A shape that works
Be careful—that knife is sharp! Don’t stick yourself with that needle!
Working with sharp things means taking special care. But some things
have to be sharp to work well. A dull knife or a needle without a point
isn’t much help at all.
Sharp things have a special shape that makes work easier. They are
really all one kind of machine—a machine called a wedge. The thin,
sharp end of a wedge can cut or push into things easily. Then the
thicker part of the wedge can push through.
Knives, saws, and scissors are wedges used for cutting. The sharp edge
pushes in, and the thicker part spreads the pieces.
Nails and needles are wedges used for pushing into or through things.
The point makes it easy for the nail to push into wood or the needle to
push through material.
Axes and metal wedges are used to split wood. A chop or a hard hit
drives the sharp edge into a log. Then the wider part spreads the wood
and makes it split.
Many boats have a wedge-shaped front, or bow. The bow cuts the water and
makes it easy for the boat to slide through.
Pounding pushes the thin end of the wedge into the log. Then the thick
part splits the wood.
