Fermi Questions C

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Re: Fermi Questions C

Postby Name » January 14th, 2019, 10:32 pm

What is the angular momentum of the solar system around the Milky Way Galaxy? (use standard SI units)
angular momentum is angular velo times moment of inertia. angular velo is v/r. v is 2pir/time. divide by r is 2pi/time. time is 2.5E8 yr or 7.5E15 sec. 2pi/7.5E15 is around 8E-16. Moment of inertia is mr^2. R is 2E17 km or 2E20 m^2 is 4E40. Mass should be appriximentaly the mass of the sun. Times 2E30 is 8E70*8E-16 is 6.4E55 or 56.
I believe my solution is correct (although I could be wrong)
How many joules are released every minuite due to lightning striking the earth?
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Re: Fermi Questions C

Postby Adi1008 » February 9th, 2019, 10:39 am

What is the angular momentum of the solar system around the Milky Way Galaxy? (use standard SI units)
angular momentum is angular velo times moment of inertia. angular velo is v/r. v is 2pir/time. divide by r is 2pi/time. time is 2.5E8 yr or 7.5E15 sec. 2pi/7.5E15 is around 8E-16. Moment of inertia is mr^2. R is 2E17 km or 2E20 m^2 is 4E40. Mass should be appriximentaly the mass of the sun. Times 2E30 is 8E70*8E-16 is 6.4E55 or 56.
I believe my solution is correct (although I could be wrong)
How many joules are released every minuite due to lightning striking the earth?
[math]10^{3}[/math] lightning strikes happen every minute and each lightning strike carries [math]10^{9}[/math] Joules. Answer: 12
A bolt of lightning appears to contain about 5 billion Joules ([url=http://www.physics.org/facts/toast-power.asp]source[/url]). Lightning strikes Earth about 100 times a second ([url=https://learn.weatherstem.com/modules/learn/lessons/36/02.html]source[/url]), which is 6000 times a minute. Answer: 13
How long (in years) does it take a galaxy in a galaxy cluster to orbit the cluster's center of mass once, assuming that the cluster has an average density 100 times that of the universe's?

edit: added units
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Re: Fermi Questions C

Postby Name » February 10th, 2019, 8:01 am

How long (in years) does it take a galaxy in a galaxy cluster to orbit the cluster's center of mass once, assuming that the cluster has an average density 100 times that of the universe's?

edit: added units
so first we have to estimate the distance from a galaxy to the center. Distance to andro is 2E22 m so lets say the average distance is E24. The volume would be 4/3pi(E24^3) or about 4E72 m^3. iirc density is 4 protons per m^3 so 1.6E73 protons 1.6E46 kg. 100 times is 1.6E48. distance^3/mass=time^2. E72/1.6E48 is about 6E23. sqrt, 8E11 years so 12??????
so (assuming the milky way represents an average galaxy), the distance is 4.8E23 according to wikipedia. Volume is 1.5E71. Density is -26 so mass =1.5E45. d^3 is 1.1E71/1.5E45 is 7.37E25. sqrt is 8.6E12 or 13. Correct me if I screwed up anything

If the volume of every person alive was instead filled with butter how many kcal does this contain?
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Re: Fermi Questions C

Postby Jjshan26 » May 27th, 2019, 3:26 pm

How long (in years) does it take a galaxy in a galaxy cluster to orbit the cluster's center of mass once, assuming that the cluster has an average density 100 times that of the universe's?

edit: added units
so first we have to estimate the distance from a galaxy to the center. Distance to andro is 2E22 m so lets say the average distance is E24. The volume would be 4/3pi(E24^3) or about 4E72 m^3. iirc density is 4 protons per m^3 so 1.6E73 protons 1.6E46 kg. 100 times is 1.6E48. distance^3/mass=time^2. E72/1.6E48 is about 6E23. sqrt, 8E11 years so 12??????
so (assuming the milky way represents an average galaxy), the distance is 4.8E23 according to wikipedia. Volume is 1.5E71. Density is -26 so mass =1.5E45. d^3 is 1.1E71/1.5E45 is 7.37E25. sqrt is 8.6E12 or 13. Correct me if I screwed up anything

If the volume of every person alive was instead filled with butter how many kcal does this contain?
uhh assume that butter is a perfect fat so it yields around 9 kcal/g and then like it's probably close to the density of water so 9 kcal/mL. We will also make the assumption that everyone alive is an adult human, which has a volume of 66.4L. Sooo use some dimensional analysis and get 6e5 kcal / person. Multiply this by 7e9 people (lets just round down because we are overestimating a lot of stuff), and then you get like 4e15 kcal, so 15.
turns out, 14 grams of butter is 100 kcalories, yielding 7.14 kcal / g. density of butter is 0.911 g/mL, so 7.14 kcal/g * 0.911g/mL = 6.5 kcal / mL (oops). The volume of an adult human is indeed 66.4L, yielding 4e5 kcal / person. Now the tricky part is like avoiding the assumption that all people are adults. So it turns out around 75% of people are over 18 so the sigfig shouldn't change too drastically. 4e5*8e9 = 3e15, or 15
Assuming perfect efficiency, how many McDonald’s Hamburgers would need to be combusted per second to equate the total amount of energy in the Universe? (hint: this question is troll)
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Re: Fermi Questions C

Postby kman1234t » May 27th, 2019, 3:58 pm

Reminds me of that one Fermi test where the answer was -infinity and my partner and I were wondering what the test writer wanted.

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Re: Fermi Questions C

Postby jimmy-bond » June 5th, 2019, 6:56 pm

Reminds me of that one Fermi test where the answer was -infinity and my partner and I were wondering what the test writer wanted.
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Re: Fermi Questions C

Postby kman1234t » June 5th, 2019, 8:47 pm

Nah. Some random invitational or practice test.


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