CANANY ONEDERRIVETENSIONCONSTRAINEDMOTIONTOSOLVECONSTRAINT

First off all your question is not clear enough..As per i have understood you can use only physical material and properties to derive a contraint ...like if you have a pully then you can make a constraint saying that the lenght of the thread is constant and does not change overtime...A +B + C = constant and find acceleration from it so to solve it .Even if the partical is in motion the question will only ask you to find acc or v at a perticular instant because with motion and increase in speed tension will increse so you can find a constraint at a particular moment only .most of the time it remains say ..but in say the string is elastic you cannot say the lenght is constant .Thats the reson constraints are made from physical properties only 

plz post  the question