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Lesson: Chapter 12 — Equilibrium of Forces

12.3. Equilibrium under three parallel forces 3 of 5

Activity 3 — the metre rule

With a metre rule and two spring balances, the relationship between three parallel forces can be shown in numbers.

Activity 3

You will need: two spring balances, a metre rule, two rubber bands.

  1. Weigh the metre rule.
  2. Hang it from the two spring balances by its two ends and keep it horizontal and in equilibrium. Then take the readings of the two balances.
  3. Look for the relationship between the two readings and the weight of the metre rule.

Figure to be added

Figure 12.10 — a metre rule in equilibrium under three coplanar parallel forces: the two Newton balances F1 and F2 hanging from rubber bands at its ends, and the weight W of the rule downward.

Observation

The sum of the two balance readings is equal to the weight of the metre rule.

F1 + F2 = W

Now take the balances out of the plane

Keeping the rule horizontal, see whether equilibrium can be maintained while pushing the two ends in opposite directions so that the two balances no longer lie in the same plane as the rule.

What you find

In every such case the rule comes back into equilibrium only when the rule and the two spring balances are once again coplanar.