Because the cord has constant length, both blocks move the same number of centimeters in each second and so move with the same acceleration. To find just this acceleration, we could model the system as a particle under a net force. That method would not help to finding the tension,
so we treat the two blocks as separate accelerating particles.
(a) Below figure shows the free-body diagrams for the two blocks.
The tension force exerted by block on block is the same size as the tension force exerted by object on object . The tension in a light string is a constant along its length, and tells how strongly the string pulls on objects at both ends.
(b) We use the free-body diagrams to apply Newton’s second law.
For or ........... 
Also, the definition of the coefficient of friction gives
For ...................................... 
Also from the component, or
Substituting from equation  into , we get
(c) From equation