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B Distance between the Earth and The Moon

  1. Dec 9, 2017 #1
    we know that the moon is far away
    the distance between us and the moon can fit all the other 7 planets and some small dwarf planets
    but how ?
    it would take us 3 days to reach the moon and the spacecraft were traveling in at least 240,000 miles
    it should take us more than a year to reach the moon right ?
    maybe less but not 3 days
    it would take us 6 months to reach mars in the nearest place to us
    or this is because there is a very big distance between moon and mars so it would take us 6 months
    am i misunderstood the concept of the distances here or what ?
     
  2. jcsd
  3. Dec 9, 2017 #2

    phinds

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    Depends on your concepts. In cosmological scales, the Earth and moon are in pretty much exactly the same place.

    I suggest that you simply haven't done the math. You have, for whatever reason, created in your head some idea of what it SHOULD be. That's not a good idea in physics, particularly when the math is so trivial.
     
  4. Dec 9, 2017 #3

    Vanadium 50

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    You do know that we sent 9 manned and dozens of unmanned missions to the moon, and we know how long it takes from experience.
     
  5. Dec 9, 2017 #4

    russ_watters

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    A friend of mine who is in her early 40s has owned one car in her life. The distance she's driven it could have taken her to the moon and back twice. Considering the scale of the universe, that to me makes the moon seem pretty close.
    Since these statements of course directly contradict each other, it is tough to know what you mean. So can you be specific?: Do you know the basic equation relating speed, distance and time? My friend; if she just kept driving at highway speed (perhaps we drove in shifts) -- how long would it take to drive a car the distance to the moon?
     
  6. Dec 10, 2017 #5

    mfb

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    The Moon is something like 240,000 miles away. Mars is between 30,000,000 and 250,000,000 miles away (depening on when you start, rough estimates). It shouldn’t be surprising that reaching something at least 100 times more distant takes months instead of days.

    240,000 miles divided by 3 days is about one mile per second. While the speed of rockets is not constant during the trip (they are faster closer to Earth), this is the speed you expect for such a trip.
     
  7. Dec 28, 2017 #6

    Chronos

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    The escape velocity from earth is 11.2 km/sec. anything with less velocity cannot leave earth orbit. The velocity necessary to achieve earth orbit at the distance of the moon is about 10.8 km/sec. From there its just a matter of timing to enable a collision with the moon. Actually the average lunar distance is slightly less [~384,000 km] than the cumulative diameters [~388,000 km] of all the planets [save earth]. Anyways that only amounts to about 10 hours to travel a distance of 384,000 km at an average velocity of 10.8 km/sec.
     
  8. Dec 28, 2017 #7

    mfb

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    You don't have these 10.8 km/s all the time. If you are in low Earth orbit with 10.8 km/s, your speed drops to something like 1-2 km/s before the Moon speeds you up again.
     
  9. Dec 29, 2017 #8

    Janus

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    Your speed would drop to ~0.19 km/s. The Moon's orbital velocity itself is ~1 km/s. The 10.8 km/s is not escape velocity from LEO, but the perigee velocity of an orbit with a perigee at LEO and an apogee at the Moon's orbit, thus your craft will be moving slower than the Moon when it arrives.
    If you left LE0 at 10.8 km/s, it would take Just a bit under 5 days to reach the Moon. The Apollo missions used a higher perigee velocity, which shortened the trip to 3 days, and allowed for a "free-return" trajectory.
     
  10. Dec 30, 2017 #9

    stefan r

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    What kind of car was that. Mine fall apart before getting back from the moon once. :cry:

    The numbers depend on your frame of reference. Is 0 meters/second the center of the Earth, barycenter, Kennedy Space Center, or LEO? Out of context I would assume "left LEO at 10.8 km/s" means adding 10.8 km/s delta-v to a spacecraft in low earth orbit.

    The distance traveled changes when intercepting a moving target. Shooting straight at the moon would be fatal to any astronauts and science equipment.
     
  11. Dec 30, 2017 #10

    Janus

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    In the first sentence of my post I state that the 10.8 km/sec is the perigee velocity for the trans-lunar orbit. This supplies the context for the later statement.
     
  12. Dec 30, 2017 #11

    Chronos

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    Driving at an average of 50 mph to grandma's house does not say anything about your initial or final velocity during the jorney to grandma's house. It only ensures your vehicle at least achieved such velocity sometime during the trip.
     
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