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A little off topic, but need help with science question:

Discussion in 'General Electronics' started by Dave, Aug 26, 2003.

  1. Dave

    Dave Guest

    Hello all,

    I know this is a tad off topic, but the 'Aviation' groups
    have few contributors, and it seems there a
    lot of thoughtful science folks on this board.

    My 9 year old asked a very hard-to-answer question,
    at least for me, and I consider myself fairly scientific
    and well-read.

    Here it is:

    Assuming unlimited gas and visibility:

    a)You climb in your helicopter in your front yard,
    say on the 45th Parallel, in Salem, Oregon.
    You zoom straight up above your house to a
    height of about 25,000 feet. We'll assume it's
    a gloriously clear day and you can still see
    Salem clearly at this point, far far below.

    b) Assuming unlimited gas, will you look down
    and see South Dakota 2 hours later, as the
    Earth has turned on its axis? And then see
    Bordeaux, France later that night, Turin Italy,
    Siberia, and the vast Pacific at dawn, and finally at the
    24th hour see Salem roll back under your chopper?
    All of this assuming your copter is going 0 MPH
    horizontally, while the earth chugs beneath you
    at 1040+ MPH (The altitude of 25k feet assures
    us there are no mountains racing towards us!)

    c) I countered that the Earth turns at 1040 MPH or
    so, so if you tilt your chopper the opposite way,
    and gun it (we have a KILLER helicopter here!)
    to go 1040 MPH, you'll still stay static. But this
    cannot be right, or an airplane would have to travel
    at LEAST 1040 when going E to W in order to gain
    at all... so that cannot be true.

    So how would this work? Would the earth simply
    pass by while you watched? Or is there some speed
    less than 1040 and greater than zero that will keep
    your chopper 'in one place'? And if the earth DOES
    just creep by, could you then stay low, and only go up
    when you see mountains coming right at you?

    Great question from a 9 year old, and our family and
    friends have had fun over 2 weeks trying to solve it
    to no avail. So I promised Jeremy I'd ask some folks
    who might have the answer!

    Thanks,

    David & Jeremy
    Tacoma, WA (above the 46th parallel, in a house that
    moves exactly the same speed as the earth)
     
  2. Some Guy

    Some Guy Guest

    Because of friction, the air you are resting on moves with the surface of
    the earth, with minor variations we call wind. Friction would keep you
    motionless relative to the air. You would move relative to the earth at the
    same speed, or slightly slower, than the wind. Think of a stick in a
    stream. It doesn't just hover over one spot on the streambed. The water
    pushes it in the direction the stream is flowing. Air is just a thinner
    river. To have a situation like you describe, you would have to get above
    the air, like in a spaceship. Even then, there would be magnetic effects,
    space dust, etc... acting on the ship that would skew the rate of travel.
    This is why we identified the Lagrange points in space. These are special
    places where a spaceship could remain motionless relative to the earth,
    where the magnetic and gravitational effects would not pull them somewhere
    else. I am not sure, but I think there are two Lagrange points. One is in
    front of earth in its orbit, the other follows earth.

    In addition, you and your helicopter would already be moving at the same
    speed as the earth when you take off. Inertia would tend to keep you moving
    in the same direction, while gravity would keep you falling toward the
    earth. Assume you used enough energy to keep yourself a specified distance
    from the earth's surface. Now assume the air does not flow relative to the
    surface -- no wind. The combination of inertia, gravity, the force you are
    exerting to counter gravity, and friction with the air are all acting on
    your helicopter, bending it's trajectory into a circle/ellipse that would
    leave you hovering over one spot on the surface.
     
  3. Dana Raymond

    Dana Raymond Guest

    Uhmm... The problem is that as the copter rises its energy must rise to
    track a given fixed spot on the earth. On the ground its angular momentum is
    lower than if its a few miles up in geostationary orbit. It has to travel
    faster to cover the increased circumference of its orbit to stay over a
    fixed point on earth. Its angular velocity must increase, hence its energy
    must increase.

    Assuming 4,000 miles radius from earth center while on the surface, and
    4,004.74 miles (25,000 feet up) while up in the air, the helicopter would be
    traveling at 4000/4004.74 the angular velocity required for geostationary
    orbit, assuming no other forces acting on it. That means that the helicopter
    would travel West at a speed of 1.24 Mph relative to the earth.

