Why is the equation for Work W=Fdcosθ ? (2/2)

In the first problem, it's obvious that only Fp is doing any work, because the other two aren't in the direction the object is moving. In the second problem, Fp isn't even in the direction of motion. But part of it, labeled Fpx, is in that direction. Only that part contributes to the work on the object. The cosθ throws away the useless (for doing work) Fpy.

Done