Newton's third law of motion confusion Suppose, there are two objects, namely, A and B. Suppose, object B is more fragile than A, such that it breaks on the application of a 5N force. If A is accelerated such that the force carried by A is 10N and made to collide with B. Since B can only tolerate a force upto 5N, it'll will break. The contact force on B due to A is apparently 10N, and the contact force of B on A is apparently 5N( because it breaks on 5N). Shouldn't the contact forces be same, according to Newtons third law? Where am I doing Wrong?
Any sort of help will be appreciated. 
Thanks.  
(P.S. :- I have just started with classical mechanics.) 
 A: Per Newton’s third law the contact forces will be equal and opposite. However, the magnitude will be limited to max possible reaction force of either B or A, which in this case is 5N due to breaking of the intermolecular bonds of B. At the microscopic level, it is the resisting intermolecular forces in B and A that are responsible for the action reaction forces.
In short, the maximum force that can be applied by A is limited to the maximum possible reaction force of B prior to failure, and vice versa.
Hope this helps.
A: Overview:-
Newton's third law is always followed. When an object applies a force on any other object then the other object also applies a force of same magnitude on the first one. For force to be applied we need at least two objects, an object cannot apply the force on nothing, it needs somebody.  
How reaction is caused?
The law of gravitation and Coulomb's law leaves no room for doubt about why action is accompanied by the reaction (force on mass $m$ by mass $M$ is $F = \frac{G~Mm}{r^2}$ and force on mass $M$ by mass $m$ is $F = \frac{G ~mM}{r^2}$). But when it comes to contact forces, it becomes a little harder to see how reaction is caused.  
Actually, macroscopic objects (like A and B which you have used in your question) are made up of continuous structures of this form  
 
I want you to see that springs which represents the intermolecular forces. Your object A and B, both consist of these types of structures, when the object A hits or pushes (although at molecular level they don't touch each other) B, then we have something like this going on  
 
You can see that collision of A has caused a compression in B (spring's length is shorter than before) so from your everyday observations you can conclude that B must be pushing back on A (springs when compressed, applies a restoring force). In reality, this pushing back on A is caused due to the repulsive intermolecular forces. So, this is how reaction is caused.  
How Newton's Third Law works when an object gets broken?
Again, consider this figure  
 
We know from the last section that there is no problem up to $5~N$ of force (I'm specifically considering your case). Now, if the force by A on B is increased above $5~N$ then you say that B gets broken. Broken can mean two things here 


*

*Object B breaks down into smaller pieces : if it gets broken down into smaller pieces, then A will apply force on these smaller pieces and as @sammygerbil has explained, third law will continue to be valid.  

*Object B disappears : if any force just a little more than (how much more is quite controversial here) $5~N$ causes B to fly away or makes it to disappear (only for theoretical purpose) then you see there is nothing for A to apply the force on. If B just disappears when the force exceeds $5~N$, then there is no object left for A to apply any force and hence there will be no action and therefore no reaction will be there.  
Conclusion:-
Newton's third law is always valid.  
P.S.:- I've not taken Quantum Mechanics into account in the above answer.  
A: As @Alchimista suggests in a comment, make the situation simpler by considering a static situation.
Suppose that object A weighs 10N and it is lowered gently onto object B which is resting on the ground. Someone (person C) carries the weight of A and releases it so that the force on B increases gradually.
Before the two objects touch the contact forces between them are zero. When they touch the contact forces increase gradually but possibly rapidly as C releases more weight. At each instant the force of A on B and the force of B on A are equal and opposite in direction, and person C bears the weight of A which is not acting on B.
Object B is fragile and breaks apart when the contact forces reach 5N. What happens next? The pieces of B get rearranged. Person C holds up more of the weight of A so that it does not fall suddenly, then releases it gradually onto the pieces of B. The process starts over again.
Suppose that B breaks into 2 pieces each of which can withstand a force of 4N. The weight of object A is released gradually. The contact forces between A and the 2 pieces of B increase until the force of A on each piece of B is 4N and the force of these pieces on A is 8N and person C is holding up 2N. Then the pieces of B themselves break and the process starts again. It repeats until there are enough pieces of B to withstand the full weight of A.
A: Object A cannot push on object B without object B pushing back on object A.  At the point that object B begins breaking, at 5N of force, it can't put any more force on object A, and vice versa.
