Question

Hey guys. I was wondering... What is the importance of Newton's first law, as they understood it in his time. Is it ok (as Susskind says in one of his lectures) if we absorb it into the second law? I heard once that philosophically it serves as a definition for inertial frame of reference thus paving the way for the second law, which only applies to inertial frames of reference unless we include "ficticious" forces. What do you guys think?

Answer 1

People in newton's time thought that a force was always needed to keep a body in motion, and why wouldn't they think so? Move a ball on the ground pushing it with your hand for some time, and then remove your hand. The ball stops after some time, as now you provide no more force to it.
Galileo performed an experiment in which he let a ball roll on ice. He noticed that the ball moved for a very long time, even after you stop pushing it. Clearly, a force wasn't needed to keep a body in motion. But what stopped the ball in the previous case? It was friction, another opposing force. Ice offers very little friction.
Newton hence interpreted these results and stated his first law. This demythed the myth that a force was needed to keep a body in motion.
The first law stated that a force is needed to change the state of a body. But why should a body need to be 'forced' so that it changes its state, i.e. why can't it happen by itself? Newton also defined the reason for that. It is inertia, the resistance of a body to change in its state. It is a quantity proportional to its mass.
The second law is nothing but a mathematical extension of the first law. You can even prove the first law using the second. So, susskind is right.
The reason they are called 'inertial frames' because the definition of inertia is perfectly valid there. In a non-inertial frame, you might get a force without change in state, or a change in state without force. So the first law somewhat describes inertial frames too.

Answer 2

Hi, thanks but I think you completely misunderstood my question. If you see carefully. People could argue that Newton's first law can be derived from the second. Just consider that F=0 means a=0 then a motion in a straight line (i.e. the 1st law) follows. If the 1st can be derived from the second then this raises the question as to why he needed to state it first.

Answer 3

Because the first law defines inertia, the second law doesn't. And inertia is really important.(as in inertial frames)

Answer 4

yes quantiatively u can say that all the aspects of 1st law is explained by the second
ie no force means body at rest or uniform motion of body
but there is a neccesity to introduce this very concept that DEFINES inertia or the tendency of bodies to obey the first law
if newton hadn't stated the first law there wouldn't have been any foundation to all the concept which came after that
it is a significant law which brings out the beauty of properties and tendency of bodies which can be very well supported by the 2nd law