Tension, for me, is a tricky thing.

After finishing a related chapter of my book and watching a video, I still can't get a hang of it.

Here is a situation: enter image description here

My knowledge is that tension, just like normal force, happens in just the same way, but with the difference of a string attached.

It is understandable that in the picture, the string attached to the hanging mass has a tension directed upward, because it's weight is directed downward. (Action and reaction)

But what intrigues me is that applying the same logic, the horizontal mass is being pulled to the right, it would have been understandable if the tension is directed to the left.

also Can I think of tension as the reaction force when you pull, and normal force when you push?

update If I was to think tension as a reaction force in the first picture for the horizontal mass, then can the frictional force be the action force? (Ehh, I don't think that makes sentence. Friction is always reaction)

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    $\begingroup$ Here's an easy way to get it right, without worrying about "why": a string can only pull. If you try to push with a string, it will just fold up. $\endgroup$ – mbeckish Jul 22 '15 at 13:54
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    $\begingroup$ Because the string "pulls" upward - if the force were pointing down, the string would be "pushing" the block. $\endgroup$ – mbeckish Jul 22 '15 at 18:00
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    $\begingroup$ Try it yourself - tie a string to a block, and grab the other end of the string with your hand. Now, try to use the string to move the block away from your hand. You can't, because the string goes limp. Now try to use the string to move the block towards your hand. $\endgroup$ – mbeckish Jul 22 '15 at 18:02
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    $\begingroup$ No, the string never pushes the mass. $\endgroup$ – mbeckish Jul 22 '15 at 18:07
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    $\begingroup$ Tension does not have a single direction. It is either tensile or compressive. $\endgroup$ – ja72 Jul 22 '15 at 18:31

We treat the string/rope like another object. This object exerts forces on other objects such as the hanging mass (in your picture). However, a string, by its very nature, can never "push" another object, it can only "pull" another object. That "pull" is a force which we give the name tension.

Thus, tension will point away from the mass in the direction of the string. In the case of the hanging mass, the string pulls it up, so the string exerts an upward force on the mass, and the tension will be upwards. In the case of the mass on the table, the string pulls it to the right, so the tension will be to the right.

So, for example, suppose you have a rope attached to a mass on a friction-less table, and you pull the rope to the right with a constant force of 1 Newton. The rope will then pull on the mass with a force of 1 Newton to the right, and the mass will start to accelerate to the right. Here, the tension, (i.e. the force with which the rope pulls on the mass) is to the right with a magnitude of 1 Newton. So, you created tension by pulling on the rope; a tension of 1 Newton.

I don't quite understand why you keep referring to the normal force. A normal force is a force perpendicular to a plane of mass; the mass we refer to here we consider to be an ideal case of a point mass, so there is no normal force. Also, the rope can be pulled in any direction, not just the direction perpendicular to the plane of the mass.

Tension is created whenever a rope exerts a force (pull) on another object. Notice that there was no friction in the example above. You don't need friction to have tension; this experiment could have been done in space.

  • $\begingroup$ I am confused as to tension is. Do you creat tension by pulling on a string and so there is a normal force, or frictional force? $\endgroup$ – most venerable sir Jul 22 '15 at 12:29
  • $\begingroup$ I am also confused as to what exactly those arrows should mean. Is it the direction of pulling. But how do you know which is pulling which? $\endgroup$ – most venerable sir Jul 22 '15 at 12:32
  • $\begingroup$ @Doeser Tension is the force (the pull) the string exerts on the mass (like I wrote in the answer). $\endgroup$ – user35687 Jul 22 '15 at 13:52
  • $\begingroup$ @Doeser I edited my answer; I hope it's clearer. $\endgroup$ – user35687 Jul 22 '15 at 14:15

Tension is an internal force in a body, such as a rope, that resists any attempt to pull the rope apart. Simply, tension arises due to intermolecular interactions, and if it did not exist, ropes would fall apart the moment you pull on them.

Now, it is necessary to distinguish between internal and external forces for a body. External forces are forces that act on the body due to other bodies, such as friction and gravity. When you look at the body as a whole, it is easy to see the effect of the force on the body. External forces allow the bulk of the body to accelerate, provided no other external forces cancel them out (equilibrium). For example: a body acted upon by the Earth's gravity:

enter image description here

Internal forces of a body are different. Internal forces exist inside a body, and the effects of internal forces cannot directly interact with anything external to the body. In other words, internal forces are forces that one part of a body causes onto another part of the same body (see later). Hence it is not as clear to see the "direction" of such forces. Internal forces do not lead to bulk acceleration, but instead cause body deformation (e.g. stretching a spring, or bending a ruler). Here is a diagram showing the internal forces acting on a block:

enter image description here

Well, you can't see anything obvious on the outside, but that not because internal force don't exist. It's because if you were to take all of the internal forces present in a body, they would sum to zero. This is a result of Newton's 3rd Law. Instead, a better way to visualise internal forces in a body is to make an imaginary cut through the body and see how the forces act on the cut face. This is better demonstrated below in the diagram, where a rope is being pulled apart:

enter image description here

By considering an imaginary cut, you look at the two halves and enforce equilibrium (if the whole rope obeys equilibrium, so must any arbitrary sub-length of rope). By doing so, you find out that in order to satisfy equilibrium, one half must exert a force on the other, and vice versa by Newton's 3rd Law. These particular forces are internal since they're forces caused by one part of the rope acting on another part of the same rope. These internal forces act at the interfaces of the imaginary cut, and, in this case, are known as the tension. To be more precise, it is the tension at the location on the rope where the imaginary cut is made. Note that you can determine the tension's direction if you look at one half of the rope, but since the tensions occur in pair, there is no obvious direction of tension at that point of the rope for the whole rope.

Looking at your example, let's make a few cuts to see the tension forces in the rope:

enter image description here

Only the forces acting on/in the rope, on the box or on the hanging mass are included in the diagram above.

In short, internal forces like tension at a particular point in a rope doesn't really have a clear cut direction, as they occur in pairs.

  • $\begingroup$ This is clearly a better answer than mine. Feel free to rewrite with any components of mine you like; I'll eventually delete mine. $\endgroup$ – Daniel Griscom Jul 22 '15 at 23:23
  • $\begingroup$ @SprocketsAreNotGears Do you have some sort of intuitive explanation how you calculated the force that the pulley exerts on the rope? I can't seem to figure that out... $\endgroup$ – Dude156 Jun 30 at 4:08

The tension on a string between two objects (note: your top diagram shows t̲h̲r̲e̲e̲ objects) is analogous to the force between two objects elastically colliding. The force exerted by the one end of the string is opposite and equal to the force exerted by the other end of the string; both forces must be parallel to the string and pointed towards its center.

However, do not confuse the string tension with a force. Even though it will have the same magnitude as the force on either end of the string, and is oriented along the length of the string, tension does NOT have a direction. You can't say "The tension is headed to the left" and be making any sense.


Tension is just a pulling force.

As we all know that a rope can't push. It exerts a force on objects like a contact force. Which is pulling force known as tension, comes into action when something is stretched to regain its position, like a restoring force.


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