well, my question isnt directly towards newton's law or anythiing, but its a question on how to solve problems using newton's law. for example, look at this problem below.. a force of 20N acts horizontally on a mass of 10kg being pushed up a fritionless incline that makes a 30degrees angle with the horizontal. What is the magnitude of the acceleration of the mass up the incline equal to? ok, for this problem, how would you know you solve this problem by using the equilibrium in respect to the x ? is it because only x equilibrium exists? or is it because its asking for the magnitude of the acceleration up the incline? sorry if this question confuses you..