Question:
Define watt. Express kilowatt in terms of joule per second. A $150 \mathrm{~kg}$ car engine develops $500 \mathrm{~W}$ for each $\mathrm{kg}$. What force does it exert in moving the car at a speed of $20 \mathrm{~m} \mathrm{~s}^{-1}$ ?
Solution:
$\mathrm{kW}=1000 \mathrm{~W}=10^{3} \mathrm{Js}^{-1}$
Power, $\mathrm{P}=150 \times 500=75000 \mathrm{~W}, \mathrm{u}=20 \mathrm{~m} \mathrm{~s}^{-1}$
$\therefore \mathrm{F}=\frac{\mathrm{P}}{v}=\frac{75000}{20}=3750 \mathrm{~N}$