text: what exactly is tension? well, i have struggled all my student life with this. when a string goes over a pulley, what is the force on the pulley if the tension in the string is t on each side? what is the direction of tension in any system? oh help please... tension in the string is the force acting on the string.  for time being forget about the pulley.  one end of a rope is tied to a pole at the ground. suppose you are pulling the rope from the other end.  you are exerting a force on the rope so as to increase its length.  if the rope is made of rubber you can see the increase in length.  if it is a steel rope, you cannot visualize the increase in length; but still it increases in its length.   if you exert a heavy force the rope breaks into two.  suppose you are not giving such a force. when the force is within a limit, the rope extends to a length which is determined by the applied force.  when the increase in length is stopped, you and the rope are at rest; not moving further.   now consider the rope alone. what are the forces acting on the rope?  two forces are acting on the rope, each at one end of the rope.  since the rope is not moving, by applying the law of forces, we come to a conclusion that these two forces are equal but opposite.  note that if the rope is moving with a varying speed, we cannot come to this conclusion.  the forces in the rope are called tension.  now suppose you know the value of the force, say 10n, that you have applied to the rope. it is obvious that this force which is pulling the rope and hence the value is 10n.  now coming to the pulley, a rope is passing through the pulley and one end carries a load and at the other end you are pulling it.  suppose the load and the end in which you are applying force is at rest ;( not moving)  ii is similar to the above case; instead of the pole to which the rope is tied, here the load is acting as a pole. (the load is also at rest).  since the system as a whole is at rest. the force you are applying must be equal to the load. suppose if the load is 10n, you are applying 10n.   actually you are applying 10 n to one end of the rope. the rope must move with this force; but for the load which is pulling it on the opposite direction. thus in the rope one end is pulled by you and the other end is pulled by the load.  the rope is under the tension of 10n force.  now you know the direction of tension in the rope also.  consider the end of the rope to which load is connected. the load is pulling the rope downward. therefore, the tension in the rope at this end is 10n down ward.  consider the end of the rope to which you are pulling. you are pulling the rope downward. therefore, the tension in the rope at this end is also 10n down ward.  the pulley is acted on by two forces on either side each equal to 10n, totalling 20n.   if there is no force acting upward on the pulley, the pulley will come down due the 20 n forces.   therefore, the pulley is pulled upward by a force of 20n by the one which is connected to the pulley.  if the load is moving with changing speed, the tension in the rope will be different.  if the concept of tension when the rope is at rest is understood, then it will be easy if even if the rope is having some acceleration.