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Lesson: Chapter 12 — Equilibrium of Forces

12.1. Introducing equilibrium of forces 3 of 4

Weight and tension

Take a sphere hanging from a strong thread. The weight of the sphere acts vertically downward. What balances it is the tension acting upward along the thread.

Figure to be added

Figure 12.3 — a sphere hanging from a thread: the tension (T) upward along the thread, the weight (W) vertically downward.

Tension

The force acting along a thread that is under a pull is called the tension in that thread.

T = W

Only while the sphere is in equilibrium.

Tension pulls, it never pushes

All a thread, a string or a rope can do is pull. It cannot push an object. So tension always acts along the thread, from the object towards the point it hangs from.

Weight and tension are not the same thing

The two are equal only while the object is in equilibrium. If the sphere is being hauled upward the tension is greater than the weight; cut the thread and the tension becomes zero while the weight remains exactly what it was.