# A block, a string and Newton's third law

So this is a general force diagram of the system shown. My question is, according to the third law, if the block is exerting a force of magnitude mg on the thread in the down direction, then the thread should exert an equal and opposite reaction on the block. So, it should exert a force of mg in the upward direction, and nothing should move, as the block experiences equal and opposite forces by the thread and the earth. But this doesn't happen always in real life. Can you please help?

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Can you give the example of some real life happenings you encounter in static conditions... – ABC May 31 '13 at 17:01
well, @userØØ7, I see problems where the block moves down, and logically, it must, under some conditions! this indicates that the block must not move in ANY case..which is false. But why? – Saurabh Raje May 31 '13 at 17:13
Aah , I knew. I asked about static.In other cases the block does not apply $mg$ on the rope.It applies $m(\vec g-\vec a)$ on rope. – ABC May 31 '13 at 17:19
@userØØ7, Well, it thought about it a bit, and I can see your point! I agree, but that seems very counter intuitive, and moreover, my teacher didnt say this in class...he just did mg - T for the FBD. – Saurabh Raje May 31 '13 at 17:25
Join that books room.. – ABC May 31 '13 at 17:28

nice question (I'm just being formal). Things are not as simple in real life, bwoy...

The reason the teacher did mg-T is because that there is a limit to which the string can hold. In real life , you obviously would not see a heavy object attached by a thin rope. That is because that "almost massless" rope does not have the tensile strength to hold the heavy object (mg>>T), where T is the maximum tension the string can hold. You already must have seen questions like "if a string can hold max. 30N then how much mass it can handle", etc. . Also, real life strings are bundles of fibre, which when get non uniformly distributed weight tend to break.

Also, by this, you might ask why things move with the third Motion. I know you know it, but just to clarify, because it acts on different bodies, and since here such a huge force acts on the string , it breaks. If the string can withstand the force, the system is stable. You are simply just questioning the tensile strength of the string.

I know you Raje,please don't pounce to attack me with your comments all of a sudden, just read and understand what I just said (wrote?) and then react. If you want me to explain this you can always contact me.

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the sting must be assumed ideal – ABC Jun 1 '13 at 7:23
Assumptions are not in real life, @userØØ7... – Rohinb97 Jun 1 '13 at 13:52
Yaar Bhandari, next time you comment, read the question once(at least). I am talking about an ideal case, ie theoretical, so keep ur practical knowledge to ur self. My question is, when the system is not at rest, therefore mg is not equal to T. And, I got a nice answer from @userØØ7, so either remove this, because I would very much like to down vote it. – Saurabh Raje Jun 1 '13 at 15:54
You clearly specified that "this might not happen in real life". And also i explained that in ideal cases you are just questioning the tensile strength of the string. So read your question first and then comment. – Rohinb97 Jun 1 '13 at 21:57
As I have previously said, I am sure you dont understand my question, and are commenting without a good reason. Please try to think about this better, and then answer. And btw, I have already got the answer in the comments to my question, so just read what is written! – Saurabh Raje Jun 4 '13 at 11:40