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Lesson: Chapter 11 — Turning Effect of a Force

11.1. Moment of a force 8 of 12

Activity 3 — the distance matters

Now for the other side of it: hold the force constant and change only the perpendicular distance.

Activity 3

  1. Use the same wooden strip as in activity 2. Turn the screw back a full turn to bring it to its original setting.
  2. Close to A, hang the spring balance from a loop made of thread and find the force that just turns the strip. Call it F1; the distance from the axis to that point is x1.
  3. Now tighten the screw by about a quarter turn.
  4. Keeping the balance reading constant at F1, slide the wire loop slowly towards D until the strip just begins to move. At the instant it starts, measure the distance x2 from O to the balance.
  5. Tighten the screw another quarter turn and, still holding the force at F1, obtain x3.

Figure to be added

Figure 11.5 — Testing the effect of the perpendicular distance on the moment: the same force F1 applied at distances x1 and x2.

Table 11.3

Situation F1 Distance to the balance (x)
Starting situation 1.5 N 15 cm
Screw turned a quarter turn 1.5 N 32 cm
Screw turned half a turn 1.5 N 55 cm

Observation

The force column stays unchanged at 1.5 N all the way down. Yet the tighter the strip became, the further out the balance had to be moved to turn it.

So the moment of a force also depends on the perpendicular distance from the axis.

What the two activities settle

Activity 2 changed only the force. Activity 3 changed only the distance. Each of them changed the moment — so the moment is a function of both, not of either one alone.

The product is confirmed

moment = force × perpendicular distance is no longer a guess. Both of its factors have been varied separately, and measured readings show that each of them changes the moment.