Using moon's rotational energy on earth

[henry3369]

Homework Statement

Suppose that some time in the future we decide to tap the moon's rotational energy for use on earth. In additional to the astronomical data in Appendix F in the textbook, you may need to know that the moon spins on its axis once every 27.3 days. Assume that the moon is uniform throughout.1. How much total energy could we get from the moon's rotation?
E = 3.15 x 10^23 J

2. The world presently uses about 4.0×1020J of energy per year. If in the future the world uses five times as much energy yearly, for how many years would the moon's rotation provide us energy?
t = 158 years

3. In light of your answer, does this seem like a cost-effective energy source in which to invest?

Homework Equations

The Attempt at a Solution

I'm just curious as to why this would not be a cost effective energy source. If the moon complete one rotation every 27.3 days, and as a result, it provides energy for 158 years, how is that not cost effective?

 

[mfb]

What happens after 158 years?

How did you get that number? How would you harvest this energy to use it?

 

[henry3369]

What happens after 158 years?

And how did you get that number? And how would you release this energy to use it?

Future energy use = 5* (4.0×10^20)= 2 x 10^21
3.15 x 10^23/ 2x10^21 = 158 years