Can one derive Newton's
second and third laws from the first law or
first and third laws from the second law or
first and second laws from the third law
I think Newton's laws of motions are independent to each other. They can not be derived from one another. Please share the idea.
You cannot derive any of the laws from each other. In particular, for each law there is a possible universe where one law fails and the other two hold.
So let's see where the third law fails. Imagine a universe with two bodies (with positions $x_1$ and $x_2$) of equal finite mass ($0< m_1=m_2 <\infty$). One exerts a constant force on the other $F_{12}$ that pulls it towards the origin with a force proportional to how far away it is from the origin ${F}_{12}=-m_2\omega^2{x}_2$ and the other exerts no force on the one ${F}_{21}= 0$. The motions are ${x}_1(t)=100$ and ${x}_2(t)=\sin(\omega t)$. The first two laws are satisfied (${F}_{21}= 0$ and $m_1$ is at rest and stays at rest, ${F}_{21}=m_1 a_1$, ${F}_{12}=m_2 a_2$), but since $ F_{12}+ F_{21}\neq 0$ the third law is not satisfied
Let's see where the second law fails. Now imagine a universe with three bodies of equal finite mass $0< m_1=m_2=m_3 < \infty$. The first two exert a constant nonzero external force on the other, each of the forces are equal and opposite $ F_{12}=- F_{21} \neq 0$. All other forces are zero $ F_{13}= F_{31}= F_{23}= F_{32}= 0.$ The motions are $x_1(t)=100$ and $x_2(t)=50$ and $x_3(t)=0$. The first law is satisfied ($ F_{13}= F_{23}= 0$, $m_3$ is at rest and stays at rest), as is the third ($F_{ij}+ F_{ji}= 0$). The second law is not ($ F_{12}+ F_{32}= F_{12}+ 0= F_{12}\neq 0 =m_2 a_2$).
Now let's see where the first law fails. Finally, imagine a universe with three bodies $1$, $2$, and $3$ of equal finite mass m. Let $d$ and $C$ be positive non-zero constants with the appropriate units. Suppose the universe has a potential energy function $$V(x_1,x_2,x_3)=-C\left(\frac{x_1-x_3-d}{2}\right)^{4/3}\;,$$ so $1$ and $3$ exert equal and opposite forces on each other $$-F_{31}=\frac{\partial V}{\partial x_1}=-\frac{\partial V}{\partial x_3}=F_{13}\;.$$ Suppose $x_1(0)=d$ and $x_2(0)=d/2$ and $x_3(0)=0$. Further suppose that $v_1(0)=v_2(0)=v_3(0)=0$. Obviously we can satisfy all three of Newton's laws by taking as solutions $x_i(t)=x_i(0)$, however, instead suppose the particles move as $$x_1(t)=d+(Kt)^3,~~ x_2(t)=d/2$$ and $$x_3(t)=-(Kt)^3$$ for $$K= \sqrt{\frac{2C}{9m}}.$$ Then the third law holds because $F_{ij}=-F_{ji}$ and the second holds because $ma_2(0)=0=F_2$ and
$$ma_1=mK^36t=mK^26Kt=m\frac{2C}{9m}6(K^3t^3)^{1/3}=\frac{4C}{3}\left(\frac{2K^3t^3}{2}\right)^{1/3}=\frac{4C}{3}\left(\frac{x_1(t)-x_2(t)-d}{2}\right)^{1/3}=F_1$$
and
$$ma_3=-mK^36t=-mK^26Kt=-m\frac{2C}{9m}6(K^3t^3)^{1/3}=-\frac{4C}{3}\left(\frac{2K^3t^3}{2}\right)^{1/3}=-\frac{4C}{3}\left(\frac{x_1(t)-x_2(t)-d}{2}\right)^{1/3}=F_3.$$
So the second and third laws are upheld, but the first law says that if no net external force acts, then the velocity is constant. This is not a property of the solution given, the velocities are all zero at $t=0$, as is the force, but yet the velocity is never constant, it is always changing, it's just changing there so slowing that $a=0$. A zero acceleration is different than an unchanging velocity. The solution $x_1(t)=d+(Kt)^3$ has a zero acceleration, but the velocity is changing. Note that the solutions $x_1(t)=d$ and $x_2(t)=d/2$ and $x_3(t)=0$ are also solutions to $F=ma$, so Newton's 2nd law allows multiple solutions with the same initial position and velocities, but the first law can pick a unique solution.
So there is an example where the 2nd and 3rd laws hold, but the 1st does not.
So none of the three can be derived from each other.
Edit I'd like to credit Abhishek Dhar's paper "Nonuniqueness in the solutions of Newton’s equation of motion" Am. J. Phys. 61, 58 (1993); http://dx.doi.org/10.1119/1.17411 for inspiring the example force law with nonunique solutions that I gave.
Ten years later Norton introduced his dome and noticed that you can have the stay-at-rest solution persist either forever, or for any finite amount of time and then spontaneous start to move. I added the symmetric force so that you can clearly see the third law unaffected. Norton disagrees with me about the meaning of the first law. Since Newton also intended to include uniform rotation as inertial motion (that's why he talks about bodies having their own force), to me Newton clearly meant zero net external force as the case for the first law and was attempting to make distinctions between an external force applied to a body and a body exerting its own preference for inertial motion. And that body's own inertia is the causal agent in what selects the solution of constant velocity in my example as opposed to one of the many solutions where the velocity changes, but merely changes in a way slowly enough where $a=0$ as it starts changing. The merely having $a=0$ approach, using the second law without the first, would say that $F=ma$ is all that matters and the bodies own inertia has no say about whether to have a uniform motion or whether to move. That allows multiple solutions if you really want to throw away the first law, plus you get Norton's motion that happens after any random amount of time. Throw out the first law and there are consequences.
