Shear, Tension and Leverage

A calendar on a hook and a shelf of books both hang on screws in the wall, but they ask very different things of them. Follow the weight: from the object, through its hook or bracket, into the screw, the plug and finally the wall. That route is the load path, and every link in it has to hold.

A load can work on a screw in two ways. Shear pushes across it, trying to slide it down the wall, like a coat hanging on a peg. Tension pulls along it, trying to draw it straight out of the hole, like a cork from a bottle. A fixing that shrugs off one can give way to the other, so the direction matters as much as the weight.