You and Your Students!
Vicki Cobb, Education World Science Editor
Learn about Isaac Newton by cutting an apple midair.
Simple Machines, Forces, Physical Science
Setting the Scene (Background)
They say Isaac Newton was inspired when he saw an apple fall. You can use a falling apple to inspire your students. Then, while you're at it, you can show them there's more than one way to slice an apple -- using principles discovered by Sir Isaac in an entertaining way.
Do this activity as a demonstration. Since it involves a knife, you will not want students doing the activity in the classroom.
Slice halfway through an apple with a table knife. Press the knife down far enough so that you can pick the apple up with the knife. If you are right-handed, hold the apple-on-the-knife at arm's length with your left hand; if you are left-handed, hold it with your right hand. With the hammer, give the knife a sharp blow on the back of the blade. (See picture.) Strike as close to the apple as you can. The knife will cut through the rest of the apple, and the two halves will fall to the ground.
Fold a piece of paper and put a knife blade in the fold. Cut an apple with the paper-wrapped knife. Use a steady downward pressure, not a sawing motion. After the apple is split, look at the paper. It will not be damaged.
Behind the Scenes
When you cut an apple, you use force to overcome its resistance to being cut. (According to Sir Issac, you need force to overcome resistance: That's the First Law of Motion.) There are two ways to overcome that resistance: by applying a small force for as long as you need or by applying a large force for a shorter period of time.
When an apple rests on a table, it is held steady and all the force goes into the apple. A small force, applied for as long as it takes, will get the job done. When the apple is in the air, a larger force is needed or the apple will travel downward with the knife. The blow of the hammer delivers such a large force that the resistance to the knife is overcome and the apple splits before it has a chance to travel with the knife.
In the second demonstration, a knife can cut both paper and an apple. But it is easier to cut an apple because apple pulp offers less resistance than paper fibers do. As you cut with a steady pressure, the force is enough to cut the apple but not the paper. The paper moves along with the knife. But if you hold the paper against the knife so that it can't move, the knife will cut through the paper.
Article By Vicki Cobb
Copyright © 2004 Education World