Where is the Disorder

Discussion in 'Physics & Math' started by Willem, Apr 19, 2019.

  1. phyti Registered Senior Member

    Messages:
    732
    What is accelerating the moon from a straight line course?
    If gravity was turned off, wouldn't objects on the surface depart in a straight line path tangent to the surface, since the earth is rotating?
     
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  3. exchemist Valued Senior Member

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    12,453
    Sure but that is a force. Force and energy are different things.

    The change in energy due to a force is F x d, distance gone under the influence of that force. In this case, the distance between the moon and Earth does not change so no work is done and the energy stays constant.
     
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  5. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

    Messages:
    13,077
    Even I know that is a stupid question

    The speed (not accelerating but constant) of the moon is counter balanced by the Earth's gravity pull

    The moon is in a constant state of free fall

    Constantly falling over the edge of Earth's horizon

    Sure turn off Earth gravity and it will head off in a straight line and not fall to wards Earth

    As I understand the current situation the distance between Earth and Moon is lengthening by about 4 cm per year

    Don't know how long it will be before it leaves us

    Gravity off, float away at a angle not straight up sure

    Earth stops spinning with gravity in place you would scrap along what ever surface you were on until you hit your personal non movable object

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    Last edited: Apr 29, 2019
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  7. phyti Registered Senior Member

    Messages:
    732
    w=work, f=force, m=mass, d=distance, a=acceleration


    w=fd=mad


    v=velocity, t=time


    v=at


    d=at2/2 =v2/2a


    w=mv2/2 =m v2/2


    g=gravitational acceleration, h=height


    To move a mass vertically a distance h,


    w=mgh


    In the graphic, the moon has two components,

    translational motion at v,

    and acceleration at g for the distance h.
     

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  8. exchemist Valued Senior Member

    Messages:
    12,453
    Fd work is distance gone in the direction of the applied force. Since the force always acts perpendicular to the velocity, there is no Fd work done.
     
  9. Write4U Valued Senior Member

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    20,069
    We'll need to slow the moon down, just a tad. Maybe attach a parachute to its trailing end. The moon doesn't rotate, so that would work just fine, even without an atmosphere.
     
  10. Michael 345 New year. PRESENT is 72 years oldl Valued Senior Member

    Messages:
    13,077
    Great idea

    Go one better, put up a solar panel (dual power and solar wind) sail which activates when Moon moving towards the sun

    Power provides energy to the batteries for the service unit on the Moon getting ready for the Mars launch

    Solar wind pushes against sail slowing Moon down so it's still around when the Mars explorers come back a few years later

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  11. phyti Registered Senior Member

    Messages:
    732
    The moon has no propulsion units to alter its straight line course (Newton), the g-field does that.
    The moon mass is displaced a distance h. That is work.
    I consulted a physics reference written by Issac Asimov.
    If I'm wrong, so is he!
     
  12. exchemist Valued Senior Member

    Messages:
    12,453
    Asimov would not make such a foolish error so it follows that you have misunderstood. If you can post a reference to it that I can read, I can go through it and help you understand it properly.

    I repeat: if the force is at right angles to the motion, there is no motion in the direction of the force and thus no work is done.
     
  13. phyti Registered Senior Member

    Messages:
    732
    In post 64, the distance h = 1.3 mm/sec of moon motion in a straight line (no gravity).
    It's a tiny displacement but continuous to produce the orbit. Here's the page...

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  14. James R Just this guy, you know? Staff Member

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    More generally, the work done by a force is $W=\bf{F}\cdot\bf{d}=|\bf{F}||\bf{d}|\cos \theta$, where $\theta$ is the angle between the force and displacement vectors. In the case of the (approximately) circular orbit of the moon, the gravitational force on the moon is always perpendicular to the Moon's displacement, so no work is done by gravity.

    Even if we take into account the slight ellipticity of the orbit, the total mechanic energy of the Earth-Moon system is still conserved, and for each complete orbit gravity does no work.
     
  15. exchemist Valued Senior Member

    Messages:
    12,453
    Exactly. In the case of a circular orbit, θ =π/2 and Cos π/2 = 0.
     
  16. exchemist Valued Senior Member

    Messages:
    12,453
    As I thought, this is a discussion of a scenario of motion in a straight line under the influence of a force. Suggest you go on a chapter or two and look at motion in a circle. If you do that you will find that what James and I are telling you is correct.
     
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