Calculating Work Using W=f*d*Cos(theta)

[laxboi33]

Homework Statement

W= f * d * Cos (theta)

Homework Equations

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.

 

[rock.freak667]

The angle θ is measured counter clockwise from the +ve x-direction. So if θ=0, then the force is acting in the +ve x-direction.

If θ=180, then the force is acting in the -ve x-direction. (This is why in the case of friction θ=180).

If θ=90, then the force is acting perpendicular to the mass it is acting on. This may mean that the force and displacement are at 90 degrees to each other and thus the work done = 0J