Does these violate Newton's third law?

Question

A 50 KG brick is put on a glass table glass. Action force and reaction force are not the same and the glass breaks because a glass's opposing force will never equal the heavier brick. So in everyday mechanics, Newton's third law is broken! Everything might have an opposing force but that is not necessarily equal. If a train collides with a car moving in the opposite direction, the train will smash the car and go on moving without damage. So this also violates Newton's third law?

Answer 1

When you say action and reaction force are not the same, you are going against the 3rd Law and it is not a true statement. The force exerted on the glass by the brick must be the same magnitude as the force of the glass on the brick. Action and reaction are not good terms to use - rather the only way object 1 exerts a force on object 2 is if object 2 exerts the same force back on object 1. Forces are always interactions and don't exist outside of these interactions. Forces always come in equal and opposite pairs. A bus runs into a bug. The magnitude of the force of the bug on the bus is equal and opposite to the force of the bus on the bug!!! The effect of that force is different: F_{B on b}/ very small mass of the bug results in an acceleration that the structure of the bug cannot handle and it goes splat. The bus, on the other hand, experiences F_{b on B}/mass of the bus with is a miniscule acceleration so that the bus's velocity is essentially unchanged. Here's another one: you attract the earth with your weight. (F_{you on earth}= - F_{earth on you}).

Answer 2

Hi Krugen K.,

Jacques D.'s explanation is spot on (so to speak, for that bug!).

But laying (note: NOT lying or lieing!) a brick at the center of a plate-glass table violates common sense, for sure. If you needed it on the table, put it near an edge (gently), and on the narrow long face. Same idea, for climbing a tree-limb or a ladder, which you aren't sure of the strength of. You stay near the trunk on the limb, and near the edge of the ladder, as you step. The stress vector in each case is directed as vertically as possible, which tends to minimize strain on the supporting member.

-- Cheers, --Mr. d.