# An atomic explanation of Newton's Third Law of Motion

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1. Jan 25, 2016

### faiziqb12

let's look at force at the atomic level to understand the newtons third law of motion. I'll use Helium atoms as an example.

Now imagine we start with one atom HE2 stationary, and throw another atom HE1 at it.It is the velocity of HE1 that affects the motion of HE2 , because the system of these two HE2 atoms is isolated , and there is no unbalanced force on each of these . According to physics both the atoms will experience action reaction forces at the same time .

for sake of simplicity lets consider a imaginary hollow box between the two atoms as an overlapping of their atomic forces. As soon as HE1 moves x distance inside this box , there will be a increase of y amount of velocity in HE2 and a correspoding decrease of y in the velocity of HE1 . But there is still a greater velocity in HE1 as compared to HE2 , so the process of changing of velocities will continue up to the time when both the atoms have the same amount of velocity , because after they attain the same amount of velocity they wont be able to enter tat box again .

So , is what i just explained right ?

i dont think so, because we know from our knowledge of head on elastic collisions of equal masses that when HE1 and HE2 will collide, HE2 will gain the velocity that HE1 had and HE1 will itself become stationary.

so can you please correct me , and provide me with the correct answer to understand the newtons third law of motion ?

2. Jan 25, 2016

### Drakkith

Staff Emeritus
This is only half the truth. In the process of the collision, the repulsive forces between the two atoms is highest when they are closest. So they don't keep moving at the same velocity. The first continues to slow down as the second continues to accelerate. The end result is that the first atom comes to a stop and the 2nd continues on at the original velocity of the first.

3. Jan 25, 2016

### A.T.

As Drakith notes, the forces are distance dependent, not velocity dependent, so there is no reason to assume the above.

This is not exactly true, as changes in EM fields propagate at a finite speed. Momentum is still conserved, when you account for the momentum of the field. But Newton's 3rd in its simple form doesn't hold.

4. Jan 25, 2016

### faiziqb12

could you please elaborate the process

5. Jan 25, 2016

### faiziqb12

thats fine but the distance itself depends on the velocity .
doesnt it ?

6. Jan 25, 2016

### A.T.

You need a quantitative description, not vague arguments.

7. Jan 25, 2016

### faiziqb12

i am sorry but i didnt get you

8. Jan 25, 2016

### faiziqb12

basically , it appears like when a moving atom reaches another stationary atom they must both move together because the molecular force between them will join them together . So , if my reasoning is right an atom will never rebound back or give its velocity to the stationary atom , but rather take the stationary atom with itself.

well that isnt quite right ?
so could you please correct me ?

i really need help with this

9. Jan 25, 2016

### Drakkith

Staff Emeritus
Imagine two balls with springs attached undergo a collision, with the first coming from the left and impacting the second, which is stationary. During the collision the springs are compressed as the balls approach each other. When the balls are at their closest, the springs have maximum compression and exert the maximum amount of force. This accelerates the two balls away from each other. As they begin to get further apart, the force exerted by the springs drops off until it reaches zero. At this point the collision process is over and the 2nd ball is now moving at the same speed and in the same direction as the first ball was prior to the collision. The first ball is now stationary.

10. Jan 25, 2016

### faiziqb12

thanks for the answer
my question is somewhat resolved ..
it looks like velocity of the moving object is is its ability to break the atomic force and reach nearer the atom , so if we have such a large velocity of the moving object that it can coincide with the nucleus and still have some velocity remaining , then how would you explain that

11. Jan 25, 2016

### Drakkith

Staff Emeritus
At that point you'd have to get into quantum electrodynamics and/or quantum chromodynamics to explain the interaction between the two atoms since the forces involved and the interactions of the atoms no longer behave classically.

12. Jan 25, 2016

### faiziqb12

why specifically cant the phenomenon be explained by quantum electrodynamics..?
and could you also give the links where i could find about , how to explain this phenomenon by quantum electrons or rather please explain yourself

13. Jan 25, 2016

### Drakkith

Staff Emeritus
I cannot, as I am not familiar enough with either QED or QCD.

14. Jan 25, 2016

### faiziqb12

sorry , i meant why specifically cant the phenomenon be explained without quantum electrodynamics or chromodynamics either..?
well , i have understood the "i dont know" form you , so this reply is just a correction of what i said

15. Jan 25, 2016

### Drakkith

Staff Emeritus
Because once you get down to the level of where atoms are colliding at high enough velocities to overcome their coulomb repulsion, classical mechanics can no longer describe the interaction. You have to take into account three different forces (EM, weak force, and strong force) along with several quantum mechanical phenomenon which don't even exist in classical mechanics. I'm sorry but I don't really know much more than that.

16. Jan 26, 2016

### faiziqb12

thanks for that ..
well , exchanging even a single drop of knowledge , means you have changed the life of another person , and then admitting that is even a more beautiful deed , people like you are very rare .
anyway , thanks !

17. Jan 26, 2016

### my2cts

Even with two He atoms we can make this as complicated as we want ;-).
Lets assume that the collision takes place at sufficiently low energy so that both He stay in the ground state. It is convenient in QM to think in terms of energy and momentum, not force. Newton's third law is a statement of momentum conservation. The total momentum, the sum over electrons and nuclei is constant in time. As He1 slams into He2, the electrons interact via the quasistatic e-e Coulomb force and the Pauli principle. The electrons of He1 slow down and the electrons of He2 accelerate. Via the quasistatic Coulomb e-n force this leads to deceleration of the He1 nucleus and acceleration of the He2 nucleus. If the collision is head-on, He1 will transfer all momentum to He2, as in Newton's cradle. During the collision the atoms deform through virtual excited states in QM language. As long as you don't care how long the collision takes, you don't have to think about this.
Note that He can form a very weakly bound vanderWaals dimer ( https://en.wikipedia.org/wiki/Helium_dimer ), so at very low energy the two atoms will stick together ! There are subtle effects involving rotation and vibration states of this dimer leading to emission of infrared light.
If there is enough kinetic energy, one or both atoms may be excited, if conservation of momentum and energy allows. Then things get really interesting, since a chemical bond between the atoms may form resulting in an excimer. Excimer lasers exploit this phenomenon. Photons will be emitted and these must be considered in the sum of all momenta. See https://en.wikipedia.org/wiki/Helium_dimer#Excimers .

Last edited: Jan 26, 2016
18. Jan 26, 2016

### faiziqb12

well , this question may be a bit awkward , but will i read about helium dimer in high school , currently i am in 9th class .. well , i havent even read the pauli exclusion principle yet that you mentioned
so , is a graduate matter of study or high school ?

19. Jan 26, 2016

### Staff: Mentor

Quick point of order here: did you move past your original question? Are you interested in newton's laws or atomic physics?

If you want to explore Newton's laws on an atomic (molecular) model, you've gone too far into atomic physics. If you want to explore atomic physics, Newton's laws don'T much apply.

20. Jan 26, 2016

### faiziqb12

thanks , maybe that could be considered as an compliment