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What exactly is tension force and how does it act?

When I saw a video on Khan Academy it said that tension force is just a force by which a force is transmitted across a rope, but when I was solving related problems, it was that the tension force is acting upwards on the body which is attached to a rope.

Also, how can a tension force be acting upwards on the lower part of the string and downwards on the lower part of the string? In wave motion, I saw that tension is perpendicular to the string and that is what causes it to stretch.

What is tension force and why does it have the directions that I mentioned and why? And how can I find its directions on bodies and in the string?

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  • $\begingroup$ "tension is perpendicular to the string" Do you mean parallel to the string? Also, you say "lower part of the string twice. I believe you need to edit your question. $\endgroup$ – Bill N Jul 24 '20 at 4:22
  • $\begingroup$ Did you ever play the game called “tug of war “ with a rope? $\endgroup$ – Bob D Jul 24 '20 at 4:31
  • $\begingroup$ @BobD another analogy is to imagine a chain of people linking hands. The people at the ends of the chain pull away. What does it feel like to the people in the middle? $\endgroup$ – StayOnTarget Jul 24 '20 at 14:38
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What we call a tension force is actually the net result of the electromagnetic forces which hold the rope together as something tries to pull it apart. At every small cross section of the rope there are forces acting perpendicular to that cross sectional area, and hence act (generally) parallel to the length of the rope. If the rope is not stretching and the weight component of the rope acting parallel to the length is small compared to the forces acting on the ends of the rope, the tension will be (nearly) constant.

And the direction of the forces which the rope exerts on things is always pulling. So, if a mass is hanging vertically in equilibrium from a single rope which is attached to some ceiling point, and its weight is large compared to the weight of the rope, the tension force which the rope exerts on the mass is up (pulling) and is equal in magnitude to the weight of the mass. The rope is also pulling down on the ceiling point with the same magnitude force, the tension.

If the rope/mass structure is accelerating upward, the tension force has a magnitude greater than the weight of the mass and is upward. If the structure is accelerating downward, the tension force has a magnitude less than the weight, but its direction is still upward because ropes ALWAYS pull.

One way (but not the only way) to think about it is tension is the magnitude with which a rope or cable can pull at any point along the length of the rope. Wherever that point is, the direction of the tension force pulls away from that point. That means that each point in the structure of the rope is in equilibrium, so it doesn't accelerate relative to other parts of the rope.

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    $\begingroup$ A string can only have a tension that is pulling. If you replace the string by a rod or a spring, however, you can have negative tension where the rod/spring is pushing on the objects at its ends. $\endgroup$ – cmaster - reinstate monica Jul 24 '20 at 13:44
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    $\begingroup$ @cmaster-reinstatemonica Technically that's not called tension. It's called compression. And with rods and sticks in compression, the force exerted isn't necessarily parallel to the length. The compression force can have a perpendicular component. $\endgroup$ – Bill N Jul 24 '20 at 14:49
  • $\begingroup$ Nevertheless, tension and compression are the same kind of force, only with different directions. I always find that it helps my thinking to not unnecessarily split the concepts in my head. Yes, I'll call a tension a tension, and a compression a compression in most cases. But I'll not consider a negative compression as a separate case from a positive one. If the object is a string, it can only carry a negative compression, and if the object is two bricks on top of one another, it can only carry a positive one. But a spring or rod can transmit a force in either direction. $\endgroup$ – cmaster - reinstate monica Jul 24 '20 at 15:04
  • $\begingroup$ @cmaster-reinstatemonica Also, a string or rope (of small stiffness) can only have tension parallel to the string. A rod/stick/etc can have tension or compression in both parallel and perpendicular components. The direction is not restricted much by the shape of the object. That's an important distinction between string-like and rigid connectors. $\endgroup$ – Bill N Jul 24 '20 at 15:31
  • $\begingroup$ And how does that conflict with what I said? $\endgroup$ – cmaster - reinstate monica Jul 24 '20 at 15:59
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what exactly is tension force?

Tension force is just a pulling force. It always pull the other object. To be very precise, tension is a scalar quantity. Tension is like what pressure is in 3D.

Why is it just a pulling force?

I am only talking with respect to string. It is only a pulling force as in Newtonian mechanics strings are inextensible and the length of the string is constant.

Suppose if two blocks connected with a string are pushed towards each other. Since the length of the string will be compressed, the tension developed in the string will be 0.

Enter image description here

The net force on the string is 0 as it's mass-less.

Now, if the length of the string is divided into $dx$ elements, each upwards $dx$ element will pull the downward element, and by Newton's third law the upward $dx$ element will be pulled downwards by the lower element.

All the picture credit goes to @satan29.

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  • $\begingroup$ What do you mean by "are in extensible"? $\endgroup$ – Peter Mortensen Jul 24 '20 at 14:57
  • $\begingroup$ @PeterMortensenp i mean that strings can't be stretched as their length is constant. In real ofcourse they can but not in newtonian mechanics(generally). They are unlike springs. $\endgroup$ – Bhavay Jul 24 '20 at 15:01
  • $\begingroup$ In extremely simplified Newtonian mechanics that's taught in school, maybe. There is no reason a string must be inextensible and that tension occurs only in inextensible strings. Can you clarify what you mean by "It is only a pulling force as in Newtonian mechanics strings are inextensible and the length of the string is constant."? $\endgroup$ – Pranav Hosangadi Jul 24 '20 at 15:33
  • $\begingroup$ @PranavHosangadi suppose if tension was a pushing force , then by Newton's 3rd law it will experience a push which will decrease the length of the string (not physically but the distance btw two points which it is attached) and it will become loose/ slack .I also tried to gave an example just below it. In order to string to remain taut it must pull. $\endgroup$ – Bhavay Jul 24 '20 at 15:39
  • $\begingroup$ Tension isn't a pushing force because it's defined as a pulling force. Tension can occur in things other than strings that don't go loose - for example if you bend a rod, one side experiences tension while the other experiences compression. $\endgroup$ – Pranav Hosangadi Jul 24 '20 at 15:42

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