Newton’s Laws of Motion, as written in his book the Principia, are actually very difficult to read.
- At the time, Newton didn’t 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 exact 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.
The 1st Law (The Law of Inertia)
“Every body continues in a state of rest or uniform velocity in a straight line,
unless an external force acts on it.”
“Every body…”
Means any physical object in the universe that has mass. It can be here on the Earth, on the moon, floating in space, wherever.
“…continues in a state of rest or uniform velocity in a straight line…”
If it is sitting still, it will stay that way. If it is moving, it will keep on moving forever at that velocity in a straight line.
“…unless an external force acts on it…”
Unless something else pushes it.
It is important that you understand the idea of “external forces”.
- Your car is stuck in a snow drift, so you ask your friend sitting in the passenger seat to push you out.
- He agrees and starts pushing as hard as he can on the dashboard…. the car doesn’t move!
- Your friend is an internal force. He would have to get out of the car and push from there to be an external force to cause your car to move.
An object resisting a change in its “state of motion” (stopped or moving in a straight line) is something that Newton called inertia.
- That’s why this law is known as the Law of Inertia.
- Basically, the idea of inertia is that however an object is moving right now, it will keep on moving that way.
Example 1: 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 force.
Example 2: A book sitting on a desk won’t start to move all on its own. A force needs to be applied to it.
Newton’s 1st Law goes against what Aristotle said, and 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 Newton’s 1st Law.
- It is called Newton’s 1st Law because he was the one that formally published it and had the mathematical proofs to back it up.