The shape of a screw
Wham! Wham! Wham! All it takes to nail two pieces of wood together is a
few strong hits with a hammer. But fastening two pieces of wood together
with a screw is a lot harder. The screw has to be turned many times to
go into the wood.
A screw has a winding edge called a thread. The thread goes from the end
nearly all the way to the top. When you turn the screw, you wind the
thread into the wood.
Turning a screw takes more time than pounding a nail the same size. But
the winding thread of the screw is much longer than the straight sides
of the nail. There is more of it to grip and hold the wood. So for some
jobs, a screw works better than a nail. It holds things together more
tightly than do nails.
A winding edge
The screw is another simple machine with a special shape. It’s really an
inclined plane that winds around a center pole. You can easily prove to
yourself that the thread of a screw is an inclined plane.
Use a sheet of paper to make a triangle. Fold over the top edge of the
paper so that it lines up exactly with the right-hand edge. Cut or tear
along the fold, and you will have a triangle. Color the cut edge on both
sides of the paper.
The edge that you cut and colored is the inclined plane—it slants. Lay
a pencil along one of the other edges of the triangle. Roll the paper
around the pencil.
When you look at the rolled-up shape, you will see that the colored edge
of the paper turns around and around the pencil. The colored edge of the
paper is an inclined plane that winds around a center pole, just like
the thread of a screw.
