Tension in a Cord

• The force exerted by the end of a taut cord, string, or wire connected to a body is called the Tension. The direction of the tension-force is exerted along the direction of the cord. The magnitude of the tension is equal to the force that one would measure if the cord were cut and a force spring-scale were inserted.
  

Tension on a Spring-Scale QT Movie (660K)
Two cords attached to the ends of a spring scale. The other ends of the cords pass over two pulleys and are connected to two 1 kg weights so that the scale is suspended horizontally and the weights hang vertically. What does the scale read? Shown is physical demonstration of the answer.

• It is normally assumed that the cord is both massless and stretchless. Under these conditions the tension is transmitted through the cord unabated. This means that the forces at both ends of a cord have the same in magnitude even if the cord changes direction through a pulley.

• At any moment the velocity and acceleration of two moving objects will be the same provided they are connected by a massless, stretchless, taut-cord.
  
  
• If the cord was not light, then the forces at both ends would have to be different. If the system is at rest (or moving at a constant speed) then the difference between the two end forces would have to be equal to the force needed to suspend the cord weight of the coard. If the system was accelerating, then the two end force would have to be different to produce a non-zero net force on the cord.

Blocks Joined by a Rope
Two blocks are connected by an inflexible cord. The blocks rest on a frictionless, horizontal surface. A constant horizontal force is applied to one of the block but not the other. The direction and magnitude of the applied force can be

altered to observe the resulting complex motion and interaction between the two blocks.