Physical Quantities and Measurement — Question 14

(i) distance = 15 km; time = 20 min

60 min = 1 hr

20 min = $\dfrac{\text{20}}{\text{60}}$ = $\dfrac{\text{1}}{\text{3}}$ h

$\text{Speed} = \dfrac{\text{distance}}{\text{time}} \\[1em] = \dfrac{15}{\dfrac{1}{3}} \\[1em] = 15 \times 3 \\[1em] = 45 \text{km h}^{-1}$ So, speed of car = 45 km h^-1.

(ii) distance = 15 km
1 km = 1000 m
15 km = 15 x 1000 = 15000 m

time = 20 min
1 min = 60 sec
20 min = 20 x 60 = 1200 sec

$\text{Speed} = \dfrac{\text{distance}}{\text{time}} \\[1em] = \dfrac{\text{15000}}{\text{1200}} = 12.5 \text{ m s}^{-1}$

So, speed of car = 12.5 m s^-1.