I am not claiming here that it always will- so, wouldn't First Law single out the solution always?
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Jun 04 '16 at 06:11
Newton's laws of motion cannot be derived from each other. They are the building blocks of Newtonian mechanics and if fewer were needed, Newton would simply formulate fewer.
The first law postulates the existence of an inertial reference frame in which an object moves at constant velocity if the net force acting on it is zero. Although it might seem you can derive it from the second law (if the net force is zero, there is no acceleration and the velocity is constant) but in fact, both second and third law assume that the first law is valid. If an observer is in a non-inertial reference frame, she will observe that the second and third laws are not valid (when you sit in an accelerating car, the Earth accelerates in the opposite direction without any force acting on it).
You also cannot derive the second law from the first one because all you know from the first law is that when an object accelerates, there is a force acting but the first law says nothing about the relation between the force and the acceleration. That's what second law is for, to say that there is a linear relationship.
The third law adds something more to the first and second laws. It deals with interactions and states that two bodies exert same but opposite forces o each other. That is something you cannot see from the first or second law and similarly, there is no way to use this to derive the second law (you cannot derive the first law because that is assumed to be valid in order to postulate the third law).
No, they're not independent, because the first can be deduced from the second. Newton's second law says that $F=ma$. The first law says that $a=0$ when $F=0$, which clearly follows from $F=ma$. The purpose of the first law is not to be an independent postulate from the second law, but just to emphasise this particular special case, which presumably would have been counterintuitive to many of the contemporary readers of Newton's work.
Other answers have tried to claim that the first law is really about the existence of an inertial reference frame, and of course you're free to interpret it that way if you want, but what it actually says is not independent of the second law.
From a modern point of view, all of newton's laws follow from the conservation of momentum. For example, for two bodies in one dimension, the total momentum is $m_1v_1 + m_2v_2$. If it doesn't change over time then its derivative must be zero, i.e. $$ \frac{d}{dt}(m_1v_1 + m_2v_2) = m_1a_1 + m_2a_2 = 0. $$ If we define $F_1 = m_1 a_1$ and $F_2 = m_2 a_2$ then this becomes $F_1 = -F_2$, which is Newton's third law. The second law is just the definition of $F$, and the first law comes from noting that if you just have one body then $mv$ can't change, so $v$ has to be constant.
The modern interpretation of Newton's First Law is about the existence of inertial reference frames, mainly to solidify the idea that such coordinate systems exist and are important.
However, I sincerely doubt this is what Newton himself had in mind when he postulated his laws. Historically, Newton probably introduced his first law to put emphasis on the fact that moving bodies do not slow down by their own accord (as was common wisdom at the time). So yes, the first law in this context can certainly be derived from the second law by setting F=0. In fact I doubt Newton even had an idea of what an inertial frame was (though it probably could have been explained to him with relative ease).
There are cases where one can derive the 3rd Law from the 2nd Law (with an additional assumption)
Consider two masses $m_1$, $m_2$ glued together which are subject to a force $F$ (this experiment is being carried out in space, in a region with negligible gravity).
By the second law of motion, the acceleration $a$ of this block is given by: $F = (m1 + m2) a$ (1).
We will now analyze each of these two conjoined parts as independent subsystems with their own force diagrams ($F_{A/B}$ represents a force exerted on $A$ by $B$).
Applying Newton’s second law on these subsystems, we get
$F + F_{1/2} = m_1a$, (2)
and
$F_{2/1} = m_2a$. (3)
Adding Eqs. (2) and (3), we get
$F + F_{1/2} + F_{2/1} = (m_1 + m_2)a$ . (4)
However, from Eq. (1), we have $F = (m_1 + m_2) a$.
Thus, combining Eqs. (1) and (4), we reach the following surprising result: $F_{1/2} + F_{2/1}= 0$. (5)
But that is the statement of the third law!
Newton’s second law alone did not bring us to Eq. (5). Along the way, in Eq. (1), we assumed that the mass of the composite object was equal to the sum of its components. In other words, we assumed that mass has an additive property. That was the extra input required for this derivation.
But Newton's 3rd law has a much wider application than this simple case which we demonstrated and conservation of momentum is related to it.
In other words, one would need further assumptions (e.g. conservation of momentum) in order to derive the 3rd law, independent from the other 2 laws.
References and full text:
You can only deduce the third law from the second one, its long and can be found here.
The first cannot be deduced from the second because it talks about initial reference frames. And with that you can see that you can't deduce any others from each other (without creating a dependence circle).
However you must keep in mind that this was not that trivial in the 17th century. You can't state that clearly that one is a derivation of the other, because you are assuming something about how the world works, and that it follows some deep mathematical consistency. It could have done other stuff too, however today those kind of actions would not be considered 'physical'. But note, Newton also invented calculus which is in a way the tool we used to derive the first and second law.
