Unit Conversion

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.