Unit Conversion
A traffic study records a vehicle moving at a steady speed of 72 km/h along a straight highway segment. What is this speed expressed in metres per second using SI base units?
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About This Question
- Subject
- physics
- Chapter
- physics and measurement
- Topic
- unit conversion
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
20m/s
Unit conversion, emphasised in NCERT Class 11, Chapter 2 (Units and Measurements), relies on replacing each unit by its equivalent in the target system while preserving the physical value. To convert from km/h to m/s, note that one kilometre equals 1000 metres and one hour equals 3600 seconds, so the conversion factor is 1000/3600 = 5/18. Multiplying gives 72 × 5/18 = 72 × 0.2778 = 20 m/s. The option 25 m/s is wrong because it would correspond to 90 km/h, not 72 km/h. The option 18 m/s is wrong as it results from dividing by 4 instead of using the correct factor. The option 30 m/s is wrong since it overestimates by applying an incorrect multiplier. A final check confirms that the metre-per-second value should be numerically smaller than the kilometre-per-hour value, and 20 is indeed less than 72, consistent with the conversion factor being less than one.
This easy difficulty physics question is from the chapter physics and measurement, covering the topic of unit conversion. It appeared in the 2025 exam.
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