A Simple Machine to Make Potato Holes
If you want a cool project that you can show to your friends, this activity is for you! Take a potato and a straw—maybe a pencil as well—and challenge your friends to drill holes into the potato with the least amount of effort. Chances are, they will not find a method that gets the job done with ease. The best method—the method that requires the least effort from you—will surprise them all! Experiment, investigate and have fun! Can you find a reliable way to get the job done? Can you explain the physics behind it?
The human body is equipped with an amazing set of tools: fingers, teeth, arms, and more. Sometimes, however, these are not enough to get a job done. We use tools to enhance the human ability to push or pull, which makes tasks easier. Think about wheelbarrows, scissors, knives, catapults and screws, which all help us get a job done. Scientists call these tools ‘simple machines.’ A wedge is one example. It has two planes; one end is thick (such as the back of the axe or the top of a needle), which gradually narrows and ends in a sharp edge (the edge of the axe or the point of a needle). Wedges are used to separate two objects, to split one object, to keep objects together or to prevent objects from moving. Examples of wedges are nails, knives, staples and door wedges. Wedges concentrate all the pushing applied on the thick end of the wedge to a smaller surface on the sides or the sharp end. The result is clear, less pushing is needed to get the job done. Try it with a knife. Push the sharp side into a brick cheese, then turn the knife around and push the back side into the cheese. Which side needs less effort to cut the cheese? The sharp side, right?
A wedge is one of six types of simple machines. The others are pulleys, levers, wheels, inclined planes and screws. These machines seem quite intuitive, but sometimes surprises happen. See where you can find some wedges in this activity, but don’t forget to be creative and get thinking about other factors at play!
- Potato, at least 1 inch thick. Preferably a fresh and hard potato
- Cutting board
- Knife to cut the potato
- Drinking straws, with bendable parts removed
- Pencil with one pointy side and one non-pointy side. A chopstick with one pointy side works as well.
- Ask an adult to cut the potato so you have at least one slice that is about 1 inch thick.
- Place the slice(s) on the cutting board.
- Your goal in this activity is to drill holes of about the size of a pencil or drinking straw into the potato slice. What methods can you think of with the materials listed for this activity? In addition to your own ideas, you can find eight methods to test below. Try each one of them; see which one(s) are easy and try to explain why.
- Here are a few tips that can help you experiment like a scientist:
- Always make a hypothesis (or educated guess) before you test a method. Completing the following sentence will help you create a hypothesis: “If I … (describe the method you are about to try), I … (expect to / do not expect to) create a hole through the potato slice because …. “.
- Use a new straw for each test. Straws might get damaged during a test, so to test the different methods under similar conditions, you will need a new straw for each test.
- After each test, take a moment to investigate the results. Was your hypothesis correct? Why do you think this is the case? Did you learn anything from this test?
- Safety Note: For safety reasons, always have the potato on the cutting board while testing.
- “Twisting a straw.” Hold the straw at about three-fourths of the way up, and try to twist the straw in. If you are wondering if the edges of a straw are sharp, rub your finger along an edge. Does it feel sharp? Do you think it will cut into the potato? Make a hypothesis, perform the test and investigate the results. Did it work? Was it easy? What have you learned from this test?
- “Twisting the non-pointy side of a pencil.” Hold the pencil about three-fourths of the way up from the non-pointy end, and try to twist the non-pointy end in the potato. Do you think this will work? Make a hypothesis, perform the test and see what you learn.
- “Twisting the pointy side of a pencil.” Turn your pencil around and, holding it three-fourths of the way up, try to twist the pointy side of your pencil in the potato. Will this be easier, or more difficult than previous methods? Make a hypothesis, perform the test and investigate the results.
- Did any of the twisting methods work? Let us try something different: jamming the item on the potato.
- “Jamming a straw.” Hold your straw about three-fourths of the way from one end (without pinching the straw) and jam the far end onto the slice. Can you force the straw through the potato? Make a hypothesis, perform the test and investigate the results. What have you learned from this test? Was your straw sturdy enough or did it bend under the force?
- “Jamming the non-pointy side of a pencil.” Repeat the previous method with the non-pointy side of the pencil.
- “Jamming the pointy side of a pencil.” Repeat with the pointy side of the pencil.
- “Jamming the plugged straw.” Back to the straw, with the sharp edges. Hold it so your thumb blocks off one end of the straw. Jam the other end onto the slice. Keep making a hypothesis before you test, and investigate after the test. Was your prediction correct? Why do you think this works or does not work?
- “Jamming a folded straw.” Now, try to hold the straw in yet another way. Fold one end of the straw over and hold it firmly closed so little to no air can escape through this end of the straw. Jam the other end onto the slice. Do you think this will work? Did it work? Why?
- Look back at all the methods you tried. Which worked well; which worked, but with difficulty and which did not work at all? Can you see a trend? Can you explain why some methods work extremely well, others are hard and still others do not work at all?
Extra: Try these methods on other fruits or vegetables, or on other materials like stones or a sponge. Will it work? Why or why not?
Extra: Can you come up with other ways to cut a hole this size through a potato? You can use other materials you find around the house, but be sure to ask permission to use anything that is not yours, and have an adult supervise you.
Extra: As explained in the background section, the methods that worked for this experiment used—among other things—a simple machine called a “wedge” to cut the hole in the potato. Can you find other examples of wedges around your house?
Observations and Results
Jamming a plugged straw worked with amazing ease, right? Jamming the pointy side of a pencil was probably successful, although it needed some more effort; twisting the pencil was likely hard or unsuccessful. These are all expected results.
Both the edge of a straw and the pointy side of the pencil are wedges. Wedges make it easier to push aside the potato flesh. On top of that, you use the power of jamming. While twisting provides a gradual force spread out over time, jamming provides an instant force. The wedge concentrated this instant force on a small surface area, the border of the straw or the point of the pencil. This allowed the straw or pencil to penetrate the potato.
The plugged straw would most impress your friends if you challenge them with this activity; it was strong enough to penetrate, while the unplugged version bended. Do you know why? When plugging the straw and jamming the other end on the potato, you trap air inside the straw. This air keeps the straw stiff. It pushes on the walls of the straw, which prevents the straw from folding. Air is a gas, and gasses can easily be compressed. It still allows the potato flesh to enter the straw, but at the same time, prevents the straw from collapsing and bending.
The plugged straw worked better than the pencil because with the straw, you only displaced a very small amount of potato to make room for the straw. With the pencil, though you displaced much more potato, displacing material is work, so it felt more difficult to do.
Ask an Expert
- The potato goes in the compost bin, straws go in the trash and all other equipment goes with the dirty dishes.
- A Really Long Straw, from Scientific American
- Suction Science: How To Break A Ruler Using Air Pressure, from Scientific American