Problem
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.
Solution
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 up a log 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. Problem solved!
It's Simple
What made the axe such 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 the machine.
You probably don’t think of an axe head when you hear the word machine. Instead, you might picture something that whirs and purrs or roars and rumbles. A blender, a car, or a bulldozer might come to mind. These are all complex machines with lots of parts.
Simple machines, on the other hand, have only a few parts. In fact, some are made of only one part. Nearly every machine, no matter how complex, contains several simple machines.
Think about a wagon. Its two sets of wheels and axles make it easy to take a friend for a ride. The handle is a lever. It lets you pull and steer with ease. And screws hold many of the wagon parts together.
Simple machines can be combined in endless ways to accomplish tasks, from sharpening a pencil to rocketing into space. But each simple machine also 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. You might not notice this shape at first. In fact, the axe head might look more like a square. But look down at the top of it. Do you see how the head starts out wide and then tapers to a thin edge? That’s a triangle. And here’s something to remember: The thinner the edge, the sharper the wedge.
So how does this wedge do its job? It starts with effort from you. It takes effort to swing the axe. This effort provides the force needed to drive the axe blade into the wood.
When you swing the axe you direct the force downward. But as the blade pushes into the wood, the wedge directs the force sideways. That’s the whole point of a wedge—it redirects force. With each downward swing of the axe, the wedge pushes outward in two directions. 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 head 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 has a head that 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, and a lot more effort from you to provide that force.
So, you may need to take a few more swings to drive the long, narrow axe deeper into the wood, but they are easy swings. You trade distance for effort. Not a bad trade!