What Causes the String to Break in Different Physics Demonstrations?

In summary: Doc Al says "has it anything to do with the speed he is pulling with?" and I was wondering if that was the answer too. In summary, the answer involves inertia.
  • #1
danielI
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At the end of this lesson. http://mfile.akamai.com/7870/rm/mit...870/8/8.01/f99/videolectures/wl99lec6-300k.rm
Why does the lower string break, and then the upper and finally the lower one again?
Has it anything to do with the speed he is pulling with?

Look at the picture below. The acceleration of the following systems are different (1 =1.4 m/s^2 and 2 = 3.27m/s^2). Is this any related to the result with the string?
Also, why does it give different answers for the two systems in the picture?

Cheers
 

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  • #2
I am unable to view the lesson in the link you provided, so I can't comment on it. (Why not just describe it?)

danielI said:
Also, why does it give different answers for the two systems in the picture?
The string tension is different for each system.

In the left picture, a 200 kg mass hangs from the string. What's the tension in the string? (Hint: It's not equal to the weight of the mass!)
 
  • #3
Great video lecture.
Anyway, the answer involves inertia.

When pulled fast, the lower string breaks because it cannot move the 2kg mass downwards as fast as the downward extension of the string. Thus, something must "give", in this case the string.

However, when pulled slowly, the 2kg mass is pulled about as fast as the lower string is being pulled, but the top string is being critically extended.
 
  • #4
Doc Al, My first theory regarding the picture was that 1 will accelerate but not system 2 since the 150kg cylinder is being pulled with a constan force. But this is wrong.

I know how to calculate system 1.

T - 150*9,81 = 150a
200*9,81 - T = 200a
+
=> a = 1.401m/s^2 => T = 1681,65N

And the second one ... Oh, I just came up with the answer as I speak (write).:rofl:
T = 200*9,81.

pallidin,
My first guess was that when pulling fast his hand is accelerating, while it isn't (almost) accelerating when pulling the thing slowly. So, am I correct to say that if he accelerates faster that g the lower one will break (because of inertia). Did I understand you?

Cheers!
 
  • #5
danielI said:
Doc Al, My first theory regarding the picture was that 1 will accelerate but not system 2 since the 150kg cylinder is being pulled with a constan force. But this is wrong.
Why in the world would you think that just because the 150 kg mass is being pulled with a constant force that it wouldn't accelerate? (By the way, in both cases the 150 kg mass is being pulled with a constant force. :wink: )

I know how to calculate system 1.

T - 150*9,81 = 150a
200*9,81 - T = 200a
+
=> a = 1.401m/s^2 => T = 1681,65N

And the second one ... Oh, I just came up with the answer as I speak (write).:rofl:
T = 200*9,81.
Looks like you figured it out.

pallidin,
My first guess was that when pulling fast his hand is accelerating, while it isn't (almost) accelerating when pulling the thing slowly. So, am I correct to say that if he accelerates faster that g the lower one will break (because of inertia). Did I understand you?
The acceleration due to gravity has nothing to do with this.

By pallidin's description, I understand that this is the standard demo of a mass hanging from a string with a second string hanging down from the mass. Here's another way to look at it. (Just another way of saying what pallidin already explained.) When you pull the string slowly, the mass has plenty of time to move and stretch the top string: the tension in the top string equals the tension in the bottom string plus the weight of the mass. So the top string reaches its breaking point first.

But when you pull the string sharply, it will stretch to its breaking point long before the mass can move enough to stretch the top string (due to the mass's inertia).
 
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  • #6
The reason I thought so was because the 200kg mass accelerates and so would the 150kg mass.
While the 200*9,81 force in system 2 doesn't do that. Ofcourse there is some logical error with my reasoning.

Also, here are all the videos.
http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/index.htm

The lectures are good for self-study and fun to watch. Like at the end of lecture 8 (Friction) :rofl:
 
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1. Why does the string break when I play my instrument?

The most common reason for a string to break on a musical instrument is due to wear and tear. As strings are constantly being stretched and plucked, they can become weak and eventually snap. Another reason could be improper installation or tuning, resulting in too much tension on the string.

2. Can weather or temperature affect the strength of the string?

Yes, weather and temperature can play a role in the breakage of strings. Extreme heat or cold can cause the strings to expand or contract, putting extra strain on them. High humidity can also cause the strings to corrode and weaken over time.

3. How often should I replace my strings to prevent them from breaking?

The frequency of string replacement depends on how often you play your instrument and the type of strings being used. Generally, it is recommended to change strings every 3-6 months for optimal sound and to prevent breakage. However, if you notice any signs of wear or damage, it is best to replace the string immediately.

4. Are there any techniques I can use to prevent strings from breaking?

Yes, there are a few techniques that can help prevent strings from breaking. First, make sure to properly install and tune the strings to avoid too much tension. Also, avoid excessive bending or overstretching of the strings while playing. Additionally, regularly cleaning and maintaining the strings can help prolong their lifespan.

5. Is it normal for strings to break frequently?

No, it is not normal for strings to break frequently. While occasional breakage is expected due to wear and tear, frequent breakage can indicate an underlying issue with the instrument or the way it is being played. If strings continue to break frequently, it is best to consult a professional for further guidance.

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