You are camping with your family and your best friend. After a day of fun in the sun, you look forward to a night next to a bright, blazing campfire. Especially because it’s your turn to help start it!
You and your friend gather some logs and crisscross them in a small pile. Your aunt tries to light one of the logs with a match. Nothing happens. She smiles. “The logs are too big to catch on fire with such a little flame.”
“We need smaller pieces,” you say. But how? The wood is too hard to break apart with your hands. “We need some sort of tool,” your friend says. “Maybe someone in the campground has a saw.” All that back and forth sawing could take a while, you think. There must be a better tool.
There is! You remember seeing a small axe in a box of camping equipment. It’s the perfect tool for the job!
You bring it to your aunt. She stands a log up and carefully swings the axe so that the blade strikes the cut wood on top. THWACK! The log splits to its halfway point. Another swing, and CRACK! It splits in two. A few more whacks reduce the half logs to pieces small enough to start the fire.
What makes the axe a good tool to split wood? It’s simple! The axe head is a simple machine. Like all machines, simple machines help us do jobs, usually by moving things faster, farther, or more easily than we can without them.
You probably don’t think of an axehead when you hear the word machine. A blender, a car, or a bulldozer might come to mind. But these are all complex machines. They have lots of parts.
Simple machines have only a few parts. In fact, some are made of only one part. Nearly every machine, no matter how complex, contains one or more simple machines.
Simple machines can be combined in endless ways to complete tasks, from pulling a load to flying in space. But each helps us do work all on its own.
A Wedge Gives You an Edge
The axe head that you used to split the wood is a simple machine called a wedge. It’s shaped like a triangle. At first, it might look like a square. But look down at the top of it. The head starts out wide and then narrows to a thin edge. The thinner the edge, the sharper the wedge.
How does it work? It starts with effort from you. It takes effort to swing the axe. This effort provides the force to drive the axe into the wood.
As you swing the axe, the force moves downward. But as the axe pushes into the wood, the wedge shape directs that force sideways. A wedge redirects force. As the axe drives deeper, the wood moves farther and farther apart, until, with one final CRACK, it splits.
With a powerful CHOP, this axe splits the wood in two. The axe head is a wedge.
It’s All About Trade-offs
Now, suppose you had two axes to choose from. One has a head that is long and narrow. The other is shorter and wider. Which would you choose?
Remember, the thinner the wedge, the sharper the edge. So, the long, narrow axe head will be sharper. Choosing this one is a no-brainer. It will be easier to drive into the wood with each swing.
But here’s the thing about a simple machine: There are always trade-offs. The long, narrow axe is easier to push into the wood, but it has to go deeper before it splits the log. The short, wide axe could split the wood without going in so deep. This might mean fewer swings, but each swing would require a lot more force. You trade distance for effort. Not a bad trade!