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Lesson: Chapter 12 — Equilibrium of Forces

12.2. Equilibrium under two forces 2 of 6

Activity 1 — the ring

With a ring and two spring balances, what equilibrium under two forces requires can be measured.

Activity 1

You will need: a ring, two spring balances.

  1. Lay the ring flat on a table and pull it in opposite directions with the two spring balances.
  2. By changing how hard each balance pulls, apply forces of various magnitudes to the ring. In every case the ring must be kept still.
  3. Read and compare the two balances in each case.

Figure to be added

Figure 12.4 — testing the equilibrium of a body under two opposing forces: a ring pulled either way by two spring balances.

Observation

In every case where the ring stays in equilibrium the two readings are equal. That is, in equilibrium the two forces on the ring are equal in magnitude.

Now try it with the balances out of line

Try to hold the ring in equilibrium with the two spring balances not lying along one line. You will find it cannot be done.

The second thing the activity shows

In every case where the ring is in equilibrium the two forces must be collinear, and they must act in directions opposite to each other.