I have a few questions about forces:

Below, is the explanation of what I am confused about

In the simplest case of an object moving in one direction and a constant force applied parallel to that direction: W = F A d where F is the magnitude of the applied force and Ad is the distance travelled. (Note that work is a scalar.) If I apply a horizontal force of magnitude 40 N to push a box 15 m across a frictionless surface, the work done on the box is W=F Ad =(40N)( 15 m)=600 J But if I apply a vertical force of magnitude 40 N on the same box as it slides 15 m across a frictionless surface, the work done on the box is zero. Why? Note that the perpendicular force was not increasing the energy of the box. The only portion of the force that counts towards the work done is . . . . Force F that component of the force In the direction of the motion: \"0'90 W=(Fcos6)Ad \"mm\