What are some limitations and fixes for a Physics ISA experiment on moments?

[ViralRiver]

We need to think of limitations regarding an experiment we did as a practice Physics ISA regarding moments, and how to overcome the limitations. I can only think of a couple limitations, but we need 5, any ideas?

Experiment: Using a 1m rule attached to a clamp by a nail (nail is a pivot) and a set of unknown masses taped together, you need to balance the rule in equilibrium. The taped masses are added to the rule by a string (it goes around the rule like a lasso. The distance had to be changed, while keeping the rule in equilibrium.

Ok, the experiment was done a long time ago so that's not exactly how it was, but it's close enough. We did this to work out the value of g (accel. of free fall). Here are some limitations with 'fixes' I have thought of:

1). You can't accurately position the masses as the string blocks the view of the markings (better word?) on the rule. Fix: Use a metal rule and magnetised masses. To accurately position the masses, mark the centre of the top face.

2). There is friction between the nail and the rule (the nail is placed inside a hole in the rule). Fix: Use a smoother nail and file the hole in the rule.

3). The density of the rule may not be constant. This means that the centre of mass may not be at the halfway point. Fix: Support the rule freely on a wire passed through a small hole. From the wire hand a plumb line to show the vertical. Mark the line of the string and repeat from a different small hole. The centre of mass lies where the two lines intersect.


Please comment on my current limitations and fixes, and give me some more suggestions. The fixes must be 'doable' with college-grade equipment.

Also, can you explain how to calculate percentage uncertainty and where a question about it might arise with this practical?

 

[jbsteele]

4). The masses cannot be accurately measured. Fix: Use a digital balance to measure the mass of each piece.5). The string may be too long or too short due to stretching/shrinking. Fix: Use a Vernier caliper to measure the exact distance between the masses.To calculate percentage uncertainty, you will need to use the formula (percentage uncertainty) = (uncertainty/measured value) x 100. For example, if you measured the distance between the masses to be 10 cm with an uncertainty of 0.5 cm, then your percentage uncertainty would be 5%. This is important to note when doing experiments as it can affect your results considerably. You might be asked about this in a Physics ISA if the experiment requires calculations that are based on the measurements you took.