1. The problem statement, all variables and given/known data W= f * d * Cos (theta) 2. Relevant equations 3. The attempt at a solution In an object moving in the x-direction how do you know when to use zero as theta or 180 as theta? From the problems I've solved I find that when there is negative acceleration or a force stopping the object then the theta is 180. When the object is in a constant velocity or it is accelerating the theta seems to be zero. Can someone explain to me when to use zero or 180 for theta in objects moving in the x-direction? Also, in what case would you use 90 degrees as theta? thanks.