This week's experiment is in response to another question by a list member. What makes things sharp? This is one of those wonderfully simple questions which most kids come up with that most adults never think to ask.
To explore this, you will need:
First, be very careful with the knife. It would not do to mess up a good science experiment by cutting yourself and bleeding all over the demonstration.
We will start by washing the apple. After all, once we cut it into pieces, it would be a shame to waste it. Once it is washed, place it on a cutting board and carefully use the knife to cut it in half. Observe carefully as you cut it. Notice how the blade slices through the apple. Cut each half again, so that you wind up with 4 similar slices of apple.
Why does the knife cut the apple so well? Lets try something else to compare. What about your finger? Try using one of your fingers to cut one of the pieces of apple. What happens? Besides crushing the apple a bit, not much. Of course, your finger is very different from the knife. Where the knife edge is very thin, your finger is wide and rounded. Let's try something else.
How about a piece of paper? It is as thin as the edge of the knife. Use the edge of a piece of notebook paper to try and cut the apple. Observe carefully as you try. If you are careful, you may actually be able to cut a small way into the apple with the paper. The thin edge concentrates the force into a much smaller area. Instead of crushing a wide area, the edge of the paper tears a much thinner pathway into the apple. Unfortunately, the paper so too soft to cut the apple easily. The paper bends or tears as you try to force it into the fruit.
Lets try adding some strength to the paper. Fold a sheet of notebook paper in half. Put the blade of the knife inside the fold of the paper, so that the sharp edge of the knife is against the fold. The knife blade will give its strength, but only the paper will touch the apple. Try cutting one of the apple pieces with the paper and knife. What happens?
The paper around the knife cuts the apple easily. The folded paper is thin enough to concentrate your force into a small area. The knife blade holds the paper in place, so that the force you are applying does not bend or tear the paper. Together, the combination easily forces the paper through the apple.
So the thinner the edge of an object is, the more it concentrates the force and the easier it cuts. The edge also needs strength, to keep it from bending or tearing. Now that we know the basics, you can try taking things a bit farther. Examine a pair of scissors and see if you can see how these ideas apply to them. How is the way that they cut different? While you are working on that, you can snack on the apple.
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