
#1
Nov2109, 11:03 AM

P: 44

I can't prove the Newton's third law.
Please tell me how to prove the Newton's third law. Thanks. 



#2
Nov2109, 11:09 AM

Sci Advisor
PF Gold
P: 2,197

You can't prove Newtons laws...
They are postulates, meaning they are assumed to be true. 



#3
Nov2109, 11:19 AM

Sci Advisor
HW Helper
PF Gold
P: 12,016

Laws of physics have the same deductive status as axioms within maths.
One cannot prove either within each respective system, but for physics, the first selection principle for what is to count as laws is that they seem to work, in the sense of generating predictions that are confirmed by experience. The second selection principle (among those postulated laws fulfilling the first) would be along the lines of parsimony/simplicity. 



#4
Nov2109, 11:49 AM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,894

How to prove the Newton's third law. 



#5
Nov2109, 05:16 PM

P: 62

How did Newton arrive at his third law? Did he just look at astronomical data and then arrive at his conclusion?




#6
Nov2109, 07:18 PM

P: 567

If we already know about conservation of momentum, it's easy. Let two objects be initially at rest. Let the two objects be an isolated system  they may begin to exert forces on each other, but no forces on them come from outside. Let one object begin to exert force F1 on the other object. Assume that the second object exerts force F2 on the first, which is automatically valid because of its generality  F2 would have the value 0 if it really doesn't exert one, and a nonzero value if it really does. Conservation of momentum, since the total momentum of the system was initially zero, it has to be finally zero. m1 v1 + m2 v2 = 0. Take the derivative of both sides: m1 a1 + m2 a2 = 0. Define the force on any object to be the derivative of its momentum: F = d(mv)/dt = m dv/dt = ma (Newton's 2nd law) . F1 + F2 = 0. F1 = F2.




#7
Nov2109, 09:54 PM

HW Helper
P: 6,929

I think it would be more accurate to state that physics laws and theory's are verified to be resonably true (or accurate) by experiment, not proven.
I don't know if Newton derived his third law mathematically from his other laws or from observation of a relationship between forces and accelerations of varying objects, or from the fact that in any closed system, all internal forces must cancel. 



#8
Nov2309, 09:03 AM

P: 196




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