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Newton's Third Law

I was wondering if anyone could explain to me a little issue I have with Newton's Third law of motion. I understand the law in general (action an reaction are equal and opposite; can be seen by a book on table not accelerating towards the ground and staying at rest as per Newton's first law).

What I do not understand is why there is a limit to this. If I place a 10N object on my desk, it will stay there, but for an object of the same volume weighing 10^6N, it would go straight through it. If Newton's third law is correct why is there no equal and opposite reaction force stopping it? Is there one, but the sheer weight breaks the table the instant the reaction force appears, thus dissipating it? I don't know...

Any help would be appreciated,
VannR
Moved to physics :smile: . They should be able to help explain it to you :wink:
Reply 2
Im not really sure but maybe its because of the maximun tensile strength of the tables material?

Basically tensile strength is the maximum tensile stress at which a material breaks, i.e. The force/area.

The heavier the object you put on the table, the greater the force (mass x 9.81).

Im not really sure, but thats what i think :smile:

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Reply 3
If the table is assumed to be a rigid surface with an infinite tensile strength, then yes you would expect the force pushing back on the object to be the same. But tables do not have infinite properties.

Every object has a limit to the amount of stress or strain it can take (The amount of pushing or pulling force) and it's determined by the strength of the bonds at the atomic level. If you were to put a 10e6kg object on a table that can only take 500kg, the weakest bonds of the table would break and collapse

An interesting read would be "The New Science of Strong Materials or Why You Don't Fall Through the Floor" By J.E.Gordon
Original post by VannR
I was wondering if anyone could explain to me a little issue I have with Newton's Third law of motion. I understand the law in general (action an reaction are equal and opposite; can be seen by a book on table not accelerating towards the ground and staying at rest as per Newton's first law).

What I do not understand is why there is a limit to this. If I place a 10N object on my desk, it will stay there, but for an object of the same volume weighing 10^6N, it would go straight through it. If Newton's third law is correct why is there no equal and opposite reaction force stopping it? Is there one, but the sheer weight breaks the table the instant the reaction force appears, thus dissipating it? I don't know...

Any help would be appreciated,
VannR


You misunderstand Newton's 3rd Law. That's the problem.

The force of the book B on the table A is indeed equal and opposite to the force of the table A on the book B. This is two different objects, A and B. This is Newton's 3rd but has nothing to do with this problem.

The reason why or not the book falls through the table is to do with the forces on the book. On the same object.
If the weight of the book (force of Earth on the book) is greater than the force of the table on the book, the book accelerates downwards. If they are equal the book stays where it is. To predict what happens to the book, you look at the forces on the book. That is, two forces on the same object.
Newtons Law refers to two equal and opposite forces on different objects.
If the book is so heavy it moves through the table, it means the table is unable to apply a force equal to the book's weight. The book's weight (Earth on book) and the supporting force of the table (table on book) are not a Newton 3rd Law pair.
(edited 9 years ago)

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