Many textbooks are pumped up about Newton’s 3 laws of motion. For me, not so much. First, (as many other’s point out) these are really Newton’s ideas about force. Second, the first law is pretty much a special case of the second law. Here are the first two laws (in my words):
Newton’s First Law:
The natural state of an object is constant motion.
Yes. I know that is not how it is normally written.
Newton’s Second Law:
The rate of change of an object’s speed is proportional to the amount of net force on the object and inversely proportional to the mass of the object.
This could also be written as the equation:
I wrote the acceleration and the force as vectors because that is what they are. If you don’t know what a vector is, just ignore the arrow symbols.
How are these the same?
Newton’s 2nd law says that the net force is proportional to the acceleration. What if the net force is zero, or if there is no force? Well, then the acceleration is zero. What happens to an object with zero acceleration? Average acceleration (in one dimension ) can be written as:
Or you could say acceleration is how the velocity changes. If the velocity does not change, the object is either moving in a straight line at a constant speed, or at rest.
So, why the 1st law?
I am going to guess (or maybe I read it somewhere, because I am really not a good guesser) that this has to do with Aristotle. Aristotle had his own laws of motion. They go like this:
- Artistotle’s First Law: If there are no forces on an object it will stay at rest
- Artistotle’s Second Law: If you push on something with a constant force, it will move at a constant speed
- Artistotle’s Third Law: If you stop pushing on a moving object, it will stop. Or…the natural state of an object is to be at rest.
Aristotle didn’t state them like this, and you wouldn’t have to write them as three laws. But that sums them up. You have to understand that Aristotle was THE man. What he said was it. His ideas about force were held as the truth for quite some time. So, I suspect that Newton’s first law is really to counteract Aristotle. Aristotle says the natural state of an object is at rest. Newton says the natural state of an object is constant motion.
Note that Newton wasn’t the only one to come up with these ideas about force and motion, but for some reason he gets the credit. Anyway, the point is that Newton’s first law is rather silly.