Intelligent assessment and revision tools for OCR A Physics

In a lab test of a small winch, a student estimates its power using P = m g h / t. They use m = 0.200 kg measured with a balance of ±0.001 kg, h = 0.600 m found as the difference between two metre-rule readings each with ±0.001 m, and t = 3.0 s with a timing uncertainty of ±0.10 s (reaction-time limited). The student can change only one aspect for a second trial. Which option gives the greatest reduction in the percentage uncertainty in P?

• A Time five identical lifts back-to-back so the total t ≈ 15 s; with ±0.10 s, the percentage uncertainty in t falls by a factor of 5 (from about 3.3% to about 0.7%), giving the largest reduction overall.
• B Double the mass to 0.400 kg; with ±0.001 kg, the percentage uncertainty in m halves (to about 0.25%), so this gives the biggest improvement.
• C Replace the metre rule with a 0.5 mm resolution rule for each height reading; the uncertainty on h (difference of two readings) drops from ±0.002 m to ±0.001 m (about 0.33% to about 0.17%), giving the largest improvement.
• D Repeat the lift three times and average; this reduces uncertainties uniformly, so it will have the greatest effect.