Instantaneous Power

Instantaneous power delivered by a force equals the dot product of force and velocity, $P=\vec{F}\cdot \vec{v}$, which for motion in the direction of the force reduces to $P=Fv$. At constant speed the engine's driving force exactly balances the resistive force, so the engine must supply power equal to the resistive force times the speed. Multiplying gives $P=1800\times 20=36000\text{ W}=36\text{ kW}$. The option 18 kW halves the result, perhaps mistakenly inserting a one-half factor that belongs to kinetic energy, not power. The option 90 kW divides force by speed incorrectly. The option 3.6 kW misplaces a decimal by a factor of ten. Note that no net work is done on the car as a whole, since its kinetic energy stays constant; the engine's entire output is dissipated against road resistance and air drag. This is precisely why fuel consumption climbs steeply at higher cruising speeds, because the power demanded by a fixed resistive force grows in direct proportion to velocity. This applies the NCERT relation between instantaneous power, force, and velocity. As a unit check, newtons times metres per second equals watts, and tens of kilowatts is a realistic engine output for highway cruising.