23 First Law
23 First Law
23 First Law
Newtons Laws of Motion, as written in his book the Principia, are actually very difficult to read. At the time, Newton didnt care too much about the readability of his book. He just wanted to get the stuff put down on paper. As a result, the way we state his laws today, and the formulas we use, are in some ways different from the way he originally wrote them. They still mean the same thing. He actually wrote the laws in a specific order for a specific reason. As we go through the laws, you should realize that he builds one on top of the other.
If FNET = 0, then v = 0
7/22/2011
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Example 1: Describe the motion of a hockey puck that is shot down the ice. A hockey puck will keep moving in the same direction at (almost) the same speed unless someone stops it or changes its direction. This would be done by applying a external force. The effect of friction on the puck will be quite small. Example 2: Describe the motion of a book sitting on a desk. A book sitting on a desk wont start to move all on its own. A force needs to be applied to it. Since it is in a state of rest, it will stay that way. Example 3: Use Newton's First Law to explain why people are injured in car accidents when they do not wear seat belts. Assume the person was in a head on collision. During the collision the car is rapidly brought to rest by a force acting against it. In the car the person still has inertia, so the person will continue to move forward at the same speed as the car was originally traveling at until a force acts against him. This force will be supplied by the steering wheel, dashboard, or windshield as they hit it. It is this force that causes injuries to his body. Newtons First Law goes against what Aristotle said, but is basically what Galileo had said a few years earlier. Aristotle had said that if you stop pushing an object, it will come to rest. He believed that at rest was the natural state for any object. Galileo told us to ignore friction and basically came up with Newtons First Law. It is called Newtons First Law because he was the one that formally published it and had the mathematical proofs to back it up.
Homework
p142 #1, 3, 4
7/22/2011
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