Activity 1 — the door
Measuring the force needed to open a door at four different places is enough to find out what turning really depends on.
Activity 1
You will need: a door fitted to its frame on hinges, a Newton balance, a rubber sucker.
- Mark four points A, B, C and D on the door, all at the same height. Choose A very close to the hinges and D as far from them as the door allows.
- Using a rubber sucker, attach the Newton balance at point A.
- Pull the balance, keeping the force perpendicular to the door. Read the balance at the moment the door just begins to turn.
- Take readings the same way at B, C and D, and record them in table 11.1.
Figure to be added
Table 11.1
| Point the hook was fixed at | Perpendicular distance from the hinge axis | Reading as the door starts to move | Force × perpendicular distance (N m) |
|---|---|---|---|
| A | Smallest | Largest | — |
| B | ↑ | ↓ | — |
| C | ↑ | ↓ | — |
| D | Largest | Smallest | — |
The last column is empty for now. What belongs in it becomes clear two pages from here.
Observation
At A, nearest the hinges, it takes the largest force to start the door turning. At D, furthest from the hinges, the smallest force is enough.
This is something you already knew — it is exactly why the handle of a door is always fitted at the edge furthest from the hinges. What the activity does is turn that habit into readings you have measured.
Why keep the balance perpendicular?
Pull at a slant and part of that pull runs along the door instead of across it, and that part does no turning at all. Keeping the balance perpendicular is what guarantees the force you read is the whole of the force doing the turning.