A few questions related to tension in a string

[parshyaa]

Its getting hard for me to deal with the problems related to tension and reason behind this is not clear understanding of the concept so i have few questions on this.

1. What causes tension in a rope/string(i think its related to electromagnetism, but i am not getting the clear idea behind this)
2. how can you explain that tension in a string increases if the mass on the end side also increases
3. Suppose a person is pulling a rope attached to a load , then during the calculations of number of forces acting on it,do i have to take force applied by man along with the tension or only tension(explain it with reason)

Thanks in advance.

 

[Herman Trivilino]

Its getting hard for me to deal with the problems related to tension and reason behind this is not clear understanding of the concept so i have few questions on this.

Can you give us an example of one of these problems?

What causes tension in a rope/string(i think its related to electromagnetism, but i am not getting the clear idea behind this)

What makes you think it's related to electromagnetism? I wouldn't recommend an attempt at answering the other questions until you understand this one.

 

[parshyaa]

Can you give us an example of one of these problems?

continuation of the problem

Solution:

I understood the solution but in this solution,we have to assume that pulling force of his friend is same as tension in the string? Why we assumed so, why didn't we took F as force applied by his friends and T as tension in the string separately, why tension in string has to be equal to force applied by person/human in this problem.
Similar problem comes when question is related to monkey climbing the rope(we assume that his pulling force is same as tension in a string).
This question is from famous indian physics book written by HC verma

What makes you think it's related to electromagnetism?

I think that the surface atoms of rope and block repells each other but applying replusion concept makes me think that then rope has to apply force in downward direction on the body and tension should be in downwards,but this is wrong,
I could not explain why tension increases as mass attached to it increases using E&M(repulsion concept)
i really don't know the answer, i don't know much about electricity and magnetism and quantum mechanics, but i could understand if someone explains it more simply.

 

[jbriggs444]

I think that the surface atoms of rope and block repells each other

Neither pure repulsion nor pure attraction are at work.

https://en.wikipedia.org/wiki/Van_der_Waals_force

 

[pixel]

Suppose a person is pulling a rope attached to a load , then during the calculations of number of forces acting on it,do i have to take force applied by man along with the tension or only tension(explain it with reason)

You might find this video helpful, especially the last example: https://www.brightstorm.com/science/physics/Newtons-laws-of-motion/tension/

 

[parshyaa]

Neither pure repulsion nor pure attraction are at work.

https://en.wikipedia.org/wiki/Van_der_Waals_force

From the above article i understood that van-der-waal forces are distance-dependent interactions between atoms or molecules and sometime force acts as attractive and sometimes it acts as repulsive then according to this, for tension to be upward force must be attractive between surface and rope molecules, if its correct then how does it explains that attractive force will be greater or tension in rope will be greater if the mass is greater.
Also could you explain my problem related to the solution of problem metioned above.

 

[jbriggs444]

From the above article i understood that van-der-waal forces are distance-dependent interactions between atoms or molecules and sometime force acts as attractive and sometimes it acts as repulsive then according to this, for tension to be upward force must be attractive between surface and rope molecules, if its correct then how does it explains that attractive force will be greater or tension in rope will be greater if the mass is greater.

Equilibrium. Stable equilibrium.

If you apply more force, you pull the molecules apart. If the molecules move farther apart, their attraction increases. An equilibrium is achieved with increased tension in a stretched rope.

This is an over-simplification, given that a rope consists of cords which may be braided from smaller strands which are spun from fibers which contain molecular chains which display the above behavior.

Edit: Of course, none of this is in any way relevant to the problem. You can analyze the problem using the properties of ideal massless inextensible ropes. The means by which those properties are achieved do not matter and will not help you solve the problem.

 

[parshyaa]

You might find this video helpful, especially the last example: https://www.brightstorm.com/science/physics/Newtons-laws-of-motion/tension/

Woow man i liked it, "strings can only pull", he was like mazor in military, but he didn't explained why F has to be equal to the tension in the string in his last problem but this video helped me, thanks for wonderfull suggestion.

 

[parshyaa]

Equilibrium. Stable equilibrium.

