A long long time ago, Sir Isaac Newton (1643 – 1727) changed our understanding of the Universe by enumerating his Three Laws of Motion. What are Newton’s Laws of Motion? What do they mean? Can you prove Newton’s Laws? You can explore answers to these questions with Jameson Kief’s riveting account of his experience with these laws in his story, Adventures with Newton’s Laws. Jameson won the 7th Grade Physical Sciences competition at the 2011 CT State Science Fair. He is a student from St. Thomas School in Southington, CT.
I chose my science fair project because two years earlier I did a project on levers and while I was researching levers on the computer, I saw a list of simple machines. In the list I found pulleys. I kept that in my mind as I saw pulleys all around me. I saw them on cranes on the side of the road and in weight machines. I also heard that pulleys had been used to construct some of the greatest architectural feats, like the Parthenon in Athens, Greece.
I wanted to test if this was true, so I looked around my grandfather’s house to see if there was anything heavy I could lift. I found a gun safe that weighed 600 pounds.
There are three types of pulleys, a simple pulley which has one rope and one pulley, a compound pulley system which has two or more pulleys and one rope, and there is a complex pulley system which has two or more ropes and two or more pulleys. I decided to use compound pulleys because they are easy to set up and I had them available.
I researched a little about pulleys and found out that what allows a human to lift large masses with pulleys was mechanical advantage. There are two ways to find the mechanical advantage; you can divide the force-out (the load) by the force-in (the force applied to the machine) or you can divide the force-out by the number of ropes, excluding the rope that changes the direction of the force.
I built a small scale frame and pulley system and attempted to lift a ten pound weight. The predicted force-in needed was greater than the force-in predicted. I did some research and found-out that this was because of friction and inertia, Newton’s second Law of Motion. The friction was caused by the rope rubbing against the wall of the pulley. I decided that I would pull the rope as straight as I could to avoid friction. The law of inertia says that an object at rest will stay at rest unless an unbalanced external force acts upon it. I used a fifty pound weight to get more dramatic results. I picked up the weight with just the spring scale, a device to measure the applied force in. I notice that the spring scale, which measures the peak force-in, measured around fifty-four or fifty-five, but the bar that measures the current force-in was hovering around fifty. It took five more pounds of force to lift the load than it took to keep the load suspended in the air. I lifted the load a few more time and discovered that if I lifted the load slowly inertia would be less of a problem.
After I finished experimenting with the small scale, I went to New Hampshire to lift the heavy safe. I originally had my pulley in a V shape, but I figured out that this configuration had too much friction. I changed to a vertical configuration. I hooked up the safe to the pulleys using cinch straps. I walked slowly backward until the safe was off the ground. I had to walk back 601 centimeters. I only took fifty-one pounds to lift 600 pounds. I believe that the actual force-in was less than the predicted force in, sixty pounds, because I used ball bearing pulleys that have virtually no friction and I walked backward very slowly.
Congratulations Jameson Kief for winning the 7th Grade Physical Sciences Competition at the 2011 CT State Science Fair!
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