Question 1 — Moments: finding the weight of an unknown mass
Balance a metre ruler pivoted on a triangular block against an unmarked mass Q, using 2.0 N and 3.0 N reference loads, and use moments to find Q's weight.
RUN IT ON THE BENCH
Your readings feed straight into the answer table below as you record them.
ON YOUR BENCH — FROM THE CONFIDENTIAL INSTRUCTIONS
Question 1 — moments apparatus
- metre ruler(1)
- triangular block(1)
- object Q(1)
- 2.0 N load(1)
- 3.0 N load(1)
THE BENCH — METRE RULER ON A PIVOT · 2.0 N LOAD
Why does y stay the same in both experiments?
y is fixed by where you set up the apparatus — Q at the 90.0 cm mark, the pivot at the 50.0 cm mark — not by anything you measure while balancing. As long as you don't disturb Q between runs, y = 40.0 cm both times, and only x changes.
Why repeat with a 3.0 N load instead of trusting the first result?
A single measurement can't tell you whether it's right — only whether it's repeatable. Swapping to a different known load gives you a second, independent route to the same W. If both agree, you've got real evidence your method works, not just a lucky number.
Why is finding exact balance so difficult?
Near the balance point, a tiny move of the load barely tips the ruler — friction at the pivot and your own judgement both blur "level" into a small range. The way round it: move past the point in each direction to narrow down the range, rather than hunting for one exact position.
YOUR TASKS · 0/3
- ✓Balance against the 2.0 N load and record x
- ✓Balance against the 3.0 N load, record x and calculate W
- ✓Compare your two values of W
YOUR RESULTS
| 2.0 N LOAD | 3.0 N LOAD | |
|---|---|---|
| y /cm | — | — |
| x /cm | — | — |
| W /N | — | — |
W = (x / y) × load, using whichever load you balanced against each time.
YOUR ANSWERS
With the ruler pivoted at the 50.0 cm mark and object Q centred at the 90.0 cm mark, determine the distance y from the 50.0 cm mark to the centre of Q [2]. Then place the 2.0 N load on the ruler, adjust its position until the ruler is as near as possible to balanced, and determine the distance x from the centre of the load to the 50.0 cm mark [1].
| experiment using the 2.0 N load | |
|---|---|
| y: 50.0 cm mark to centre of Q /cm | |
| x: 50.0 cm mark to centre of the 2.0 N load /cm |
Calculate the weight W of object Q, using the equation W = (x / y) × 2.0 N. Give your answer to a suitable number of significant figures for this experiment.
Remove the 2.0 N load without moving object Q. Repeat the balancing procedure with the 3.0 N load and record y, x and W = (x / y) × 3.0 N.
| experiment using the 3.0 N load | |
|---|---|
| y: 50.0 cm mark to centre of Q /cm | |
| x: 50.0 cm mark to centre of the 3.0 N load /cm | |
| W /N |
State and explain whether your two values of W are equal within the limits of experimental accuracy. Refer to the values of W in your answer.
Explain how you ensure that the centre of object Q is directly over the 90.0 cm mark of the metre ruler. You may draw a diagram.
It is difficult to find the position of the load to obtain the exact balance of the metre ruler. Explain how you try to overcome this difficulty.