-4

We all know Newton's third law.

But then why this law doesn't work when we place a 1000KG heavy container on a small weak table. The table will break. Pressure is exerted on table by container, and table exerts force on the ground.

Similarly a heavy pillar will break a weak wall while leaning upon it.

AnjumSKhan
  • 187
  • 7
    I don't see how you get from the table breaking to assuming that Newton's third law is invalid. – David Z Dec 14 '14 at 14:03
  • I am asking this because when body rests on table we say it is because of normal reaction which is equal and opposite. But what if body is very heavy ? – AnjumSKhan Dec 14 '14 at 14:05
  • I would appreciate if you can explain the answer instead of down voting. – AnjumSKhan Dec 14 '14 at 14:05
  • 1
    I would appreciate if you ask a question. – pfnuesel Dec 14 '14 at 14:10
  • Questions are answered by some sensible people which you can read below. – AnjumSKhan Dec 14 '14 at 14:39
  • @AnjumSKhan i hope you ask questions on this forum next time you post. We just deduced something from your statements and answered to an extrapolated question. – Gowtham Dec 14 '14 at 15:02
  • 2
    Equal and opposite forces do not imply immobility. You can have motion because the forces are acting on different bodies. See related posts on the side ---> – John Alexiou Dec 14 '14 at 15:33
  • The table will exert a normal force to hold up the stuff on it. This hold-stuff-up force has a limit, which is when the table cannot withstand it and breaks. The the 1000 kg does not have to push with 1000 kg but just with what is enough for the table to break. – Steeven Dec 14 '14 at 18:52
  • Consider a 10000 kg weighing body is placed on a weak small table. Naturally the table will break and the body will accelerate through it. In that case for the body we can write mg-N = ma , where a is accel of the body. And thus we can find the normal force N by table on the body . – AnjumSKhan Dec 16 '14 at 14:11
  • possible duplicate of With Newton's third law, why are things capable of moving? – Brandon Enright Dec 16 '14 at 16:26

3 Answers3

2

You're confusing Newton's 1st and 3rd laws.

Newton's 3rd law pair to the weight of the mass (ie the force of the Earth on the mass) is the force of the mass on the Earth. The 1st law pair would be the contact force of the table on the weight and the force of gravity on the weight. As the table is unable to provide a large enough force to prevent the weight from falling, Newton's 2nd law applies.

Joe Rowing
  • 46
2

I guess you are asking this: Why is the third law correct if the container makes to the table a force of 1000 kg but the table reacts with a smaller force? The error is that 1000 kg is the weight of the container, not the force it will do to the table. If the table doesn’t break, when they are at rest, it will make a force of 1000 kg to the table and the table a force of 1000 kg back (called the normal force). But if the table doesn’t resist and breaks under a force above 10 kg, then the container will make on the table a force of 10 kg, not 1000 kg. A force is an interaction between two objects. You cannot have a "force" of 1000 kg with you and apply it onto any object you want by touching it. It doesn’t work like that.

  • Probably best explanation read anywhere so far. "You cannot have a "force" of 1000Kg with you and apply it onto any object you want by touching it. It doesnt work like that." Best line read. – AnjumSKhan Dec 14 '14 at 18:18
  • So, my understanding is that "3rd Law" is immaterial. Can u give one example where 3rd law governs the happenings. – AnjumSKhan Dec 14 '14 at 18:34
  • Conservation of linear momentum in an isolated system of more that one particle requires teh third law. That is because, if the third law is valid, the internal forces cancell each other. Without the third law we would not have conservation of linear momentum. –  Dec 14 '14 at 22:53
  • So this means that normal reaction is calculated using N-F=ma. Consider a 10000 kg weighing body is placed on a weak small table. Naturally the table will break and the body will accelerate through it. In that case for the body we can write mg-N = ma , where a is accel of the body. – AnjumSKhan Dec 16 '14 at 12:28
1

A table or a weak wall can withstand only a certain amount of force depending on its material. If we provide more force it is going to fail or break . nothing to do with 3rd law. If the table can withstand 1000KG it will and there will be a normal force according to 3rd law. hope this helps clear things up. Or you should just read about "Failure of materials " from wiki for more info.

Gowtham
  • 391