1. The problem statement, all variables and given/known data I have an incline with a degree of Alpha and it is with friction that is equal to (Us = tan theta) It is accelerating to the right. Prove that this equation if it is true then the block will slide. a > g tan(Theta - Alpha) 3. The attempt at a solution That was my thought of forces.So I figured out that if I want them to stick together then it should have the same acceleration and orientation. So the Net of forces should be ma and has the same orientation as (a) I have encountered a problem before with 2 stacked objects and found that the static force keeps the blocks together by giving it the same acceleration ( Correct me if I am wrong, I kinda get confused on how that happens) and also I cant find any force that could be acting that way so it produces the same acceleration and orientation. Anyway as a result, I came up with these equations. Friction - mg sin alpha = ma cos alpha (1) mg cos alpha - N = ma sin alpha (2) Friction = Us N Then by putting 2 in 1 Us (mg cos alpha - ma sin alpha) - mg sin alpha = ma cos alpha Us as mentioned is equal to tan theta Tan theta mg cos alpha - tan theta ma sin alpha - mg sin alpha = ma cos alpha Divide all of it by cos alpha and m Tan theta g - tan theta a tan alpha - g tan alpha = a Do some math and you will come up with this g(tan theta - g tan alpha) = a ( 1 + tan theta tan alpha) a = g (tan theta - g tan alpha) / ( 1 + tan theta tan alpha) a = g tan( theta - alpha) If I want it to slide then a bit more acceleration will make it slide then a > g tan(tehta - alpha) I highly doubt that the solution is right but I hope so. (Ehhhh worth trying right??) My conflict is with the static friction how it makes things move because that what I understood from this problem. http://www.uwgb.edu/fenclh/problems/dynamics/multi/1/ He put a value of f and saw if that value of f equals to the static friction or no. I hope someone clear this misconception for me. Thanks.