Work done by a constant force is defined by W=F(delta d)(cos theta), where F is the magnitude of force and delta d is the magnitude of the displacement (not distance). (I got the definition from a textbook) Now consider the work done by friction for, 1) a box sliding across the floor to the right by 5m and and slides back to the original position The magnitude of the displacement is 0, so the work done by friction is zero? This doesn't seem to be true...how come I got the wrong answer? Now consider the work done by gravity for, 2) a rocket near the surface of the earth travels in a straight line 40 degrees above the horizontal for 1000m When using W=F(delta d)(cos theta) to find the work done by gravity, should I substitute the magnitude of the displacement (1000m) or just the magnitude of the vertical component of the displacement (1000sin130 m)? Can someone clarify these concepts? Thanks!