1)In 2003, Itlaian households consumed a total of 153.6*10^-15J of energy (the l

Question

1)In 2003, Itlaian households consumed a total of 153.6*10^-15J of energy (the largest amount of any sector in the Italians economy). How many days would it take for the sun to put that amount of energy onto a region 20 km by 20 km (0.15% of Italian's total land area)? Assume that energy from the sun arrives at the surface of the earth with an average intensity of 685 W/m^2

2)A bat uses ultrasound at a frequency of 40 kHz to locate a hovering (stationary) insect prey. If the bat is flying directly towards the insect at a speed of 4 m/s, at what frequency does the bat receive the ultrasound reflections from the insect? Take the speed of sound to be 340 m/s. (Quote ve signicant gures in your answer.)

Explanation / Answer

a) 20 km * 20 km = 4 * 10^8 m^2

Total energy obtained = 685 * 4 *10^8 = 2.74 * 10^11 Joules per sec

Energy per day = 2.74*10^11 * 60 * 60 * 24 = 2.367 * 10^16

Energy consumed = 153.6 * 10^15 J = 1.53 * 10^17 J / 2.367 * 10^16 = 6.488 days

So about 7 days are required.

b) This is an example of the doppler effect

Stationery observer and moving source

f ' = f ( V - 0) / (V - Vb)

f ' = 40,000 ( 340 / 340-4) = 40,476 Hz

The frequency heard by the bat = 40.476 kHz

Thissite will help you more :

http://physics.bu.edu/~redner/211-sp06/class19/class19_doppler.html

Please rate it ASAP. Thanks

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