This problem follows up on a discussion from lecture. A wind turbine with an efficiency of 45% for converting wind energy into electrical energy is designed to yield 10 kW of electrical power in a 20 mi/h wind. [The density of air is 1.2 kg/m3.]

What length rotor blades does this wind turbine have?

4.1 m

Explanation:

10 kW = 10000 W

20mi/h = 20*1.6 km/mi = 32 km/h = 32 * 1000 (m/km) *(1/3600) hr/s = 8.89 m/s

The power yielded by the wind turbine can be calculated using the following formula

where is the air density, v = 8.89 m/s is the wind speed, A is the swept area and is the efficiency

The swept area is a circle with radius r being the blade length

winston churchill after the dunkirk evacuation

Explanation:

Winston Churchill delivered these lines in his speech after the dunkirk evacuation which was decisive in WWII, Dunkirk is a beach in north France where the Allies troops were trapped, with the help of civilian boats and the english navy they managed to evacuate and bring back to England the majority of the troops. Churchill knew that thye´d fought till the end to defeat Hitler, and that leaving those troops alive was a mistake for Hitler.