Activity 2 — the force matters
What happens if the perpendicular distance is held constant and only the force is changed?
Activity 2
You will need: a fairly long wooden strip, two rubber washers, a screw with a nut, a hole punch, a Newton balance, and a table or board.
- Punch holes at points O, A, B, C and D, spaced 15 cm apart.
- Using the rubber washers and the screw with its nut, fix the strip to the board at O. The axis of rotation is the straight line running along the shaft of the screw.
- Fit loops made from pieces of wire into the holes at A, B, C and D.
- Attach the balance to the loop at D and, keeping it perpendicular to the strip, measure the least force that just makes the strip turn.
- Now tighten the strip by turning the screw half a turn and measure that force again. Tighten it another half turn and take a third reading.
Figure to be added
Figure to be added
Table 11.2
Suppose the values below were obtained. Depending on the apparatus you assemble, your own readings may differ from these.
| Situation | Force |
|---|---|
| Starting situation | 2 N |
| Screw tightened by half a turn | 5 N |
| Screw tightened by a full turn | 9 N |
Observation
As the strip is tightened step by step, the force needed to turn it increases. The place the force was applied never changed, so the perpendicular distance stayed constant.
If the distance was constant and the force needed grew, then the moment needed grew with it. So the moment of a force depends on the magnitude of the force.
Why tighten the screw at all?
Tightening the screw increases the friction between the strip and the board. That friction acts against the strip turning, so a larger moment is needed to overcome it. What the activity really does is create a situation that demands different forces at one and the same distance.
Only one quantity was allowed to change here. Had the perpendicular distance been altered too, there would be no telling which of the two the changed readings came from.