One of my goals with these science videos is to provide support to the homeschool community. After years of writing the Experiment of the Week, and presenting science shows for homeschool groups, I have been impressed with how strongly homeschool families are involved with hands-on science. I have also seen the need for a science curriculum that fulfills their educational needs. Starting with these videos, I hope to offer the homeschool community some of the science education support that they need, as well as a fun way to get students involved with scientific discovery.
I also try to use activities that involve common items. Most homeschoolers don't have access to test tubes, flasks, chemicals, microscopes and other scientific equipment. In the future, I hope to be able to guide them to inexpensive sources for science equipment to enrich their science curriculum.
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This week's experiment is a wonderful demonstration of the physics of inertia. Newton's first law of motion tells us that an object that is sitting still will remain stationary until something pushes or pulls on it. Newton's second law of motion tells us that the heavier an object is (the more mass it has), the more force it will take to get it moving. While this may seem a complicated idea, we will see that it is mostly common sense.
You will need:
- a heavy book
- yarn or string
- a door with a doorknob
Be sure that your string is strong enough to support the weight of the book, but thin enough that you can break it with a hard pull. Cut two lengths of string, each long enough to tie around the book with a couple of feet left over. Tie one end of the first string around the book. Tie the other end of the string around the doorknob, so that the book hangs freely from it. Tie one end of the other string around the book and let the free end hang downwards.
Get a firm grip on the end of the lower string. You may want to tie it to the center of a pencil or something else that you can use as a handle. You can now decide which string you want to break, the upper or the lower string. If you want to break the upper string, then pull gently on the lower string, slowly pulling harder and harder. Eventually, the upper string will break, letting the book fall.
If you want to break the lower string, then give the lower string one quick, hard jerk. The upper string will continue to support the book, and the lower string will break.
Why does it work this way? Can you figure it out? If you pull slowly, the lower string is subjected to the force of your pull, but the upper string is subject to the force of your pull, plus the weight of the book. As both strings have the same strength, the upper string will break first.
If you jerk quickly, then the mass of the book resists a sudden movement, due to inertia. The heavier the book, the more it will resist moving. In this case, the lower string is subjected to the force of your pull, but the upper string is only subject to the weight of the book, until its inertia is overcome and the book begins to move. With a quick jerk, the lower string breaks before the force overcomes the book's inertia.
Be sure you have several pieces of string, so you can try breaking both upper and lower strings. This experiment has applications in the "real world." Here in St. Augustine, cars are allowed to drive on the beach. Frequently they get stuck in the sand and someone winds up pulling them out. Once the tow rope is attached, it is important to add tension to the rope slowly, in order to pull the car out. If your car pulls too quickly, the rope will break before you overcome the inertia of the stuck car. I have seen this happen on several occasions.
Have a wonder-filled week.
