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Q & A: Newton's Laws of Motion

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Most recent answer: 05/02/2020
Q:
why is newtons 1st law of motion important and what would happen if we didn't have it?
- Samantha (age 13)
Louisville Kentucky
A:

Newton's first law embodies an important conceptual difference between Aristotelian (classical Greek) physics and Newtonian physics.  The essential question is, "What happens when nothing is happening?"  That is, how does an isolated object, not interacting with other objects, move?

Here is Newton's statement of the first law in his Principia Mathematica:

"Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by forces impressed."

It tells us that when nothing is happening, an object's velocity is constant.  This is completely different from the Aristotlean concept of "natural motion", in which material objects naturally come to rest at the center of the Earth.  This means, for example, that gravity is one manifestation of natural motion.  Similarly, celestial pbjects naturally move in circles (plus epicycles) around the Earth, because they are made of a different kind of material.

The first law leads to the concept of inertial reference frames and Galilean realtivity,  An inertial reference frame (think of this is an observer in some mode of motion, for example on an airplane) is one in which the first law holds (that is, when the velocity of the plane is constant).  Galilean relativity says that the laws of physics are the same in all inertial reference frames.

The first law also underlies the concept of inertia, which, in its modern form was first developed by Galileo, Kepler, and Descartes.  Inertia tells us that to change an object's velocity, one must exert a force.  This concept is given quantitative meaning to the second law, F = ma.  However, the second law would not make sense if the first law didn't hold.  One must have a = 0 when F = 0.

 


(published on 05/02/2020)

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