A car moved 839 m in 47 s. Using the formula $$speed = \frac{distance\ moved}{time\ taken}$$ calculate the speed of the car to the appropriate number of significant figures.

Question

BlackTom AI · Accepted Answer

Speed = distance ÷ time, so start by dividing the given numbers: 839 m ÷ 47 s.

Relevant concepts:
- The formula for average speed is speed = distance / time (units: m/s).
- When multiplying or dividing measured quantities, the result should be reported with the same number of significant figures as the least precise measurement.

Step-by-step calculation:
1. Compute the quotient with full precision: 839 ÷ 47 = 17.851063829787234...
2. Count significant figures in the given values: 839 has 3 significant figures; 47 has 2 significant figures. The least number is 2 significant figures, so the final speed must be reported with 2 significant figures.
3. Round 17.8510638... to 2 significant figures: look at the first two digits (1 and 7 → “17”), then check the next digit (8) which is ≥5, so round up. 17 rounds up to 18 when keeping two significant figures.

Why the correct answer is 18 m/s:
- The unrounded calculation gives 17.851... m/s, but applying the significant-figures rule (limited by time 47 s → 2 s.f.) yields 18 m/s. The value 18 has exactly two significant figures and correctly reflects the precision of the input data.

Why other possible forms are not preferred:
- Writing 17.85 m/s or 17.9 m/s keeps too many significant figures (3 s.f.), overstating precision because the time measurement (47 s) is only to 2 s.f.
- Writing 18.0 m/s would imply three significant figures; without justification (the inputs do not support that precision) this is not appropriate.

Conclusion: The speed, reported to the correct number of significant figures, is 18 m/s, which matches the provided answer.

A car moved 839 m in 47 s. Using the formula $$speed = \frac{distance\ moved}{time\ taken}$$ calculate the speed of the car to the appropriate number of significant figures.数值题

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