# Newton's Second Law and Tension - very confused

1. Aug 10, 2009

### skydiver

This is not a homework problem so I am ignoring the template.
I was watching Walter Lewin's video lectures and this experiment he performs intrigues me. I have given it a thought for over an hour but have no answer as to which and why will that string break.

I have uploaded the experiment clip - can someone please spare five minutes and explain.

Thanks so much.

Embedded clip:

Last edited by a moderator: Sep 25, 2014
2. Aug 10, 2009

### skydiver

Please let me know if one has trouble watching the video.

To summarize, a block of mass of 2kg is suspended with a massless string. Let's call that string the top string which will have tension T1 of approx 20N.
Now there is another same kind of string attached to the bottom of the block and since nothing is hanging over that string it's initial tension T2 would be 0

Now the professor (as seen in the clip) pulls on the second string and asks which string will break. The bottom string breaks.
He repeats the experiment and now the top string breaks.

He repeats one more time and now the bottom string breaks and ends the lecture there.
But he doesn't explain why and which string will break.
So I am confused.

EDIT: I have uploaded the same clip with a different format so you may try this one if the first one doesn't play.

Last edited by a moderator: Sep 25, 2014
3. Aug 10, 2009

### Hootenanny

Staff Emeritus
HINT: Does he exactly repeat the experiment three times?

4. Aug 10, 2009

### Pupil

You left out a detail. Look at how quickly the professor pulls the string the first time and second time. How do they differ? Could that difference matter?

5. Aug 10, 2009

Look at the speeds with which he pulls the strings.It's the old pulling the tablecloth away from the set table trick.

You just beat me to it pupil

6. Aug 10, 2009

### skydiver

Yes, thank you.
I did notice that the difference in his pulling techniques.
But I am worried - what if this question is asked in an exam. What will the correct answer be?
I am assuming the top one in this case as T2 is increased it is added onto T1.

7. Aug 10, 2009

### <---

Unless you are given the applied acceleration there is no right answer.

8. Aug 10, 2009

### PhanthomJay

This problem must consider impulsive forces and the fact that the strings are not ideal inextensible strings, but rather, they must deform (stretch) under load. When the lower string is pulled slowly at constant speed, Newton 1 applies, and the lower string tension will be the pulling force, T, wheras the upper string tension will be the pulling force PLUS the objects weight (T + mg), therefore, the upper string, seeing more load, must break first.
Now when the lower string is pulled abruptly (very quickly), with enough force to break the lower string, the force is impulsive in nature, that is, it acts over a very very short time period, and stretches the near massless lower string to its breaking point. But the upper string never sees this high force, because since the displacemnt acts over such a short time period, its stretch is very small (s =1/2 at^2, where t is miniscule, thus s is miniscule). This small displacement of the upper string is not enough to stretch it to its breaking point.