Question

[Extra point]A typical comet contains 10¹³ kg of water ice. How many comets would be needed to account for the 2 × 10²¹ kg of water presently found on our planet? If this amount of water accumulated over a period of 0.5 billion years, how frequently must Earth have been struck by comets during that time?

Chapter 5

(10) Because of tidal forces, the Moon is in a synchronous orbit around Earth

True

False

(11) Explain how the Moon produces tides in Earth's oceans.

(13) What does it mean to say that the Moon is in a synchronous orbit around Earth? How did the Moon come to be in such an orbit?

(14) Sunlight absorbed by Earth's surface is reemitted in the form of

ultraviolet radiation.

microwave.

infrared.

visible.

(15) According to Figure 5.5 in the textbook (Earth's Atmosphere), commercial jet airplanes flying at 10 km are in

the ozone.

the mesosphere.

the troposphere.

the stratosphere.

(16)[EXTRA POINT] Without the greenhouse effect, Earth's average surface temperature would be about 250 K. With the greenhouse effect, it is some 40 K higher.

Use this information and Stefan's law to calculate the fraction of infrared radiation leaving Earth's surface that is absorbed by greenhouse gases in the atmosphere. (Sec. 2.4 in the textbook)

(17) Give two reasons geologists think that part of Earth's core is liquid.

Answer 1

1) (a) 2x10^21 / 10^13 = 2 x 10^8 comets (answer)

because when you divide powers with the same 'base' you subtract the exponents... 10^21 /10^13 = 10^(21-13) = 10^8

2 x 10^21 kg... of water over 0.5 billion years... is the same as 2 x10^8 comets hitting the Earth over 0.5 billion years...

(b) 2x10^8 comets per (0.5 billion years) means ... the Earth was hit by comets 4 x 10^8 times per billion years...

Thus 4 comets per year