    Of course, the helicopter would pick up the required additional speed if it
    maintained geostationary orbit while it rose above the earth, but the
    problem stated "You zoom straight up above your house". If the path through
    space was straight, then the angular velocity was constant. If the
    helicopter tracked the spot on earth while is rose, then its actual path
    would have been curved through space.

    Dana Frank Raymond
     
  4. Charles Jean

    Charles Jean Guest


    Imagine you are in a airline terminal on one of those people walkway
    gizmos, moving at about 2 feet per second. You jump straight up,
    perpendicular to the surface of the walkway. You land in the same
    place you started from, just like you thought you would. Jumping
    higher doesn't help. Attaching springs to your feet doesn't help.
    You always land in the same place you started from. While all this is
    happening, you had a friend set up a video camera across the hall who
    videotaped the whole session. He shoots the scene from your waist up.
    What the videotape shows is you jumping up AND FORWARD, following a
    parabolic curve! Forward velocity(from the mover, RELATIVE to the
    wall). Vertical velocity(up, from your feet or springs; down,
    increasing all the time from gravity; net velocity RELATIVE to the
    wall). If the VCR had taped lower to show the people mover, you would
    see the track moving under you while you were in the air, and position
    itself under your feet just before you touched down. It's all
    relativ(ity).
    In reality, several planes can maintain a sustained speed greater than
    the earth's rotational speed of about 1000 mph(at the equator). The
    first was the B57 Hustler, back in the late 50's. They pointed it
    west, throttled forward, and watched the sun rise from the WEST!

    France launches its Arienne(sp?) rockets from some island(I forget
    which) close to the equator, to get the highest "kick" from the
    earth's rotation. Check the location of Cape Kennedy, too.

    What happens when you're 20 feet south of the North Pole and start
    running E to W around it? Remember the B57.

    Getting into orbit is a whole 'nuther story. I'm tired and going to
    bed.

    If God hadn't intended us to eat animals,
    He wouldn't have made them out of MEAT! - John Cleese
     
  5. Dana Raymond

    Dana Raymond Guest

    Yes, I understand what you are saying, but I think its a matter of degree.
    Jumping a few feet off of a peoplemover is not the same thing as jumping
    25000 feet. If you think of the copter on the earth as one orbit and at
    25000 feet as another, you'll see that the circumference of the two orbits
    are different. The additional energy required to travel faster in the higher
    orbit has to come from somewhere, right?

    Explain where that extra enegy comes from and you'll convince me. Anyway,
    facinating topic for discussion!

    Dana Frank Raymond

    Cleese
     
  6. Ghost Chip

    Ghost Chip Guest

    Science and electronics both deal with energy.
    The extra energy that the helicopter gains to rise in altitude, (potential
    energy), comes from all the fuel burned to get there. It gets most all of
    it back as it comes down, (SPLAT). To not crash, it must spend additional
    energy, fuel, to not let all that potential energy convert to kinetic
    energy, velocity, going straight down. Essentially this is what allows a
    glider to glide for a long distance against the air drag. The potential
    energy converts to kinetic energy (speed) to make up for the speed lost due
    to drag. The glide slope determines the gain/loss of speed. The only way a
    glider can rise, is to steal energy from air movement or wind. This is why
    windy days make for better gliding and why birds can soar for hours without
    flapping. A form of solar energy use.

    Wow, what questions will the child have when he's 18?

    Ghost
     
  7. Dave

    Dave Guest

    You guys are ALL great in your answers! And funny thing, we STILL
    don't really have 'the' answer!

    I am going to print all the replies for Jeremy to read. And yes, I can
    hardly wait for the questions when he turns 18. Of course, my answer
    will be 'duh' most of the time....

    Dave

    ps - keep the hypotheses coming!
     
  8. Jibaro

    Jibaro Guest

    the answer today is YES. (he/she is 9)
    in 9 years the question will be "where can I find girls?" for boys, or
    "what's the shortest distance to the mall?" for girls.
     
  9. Ghost Chip

    Ghost Chip Guest

    Ans 1: No
    Ans 2: No
    Ans 3: It obeys the laws of physics.
    Ans 4: No
    Ans 5: No
    Ans 6: Pilots choice
     
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