Many forces called true are also invisible just like the centripetal force or other pseudo forces. 


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*Then what is making them special and different?

 A: Real forces are those which arises due to actual interaction between objects.But pseudo forces are not result of any interaction between objects rather it arises due to change of frame of reference and that is why it is pseudo.For example let us consider a person standing in a lift and suddenly the lift is accelerated downward.Now you can see that from a reference frame out side the lift there is only one force acting on the person which is the gravitational force.But if you now change your reference frame and see the motion of the person from inside the lift then you will see that the person is moving upward.So from inside the lift it will seem that some kind of force is acting on the person upward but which is actually not true.Rather it is the effect of the acceleration of lift downward and the fact that you are observing the motion of person from this accelerated frame.
That is why while calculating motion of an object inside a lift which is accelerating downward we  add an upward pseudo force on the object .
A: A real force has a reaction force. A pseudo force has no reaction force.
