FORCE AND POWER
Each of the four jet engines on an Airbus A380 airliner develops a thrust (a forward force on the airliner) of 322,000 N (72,000 lb). When the airplane is flying at 250 m/s (900 km/h, or roughly 560 mi/h), what horsepower does each engine develop?
IDENTIFY, SET UP and EXECUTE:
Our target variable is the instantaneous power P, which is the rate at which the thrust does work. We use Eq. (6.18) (P=F_{\|} v). The thrust is in the direction of motion, so F_{\|} is just equal to the thrust. At v = 250 m/s, the power developed by each engine is
P=F_{\|} v=\left(3.22 \times 10^{5} \mathrm{~N}\right)(250 \mathrm{~m} / \mathrm{s})=8.05 \times 10^{7} \mathrm{~W}=\left(8.05 \times 10^{7} \mathrm{~W}\right) \frac{1 \mathrm{hp}}{746 \mathrm{~W}}=108,000 \mathrm{hp}
EVALUATE: The speed of modern airliners is directly related to the power of their engines (Fig. 6.27). The largest propeller-driven airliners of the 1950s had engines that each developed about 3400 hp (2.5 × 10^6W), giving them maximum speeds of about 600 km/h (370 mi/h). Each engine on an Airbus A380 develops more than 30 times more power, enabling it to fly at about 900 km/h (560 mi/h) and to carry a much heavier load.
If the engines are at maximum thrust while the airliner is at rest on the ground so that v = 0, the engines develop zero power.
Force and power are not the same thing!