If you apply more force, you pull the molecules apart. If the molecules move farther apart, their attraction increases. An equilibrium is achieved with increased tension in a stretched rope.

I understood this, thanks

Of course, none of this is in any way relevant to the problem. You can analyze the problem using the properties of ideal massless inextensible ropes. The means by which those properties are achieved do not matter and will not help you solve the problem.

Actually problem was not with the understanding the problem, i understood it but the problem was that: why force applied by man has to be equal to the tension in the string,could you explain it using your above reasoning.

 

[jbriggs444]

If an equilibrium has been achieved then the force applied by the man must be equal to the tension in the string.

This follows because ...

If the force applied by the man is not equal to the tension in the string then the end of the string will be subject to an unbalanced net force and will accelerate. If the end of the string accelerates then it is not in equilibrium.

All that remains is the question of whether an equilibrium is achievable. As long as the exerted force is less than the breaking strength of the rope, experience shows that it is.

 

[parshyaa]

If an equilibrium has been achieved then the force applied by the man must be equal to the tension in the string.

This follows because ...

If the force applied by the man is not equal to the tension in the string then the end of the string will be subject to an unbalanced net force and will accelerate. If the end of the string accelerates then it is not in equilibrium.

All that remains is the question of whether an equilibrium is achievable. As long as the exerted force is less than the breaking strength of the rope, experience shows that it is.

This explanation looks righ and it satisfies me but can you provide me the link or any source where this thing is mentioned about the equilibrium.

 

[Herman Trivilino]

Suppose a person is pulling a rope attached to a load , then during the calculations of number of forces acting on it,do i have to take force applied by man along with the tension or only tension

I suggest you draw three well-separated free-body diagrams; one for the man, one for the rope, and one for the load. Use Newton's Third Law to recognize that there are equal-but-opposite pairs of forces involved. For example, the force that the man exerts on the rope is equal but opposite to the force that the rope exerts on the man. Likewise, the force that the load exerts on the rope is equal but opposite to the force that the rope exerts on the load. The man doesn't exert a force on the load and the load doesn't exert a force on the man because the man is not in contact with the load.

It is only by going through an exercise like this that you can understand the role played by the tension in the rope. Especially illuminating is the free-body diagram of the rope, and an understanding of the role the rope's mass plays in the analysis.

 

[jbriggs444]

This explanation looks righ and it satisfies me but can you provide me the link or any source where this thing is mentioned about the equilibrium.

I am not aware of any published source.

This sort of thing is an obvious bit of reasoning that goes into the static behavior of essentially every simple mechanical device that one can think of. It is the same explanation for the why the force your shoes exert on the floor is equal to your weight. Or the fact that the torque a wrench applies to a bolt is the same as the torque that the mechanic applies to the wrench.

 

[Herman Trivilino]

I am not aware of any published source.

This sort of thing is an obvious bit of reasoning that goes into the static behavior of essentially every simple mechanical device that one can think of. It is the same explanation for the why the force your shoes exert on the floor is equal to your weight. Or the fact that the torque a wrench applies to a bolt is the same as the torque that the mechanic applies to the wrench.

Well, that's covered in virtually every calculus-based introductory physics textbook, but any textbook used for the introductory Statics course that engineering students are required to take will have it stated and explained quite prominently.

 

[Nik_2213]

"strings can only pull"

Um, in real world, you must often contend with 'lain' cordage twisting, too, and such torque changing with tension. Hence use of braided paracord, mono-fil, or a braided sheath over twisted core. And don't forget elasticity, hysteresis, pre-stretching etc etc etc.

IIRC, the first Apollo astronauts took a 'trick' rope along, which could be twisted unto rigidity. The worry was that, despite all US' & USSR's un-manned landers' reports to the contrary, the Lunar surface might yet have *some* 'dust-pools', requiring self-rescue equipment. In fact, the endless shower of micro-, nano- & pico-meteorites had sintered the regolith 'really, really hard'...

FWIW, there's an old, old puzzle solved by wetting then freezing a length of kitchen string, which may then be *pushed*...