# Which string cutten? (impulse)

1. Jul 4, 2012

### persia7

suppose a metal ball is hanging by a string and there is another string is hanging at buttom of the ball if you pull buttom string very fast which of strings (up and buttom) will be cutten ?

2. Jul 4, 2012

### Danger

There are too many undefined parameters in your question. Method of attachment is one example thereof. I admit that I don't really have a clue about this, but it seems to me that even the lengths of the string segments would be relevant. (The longer the string is, the more likely it is to break.)

3. Jul 4, 2012

### persia7

i know the answer, but i dont why. if you pull very fast(impulse) the buttom string cutten and if you pull slowly(static) the upper cut, but why?

4. Jul 4, 2012

### Danger

Have you repeated this experiment often enough to be sure that the results are guaranteed? If so, will you please describe the set-up in more detail? I'm not disputing your conclusion; I just don't know exactly how you arrived at it.

5. Jul 4, 2012

### Staff: Mentor

It is reasonable to assume the strings are sound and fit for purpose, are securely attached, and are of equal breaking strain.

It is a very good demonstration, persia7.

6. Jul 4, 2012

### CWatters

Initially the stress & strain is higher in the upper string as there is no load on the lower one.

If an impulse load is applied to the lower string then the inertia of the metal ball causes a delay between the rising stress in the lower string and rising stress in the upper string. If the impulse is short but large enough the strain in the bottom string will reach breaking point before the top. So lower string breakes first.

If the load is applied slowly then the strain increases at the same _rate_ in both strings BUT the top string had a head start so it reaches breaking point first. Upper string breakes first.

7. Jul 4, 2012

### persia7

i agree with CWatters , but can you explain with formula what happen when impulse load applied on lower string?

8. Jul 4, 2012

### DaveC426913

For a brief but strong tug, the ball might as well be a brick wall. You yank on a string that's attached to a brick wall, it's going to break. All this happens before the ball's momentum can be overcome to affect the top string.

9. Jul 4, 2012

### persia7

please explain with momentum or ... formula(mathematical)

10. Jul 4, 2012

### sophiecentaur

The length and modulus of the strings (identical material, I assume) will affect the result, as well as the 'speed' and force applied. A helpful model would be to treat the strings as springs and the Spring - mass - spring - support would have different natural modes of oscillation. I don't think the calculations would be that hard to do (you'd need a bit of motivation). First write an equation of motion and solve it for the conditions. In the end, it's coupled oscillators. For a slowly applied force, the result will always be that the top string will break, of course.

11. Jul 5, 2012

### persia7

can you tell me a book help me?

12. Jul 5, 2012

### CWatters

Better still watch the movie..

Last edited by a moderator: Sep 25, 2014