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Homework Help: Trip to the Moon

  1. Feb 21, 2014 #1
    1. The problem statement, all variables and given/known data

    You plan to take a trip to the moon. Since you do not have a traditional spaceship with rockets, you will need to leave the earth with enough speed to make it to the moon.

    On your first attempt you leave the surface of the earth at v = 5534 m/s. How far from the center of the earth will you get?

    Mearth = 5.9742 x 1024 kg
    Rearth = 6.3781 x 106 m
    Mmoon = 7.36 x 1022 kg
    Rmoon = 1.7374 x 106 m
    Dearth to moon = 3.844 x 108 m (center to center)
    G = 6.67428 x 10-11 N-m2/kg2

    2. Relevant equations

    PE = GMEm/r

    KE1 + PE1 = KE2 + PE2

    3. The attempt at a solution

    [itex]\frac{1}{2}[/itex]m (5534)2 = GMEm/r

    r = 2.6 x 107
  2. jcsd
  3. Feb 21, 2014 #2
    If you get 26 000km away fromt he Earth into space travelling at 5.5 km/s then you would have escaped the Earth's gravity. Thing is, you need to be faster to escape.

    If this is like a slingshot thing where you are given the initial velocity of 5534 m/s, then you definitely can't escape. Since you said you don't have rockets, you need to be atleast AT the same speed as escape velocity for a, whatchamacallit, unpowered ship? You need to be faster than the escape velocity if you wanna make it to the Moon, much faster if you want to make it in a reasonable amount of time.

    gravity is decelerating your ship by g m/s2 , find the time it takes for the ship's vertical velocity become 0.
    Last edited: Feb 21, 2014
  4. Feb 22, 2014 #3
    So since you don't escape the Earth's gravity, I just used mgh for PE instead of the universal gravitation and was able to get the correct answer. Thanks for pointing that out.
  5. Feb 22, 2014 #4
    Why do you think PE1 = 0?

    No, you would not. Geostationary satellites are at about 36,000 km, but they are pretty much bound to the Earth.

    That is only true when you are close to the Earth, which is not obvious in this case.
  6. Feb 22, 2014 #5


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    I assume this is just a set-up for a math problem, not an actual ENGINEERING thought experiment, but just in case it IS intended to be an engineering concept, what DO you plan on using for the launch? Since once it leaves the launch mechanism, it will be on a purely ballistic trajectory, you have to gain all your needed speed in a short amount of space/time. How are you going to keep the astronauts from being crushed by the G forces? A big enough explosion on the ground works for a math problem, but obviously not for an engineering problem since it would likely turn the astronauts into a crushed blob.
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