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Homework Help: Universal Gravitation, Please help

  1. Nov 27, 2006 #1
    Most likely a basic question to most, been awhile since ive done any of this.

    If i remember right the formula states, F = G*M_1*M_2/d^2

    where d = distance between M_1 and M_2

    Im creating a planetary simulator for fun, i get the initial position of the earth using the 7 variable Astrological Guides technique.


    I then get a position + 1 day in the future call it pos2;

    Velocity = pos2-iniPos;

    Im using the grav-constant 6.67E-20 km^3/kg*s^2

    1)So grav_constant*M_1*M_2 = grav_const*5.9742E24*1.988435E35
    = 7.923498687E35

    2)The distance between the the two is approx 150 000 000, according to the formula then d^2 = 2.25E16

    Force = grav_constant*M_1*M_2/d^2 = 3.52155E19 km*Kg/s^2

    Huge number!

    Forward Euler update shows us that the 'unit_vector' pointing towards the sun multiplied by the force should give us our update method.

    updateVelVector = oldVelVector + unit_vector*force*dt

    Where dt = change in time

    However this doesnt work worth a damn,

    Somewhere ive made a mistake and i just cant see it,

    Please help!

    PS i tried reducing the problem space to some number which i thought would be easier to use. I decided to decrease the distance to 1 km, basically this turns the problem into earth_mass/1.125E16 and sun_mass/1.125E16. This is wrong too, im very stuck!
    Last edited: Nov 27, 2006
  2. jcsd
  3. Nov 27, 2006 #2


    User Avatar

    Staff: Mentor

    force * dt is the impulse of the force during the interval dt, which equals the change in the object's momentum (mv), not the change in velocity.
  4. Nov 27, 2006 #3
    Could u explain a little further?

    I thought the update form for velocity was

    vel = oldVel + unit_vec*force*dt;
    pos = oldPos + vel*dt;

    Hmmm, arggg.

    thanks for the help,
  5. Nov 28, 2006 #4
    Sorry guys my mistake,

    For my initial velocity i forgot,


    Seconds not days, arggggg!

    thanks for the help,

    Great forum!

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