Tension in an inextensible string

mufc802

mufc802

mufc802

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I'm a bit confused about what direction the tension supposed to be.

2nrihrn.jpg

This is showing a vehicle accelerating. (Ignore the slope)

ogjg3a.jpg

This is showing a vehicle de accelerating.

Can someone explain; how come when you de accelerate, the tension direction is the opposite?
 
Its action and reaction, when the car is decelerating the reaction to this is for the tension to be applied in the opposite direction. Think of it as a metal bar, when you decelerate the force is going to be acting on the bar to attempt to compress it between the car (which is slowing) and the trailer (which is still going at the same speed). Therefore the tension must act in the oposite way to oppose the compression.
 
Dogbreath said:
"Tension" in this case is simply force.

F=ma

If the sign of 'a' changes and the mass stays constant, then F will change sign.
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Ah, so when the acceleration is negative, the force must also be negative meaning the direction is opposite.

There are a lot of questions asking me to work out the tension in the string, I just need to remember it is a force pulling away from the mass if it's accelerating.
 
mufc802 said:
Thanks, it makes more sense when I think of it as a metal bar. So is tension the opposite of compression?
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Tension is the force applied to an object once it has reached its extendable limit. By this I mean, if you have a piece of string tied to a wall and coiled up, and you pull on it, then it will extend to its limits with the force, so it is taught - then any additional force will become tension within the string.

It is slightly different in these questions as the string is always considered to be taught and thus the force is the tension within the string. I always found it easier to think of it as a metal bar which you can then think about how the forces would affect this as the car accelerates/brakes etc.

[Edit] I would bet that the questions also state that the string is inextensible - this ensures there is no 'springyness' in the string (which would then be moving into basic physics with Hooke's law and elastic and plastic regimes).

[Edit2] Just realised thats in the title - knew I had read it somewhere!
 
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I recognise the second question, m1 edexcel?
Think of it as a metal bar, when decelerating the trailer will be pushing forwards on the car in a compressive manner.
 
Dr Delphi said:
Tension is the force applied to an object once it has reached its extendable limit. By this I mean, if you have a piece of string tied to a wall and coiled up, and you pull on it, then it will extend to its limits with the force, so it is taught - then any additional force will become tension within the string.

It is slightly different in these questions as the string is always considered to be taught and thus the force is the tension within the string. I always found it easier to think of it as a metal bar which you can then think about how the forces would affect this as the car accelerates/brakes etc.

[Edit] I would bet that the questions also state that the string is inextensible - this ensures there is no 'springyness' in the string (which would then be moving into basic physics with Hooke's law and elastic and plastic regimes).

[Edit2] Just realised thats in the title - knew I had read it somewhere!
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You mean taut? I noticed this word in the book and didn't know the definition so that makes sense now. The strings are always fully extended any any additional force means there is always tension.
rogan said:
i think this was my m1 exam....
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Yes it is from the Edexcel book.

If you already took the exam, would you say this is the hardest section to learn compared to vectors, statics and moments which I haven't started yet?
 
mufc802 said:
You mean taut? I noticed this word in the book and didn't know the definition so that makes sense now. The strings are always fully extended any any additional force means there is always tension.
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Ahh yes I was writing it and thought it didn't read correctly - my mind had gone blank :p
 
mufc802 said:
You mean taut? I noticed this word in the book and didn't know the definition so that makes sense now. The strings are always fully extended any any additional force means there is always tension.

Yes it is from the Edexcel book.

If you already took the exam, would you say this is the hardest section to learn compared to vectors, statics and moments which I haven't started yet?
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If you understand vectors, then moments are a easy concept.
I did this exam, but what the hell is a static?
 
Judgeneo said:
If you understand vectors, then moments are a easy concept.
I did this exam, but what the hell is a static?
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Statics of a particle so the particle is in equilibrium all the time. I guess you have to calculate the force to keep it in equilibrium. I haven't got a clue what a moment is but I've had a look over vectors so hopefully I will be okay.:)
 
mufc802 said:
Statics of a particle so the particle is in equilibrium all the time. I guess you have to calculate the force to keep it in equilibrium. I haven't got a clue what a moment is but I've had a look over vectors so hopefully I will be okay.:)
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If you have any problems with either of those feel free to send me a trust/msn/skype - I'm currently tutoring A-Level maths students, and M1 is a forte of mine since its basically physics.
 
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