What is the Tension? Tension is a force that exists in wires, ropes, or other types of connecting materials. For the cases considered at the introductory level and in this tutorial, we consider undiminished tensions because we are assuming MASSLESS ropes and wires.

The easiest way to think about tension is to consider holding a weight on a string.(like in the diagram) I exert a force to hold the rope and weight in the air.
Now, I am in contact with only the top of the rope, so that piece (labelled 1 in the figure below) must exert a force upward on the piece just below it (2) while (2) exerts a force on (1) (see Newton's Third Law ). Now, for (3), there is force provided by (2) pulling it up, as well as (4) pulling down on it. (4) has a force because the (3) pulls up on it and (5) pulls down on it ... and so on and so on until we reach the last piece of the rope (8). It has no rope to pull up on, but it is in contact with the mass!
Hence, it must be exerting a force upward on the mass equal to the force I apply to the top of the rope.


This value is what we call the tension in the rope. The tension is also the force that the rope pulls on me, by Newton's 3rd Law. So, a free body diagram of my hand and the mass reveal the following:

If one thinks about it, it does make some sense. I am exerting a force on the mass THROUGH the rope. By Newton's Third Law, the mass must exert a force on me. The only way it can do that is to to exert it through the rope.
To review: TENSION