# Redundancy of Newtons first and second laws of motion.

Doesnt the Newton's first two laws of motion seems redundant; implying the same thing again and again. Or why could he not given a single law combining the two itself.

Note by Karthikeyan Rajaseker
5 years ago

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I'll chime in here. Newton's First Law is the statement that if there is no force there is no acceleration and objects keep moving at a constant velocity. It does not specify how the acceleration changes upon application of a force. That is the content of the second law. There have been alternative theories posited, even recently, that satisfy the first law but give a different rule for the second. The fact that you can even do this indicates that the two laws contain different (but related) concepts. If you want to read about one of these theories, called MoND (for Modified Newtonian Dynamics), see here. This theory was invented because it did nice things to galaxy rotation curves and removed the need for dark matter, which made people sit up and take notice. (Unfortunately it doesn't work for all the other dark matter observations we now have). It winds up being very unpleasant to make this theory fit int he framework of relativity though, so it's now (mostly) an historical curiosity.

Staff - 5 years ago

well it seems like they are very related. the first one dealing with inertia, and the second being F=ma. really, you could get the first one from the second. the first one is saying if the force is zero, velocity will be the same, or in other words the change in velocity will be zero. since acceleration is the change in velocity, this means acceleration would be zero. plugging this into F=ma, we get F=0, which gives us the law of inertia. you can't get the second from the first, though.

I found this on wikipedia:

Newton's laws hold only with respect to a certain set of frames of reference called Newtonian or inertial reference frames. Some authors interpret the first law as defining what an inertial reference frame is; from this point of view, the second law only holds when the observation is made from an inertial reference frame, and therefore the first law cannot be proved as a special case of the second. Other authors do treat the first law as a corollary of the second

- 5 years ago

Newton havent used the word "inertial frame" or "frame of reference" in his Principia. How could one come to such conclusions without newton mentioning them. And what is the significance of continuing with the first law even in 21st century, even after you said one can arrive at first law frm second?

I am in no way an even moderately intelligent individual on this matter, so this is merely conjecture.

First of all, the two laws do state different things (the first law states that objects like to continue consistent motion or lack thereof, and the second law states that greater mass requires greater force to accelerate the object).

I suppose the main reason would be that Newton's First Law creates this idea the law of inertia which has different applications from the second law. You would use inertia to describe something like why ketchup or another viscous fluid requires quick acceleration (shaking the ketchup bottle hard) and then quickly stopping. The law of inertia states that the ketchup inside the bottle is moving when you accelerate the bottle, but when you stop the bottle it doesn't stop accelerating, and thus plops to the opposite side. Whereas the Newton's Second Law would explain why kicking a brick wall doesn't make it move while kicking a soccer ball does. I believe that because they have different applications they remain as separate laws, instead of some overarching rule.

Then again, that's just me. Anyone want to add to this?

- 5 years ago

newton's first law gives the condition of a body until an unbalanced force acts on a body. once the force acts on that body,its change in state. first law not gives how the motion is changing while applying external force. but newton's second law gives how its motion changes in terms of change in velocity(acceleration) of the body. That Newton's 1 law gives definition of FORCE, but 2 law gives HOW MUCH THE FORCE ACTS ON THE body (i.e MEASURE of FORCE)

- 5 years ago

Why we need a first law, when second itself can be taken as defining force[as mass times acceleration]???

mass times acceleration gives how much force acts... that is gives quantitative definition of force. but first law gives qualitative definition of force

- 4 years, 12 months ago

first of all newtons laws of motion cannot be proved!!!!!!!!!!!!!!

- 5 years ago