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Homework Help: Innacurate movie physics project

  1. Jun 7, 2006 #1
    Hi, im in 11th grade and am currently in my first year of physics.

    I need to use Coulomb's laws to prove that creating a repulsive force of 110833 newtons would take an impossible amount of energy to generate...but its not really that simple, please read on.

    Our teacher recently assigned us a project in which we have to find two physically impossible scenes in movies and analyze how they are physically impossible (using the various equations and such that we learned throughout the year) The analysis need to be mathematical, not merely conceptual (ie, i cant just use the flying scene from crouching tiger hidden dragon and state that human flight is impossible).

    For my two scenes, i decided to use 2 scenes from the matrix movies, one of them i am having no problem analyzing, the second one is quite difficult though.

    It's the scene in matrix 3 when neo and trinity are flying their ship and they accelerate upwards very fast. I deemed this scene impossible because since the ships levitate the same way antigrav lifters do (by charging air particles and using repulsive forces to thrust upwards) I realized that it would take an impossible amount of energy to generate an electric field that could create a repulsive force that is that huge. I am just having trouble proving this using Coulomb's laws. I was hoping someone could help me.

    This is the information i have so far
    Ship's approxomate mass: 45,000 Kg (mass of a Boeing 737)

    Acceleration: 2.46 m/s at 40 degrees relative to the ground (the ship is rising on a slant, much like how a plane takes off)

    Net force on the ship (found by multiplying mass by acceleration): ~110833 N

    So basically, i need to use Coulomb's laws to prove that creating an electric field with a repulsive force of 110833 newtons would take an impossible amount of energy to generate...any ideas on how to do that?

    So far, i tried using F = [k(q1)(q2)]/r^2 and I got (q1)(q2) = 1.23x10^13 Coulombs (which is a HUGE amount of charge)
    BUT... thats assuming that the radius would be 1 meter...i have no idea what to plug in for the radius.

    Also, i assumed that q1 and q2 are equal but opposite (therefore 1.23x10^13 is simply q^2) but is this a correct assumption? I'm not even sure what im doing.

    I'd like to somehow convert this to Joules or Watts, so that i can compare it to the output of a powerplant, thereby showing how impossible it is (ie "powering this ship would take an amount of energy equivalent to the output of 17 nuclear power plants")

    I really have no idea where to go from here. Any help would be greatly appreciated.

    Thank you very much for your help.
    Last edited: Jun 7, 2006
  2. jcsd
  3. Jun 7, 2006 #2

    Chi Meson

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    First of all, to get a repulsive force, you need to have the two ships gain the same charge, not opposite charges (opposites attract, but you knew that, right?)

    One way to go about it, pretend that both ships gained an equal amount of charge from the earth. Use the equation by which the energy of a charged capacitor is determined. Estimate the profile surface area of the ships to determine an approximate capacitance.

    You are totally correct that there would be an absurd amount of charge required.

    PS, is this by any chance an IB physics class?
  4. Jun 7, 2006 #3
    Hi, thanks for your speedy response.

    I dont think i explained the problem very well, you see there's only ONE ship and it has electrical plates all over it that are suppsoed to lift it off the ground (using technology very similar to lifter technology).

    Oh and its not an IB physics class, its just a regular junior level phys class (no college credit here ;-)

  5. Jun 7, 2006 #4

    Chi Meson

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    OK then, repulsive force needs two things. What specifically are the two things getting charged?
  6. Jun 8, 2006 #5
    the 2 things getting charged are the particles in the air, and the plates on the ship (the will have equal charges and repel)

    this is what im thinking:

    1. I use F = kq1q2/r^2 to get (q1q2). Assuming r = 1 meter (the charge plates charge particles roughly 1 meter away)

    since q1 = q2 i can square root the answer to get Q

    2. then i use Q = CV to get capictance in farads

    3. then i use C = KEoA/r
    to get the approxomate surface area of the kind of capacitor that would hold this ridiculous charge.

    That sound about right?
    Last edited: Jun 8, 2006
  7. Jun 8, 2006 #6
    yes but not only do you need to take into account the force of repulsion but you also need to use the force of attraction due to gravity,

    F=mg or for a gravitational


    where M is the mass of the earth and m is the mass of the ship.

    so in order to find the net force on the ship, in motion,
    i think this is correct.

    [tex] F_{net}=m(g-a)[/tex], so this is the net force.
    Last edited: Jun 8, 2006
  8. Jun 8, 2006 #7
    sorry my latex is not upto sctratch it was meant to say


    Fn is the net force


    ps. model the ship as a point charge
    Last edited: Jun 8, 2006
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