A Titan IV rocket has put your spacecraft in a circular orbit around Earth at an altitude of 390 km. Calculate the force due to gravitational attraction between the Earth and the spacecraft in N if the mass of the spacecraft is 2050 kg.
+12530 A Titan IV rocket has put your spacecraft in a circular orbit around Earth at an altitude of 390 km. Calculate the force due to gravitational attraction between the Earth and the spacecraft in N if the mass of the spacecraft is 2050 kg.
Omi67:
I think r should be the radius of the Earth from the center to the spacecraft:
6,371,000 meters + 390,000 =6,761,000^2 =45,711,121,000,000.
+2511 Guest #1 is correct. You do need to add Earth’s radius to this distance – it is part of the “trickiness” needed to solve it. I made the same mistake. My mentor caught it and asked me if I thought a satellite orbiting the Earth once every 78 seconds might seem a little fast?
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Last year, my professor assigned this question as one of several related homework questions for my Physics for non-majors class. The question asked for the gravitational force, orbital velocity, and period. After completing the assignment, I sent it to my mentor for review. After a few error corrections, the Troll Master said I should ask my professor about the use of the Titan IV.
I reworded his rhetorical comments and wrote them on a post-it note attached to my homework:
I know this is a hypothetical question and it’s irrelevant how the satellite obtained its orbit, but why would the engineers use a $450 million Titan IV, capable of placing 12 times the payload in that orbit, when a $51 million Delta II could easily do this?
This class was usually taught by an undergrad, but my professor replied:
Ginger,
This was a government sponsored launch and spending 10 to 20 times more than necessary is par for the course. In fact, the engineers might have received a bonus for keeping costs down.
After 8 years of teaching physics classes, you have the unique distinction of being the first physics’ troll in the non-majors class. You’ve renewed my hope for the modern world.
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LancelotLink told me this was a great accolade and I should put it on my resume. 
Before I met my favorite troll, the only humor I could ever see in physics was in the “Big Bang Theory.”
