You would have noticed that when a bundle tied with a string is lifted quickly by holding it, the string breaks. Can you now explain why the string breaks in this case?
Answers
Answer:
If something should accelerate, we are in Newton's 2nd law. If not, Newton's 1st law. Let's write it out with the forces from each string and weight w present:
−Fup+Fdown+w=0−Fup+Fdown+w=ma
(I hope it's okay I've put the y-direction downwards.)
If you pull slowly down, no significant speeding up happens of the box. Fdown has some constant value. It all balances out. The 1st law.
If you pull fast down, the box tries to speed up fast to follow along. That means large a. That requires large force to cause it. And the force, that tries to cause is the Fdown.
Look at those two equations again. In the first case Fup=Fdown+w, so the upper string breaks. In the second case Fup=Fdown+w−ma. Hmm, here is being subtracted the part ma...
So, is Fup becoming smaller? No, of course not, it has it's tension and only grows as you pull downwards. Rather Fdown becomes larger. Because it tries to cause the a.
And as you see, it tries to but simply can't apply enough force to cause that acceleration. The necessary force in the lower string is more than the strength of the string, so it breaks.
Explanation:
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