Key concepts
Physics
Simple machines
Wedges
Air pressure

Introduction
Looking for a cool and surprising project to show to your friends? Give this activity a stab! 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—one that requires the least effort from you—will surprise everyone! How? Investigate and have fun! Can you find a reliable way to get the job done? Can you explain the physics behind it?

Background
The human body is equipped with an amazing set of tools: fingers, teeth, arms and more. Sometimes, however, these are not enough to do the job. We use tools to enhance the human ability to push or pull to make tasks easier. Think about wheelbarrows, scissors, knives, catapults and screws, all of which help us complete various tasks. Scientists call these tools simple machines. A wedge is one example. It has two planes: One end is thick (such as the back of an ax or top of a needle); it gradually narrows into a sharp edge on the other end (the blade of an axe or a needle point). Wedges are used to separate two objects, split one object, keep objects together or prevent them 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 or tip. The result is clear—less pushing is needed to get the job done. Try it with a butter knife. Push the cutting-blade end into a brick of cheese, then turn the knife around and carefully try to push the handle tip into the cheese. Which side needs less effort to cut the cheese? The sharper 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!

Materials

  • Potato, at least one-inch thick (Preferably it should be fresh and hard.)
  • Cutting board
  • Knife to cut the potato (An adult should help with this step.)
  • Drinking straws, one for each method you try (Any bendable parts should be carefully removed.)
  • Pencil with one pointy side and one flat side (A chopstick with one pointy side works as well.)

Preparation

  • Ask an adult to cut the potato so you have at least one slice that is about one-inch thick.
  • Place the slice(s) on the cutting board.
  • Your goal in this activity is to drill holes 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 try 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 approach this activity like a scientist:
    • Always form a hypothesis (or educated guess) before you try a method. Completing the following sentence will help you create one: “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 try 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?
  • For safety reasons, always have the potato on the cutting board while testing.

Procedure

  • “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 flat side of a pencil”: Hold the pencil about three fourths of the way up from the flat end and try to twist the flat end into the potato. Do you think this will work? Make a hypothesis, perform the test and see what you learn. What happened on this test?
  • “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 into 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 flat side of a pencil”: Repeat the previous method with the flat 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. (Use a new straw for this test.) Hold it so your thumb blocks off one end of the straw. Carefully 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 did it work or not work?
  • 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 of varying hardness (with permission, of course, and have an adult supervise you). Will it work? Why or why not?
  • Extra: Can you come up with other ways to cut a hole this size through a potato? See if you can use other materials you find around the house (but be sure to ask permission, and have an adult supervise you).
  • Extra: As explained in the background section, the methods that worked for this activity 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.

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 used the power of jamming. Whereas twisting provides gradual force spread out over time, jamming provides instant force. The wedge concentrated this instant force on a small surface area—the border of the straw or the pencil point. 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 is strong enough to penetrate whereas the unplugged version bent. 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 gases 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, although you displaced much more potato, displacing material is work, so it felt more difficult to do.

Cleanup
The potato can go in the compost bin. Straws can go in the trash and other kitchen equipment can go with the dirty dishes.

More to explore
Simple Machines: Facts, from Science Trek, Idaho Public Television
A Really Long Straw, from Scientific American
Suction Science: How to Break a Ruler Using Air Pressure, from Scientific American

This activity brought to you in partnership with Science Buddies

Science Buddies