Scope of the Problem:
Generally the problem asks about forces or accelerations
How to attack at the Problem
1) Draw one free body diagram for each object in problem
Read the following two post to get the feel about free body diagram
Different Type of Force and There Origin
How To draw Free Body diagram
Read the below Video for Free body diagram also
This can be done in these simple steps
1) First draw a free body diagram of the body showing all the forces on it.
2) Carefully choose a X-Y coordinate system so that direction of the motion or direction of force lies in X or Y direction. This will simplfy the Problem
3) Applying Newton’s law find out any unknown forces
A) Normal Force: When we put a book on a table ,the molecules of the book exert downward forces on the molecules of the table.The molecules composing the upper layer of the tabletop move downward until the repulsion of the molecules below balances the forces applied by the book.
A non inertial Frame of Refrence is the frame of refrence which is having acceleration a. Example Free Falling elevator
Newtons law are valid in inertial frame of refrence only. We cannot apply them as it is in non inertial frame of refrence as it left out the acceleration of the object because of the frame of refrence.
Let me provide simple example to illustrate this.