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?
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
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!
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.
More to Explore
Sabine De Brabandere, PhD, Science Buddies
Science Buddies |
Simple machines, wedges, air pressure
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