# A simple tension question.

1. Oct 7, 2006

### dEEP6ix

Is tension an external force or a reaction force? Is there an exact definition for tension? Which of the following statement is correct? A tension on a spring(horizontal) with one end hooked to a wall produce an reaction force that is equal to the magnitude but opposite to that of the tension OR An external pulling force on the spring produce a tension in the spring that is equal to the magnitude but opposite to that of the pulling force.

Last edited: Oct 7, 2006
2. Oct 7, 2006

### OlderDan

Tension is the name given to the forces acting within an object tending to pull the object apart. Fundamentally, these forces are related to the electomagnetic forces that hold molecules together. We don't usually care about the fundamental nature of these forces, but we do care about the macroscopic effects on objects. Whether any force is an action or reaction force depends on your point of view. Whether it is internal or external depends on how you define the boundaries of your system.

At any point in a string (or rope , rod, or what have you), if there are forces acting at the ends (or anywhere else along the length) tending to move the string in different directions the string will be under tension. If the mass of the string is negligible, the force acting in one direction on some bit of string will be exactly countered by a force acting in the opposite direction. If the mass is not negligible, the force on one side of a bit of the string may be greater than the other side causing that bit to accelerate. In the first case, the tension in the string is uniform. In the second case, the tension varies along the length of the string.

Every bit of the string exerts a force on an adjacent bit that is equal and opposite to the force the second bit exerts on the first. This is an action-reaction pair. It makes no difference which force is considered the action and which is the reaction. These forces always come in pairs; you cannot have one without the other. If our "system" is defined to be these two bits of string, then these are internal forces; the forces on the outside of these two bits would then be external forces If the system is expanded to include all the bits of the string, then all the forces the bits exert on one another are internal forces. The force applied by the object that is connected to the end of the string that is pulling on that very last bit of string would be an external force.