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Lesson: Chapter 11 — Turning Effect of a Force

11.2. Couple of forces 2 of 7

One balance, then two

A wooden strip and two Newton balances are enough to measure how much more effective two forces are than one.

The demonstration

  1. Punch a hole through the middle of a wooden strip and fix it to a table with a screw. Call its two ends A and B.
  2. First attach a spring balance at A only, and measure the force at the instant the strip just turns. Call it F.
  3. Now attach a spring balance at each of A and B and pull them in opposite directions. Measure the reading on one balance at the instant the strip just turns.

Figure to be added

Figure 11.7 — Demonstrating a couple of forces: in the upper drawing a single balance at A; in the lower one two balances, X at A and Y at B, pulling in opposite directions.

Observation

If turning the strip with one balance took a force F, then when two balances pull in opposite directions the force needed on each one is only F/2.

Why does the force halve?

Each force produces a moment of its own, and both moments act the same way round. If the force at A turns the strip clockwise, then the force at B — pulling the opposite way, on the opposite side of the screw — turns it clockwise too.

Opposite forces, one and the same turning

The two forces are not working against each other — they add together. That is why each of them can be half the size while the total moment stays the same.

So to turn a body about an axis it is easier to apply two equal forces in opposite directions than to apply a single force.

How this particular pair of forces is defined exactly is the next page.