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Force to move mass in space

  1. Jul 4, 2012 #1
    If I have a drive that works in space without the need of any out side contact.
    In other words there is no equal and opposite force. How many pounds of force would I need to move a 40 billion ton rock to a speed of one mile an hour? If there is a program to work it out that would be great as well.
    Also what would be the best high temp and high strain insulators? I would like something what would work with molten copper or aluminum.
  2. jcsd
  3. Jul 4, 2012 #2

    Doc Al

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    Staff: Mentor

    Oh really?
    Newton's 2nd law will tell you the net force needed to produce a given acceleration.
  4. Jul 5, 2012 #3
    I'm not sure how your hypothetical drive is supposed to work so I won't address that.

    Use as much or as little force as you want. The more force you use, the less time it will take to achieve your desired change in velocity.

    What's important is the amount of energy needed, or impulse- which is more convenient in this case.

    An impulse (I), which is Force • time, will result in an equivalent change in momentum (mass • velocity). Both are measured in units of Newton seconds:

    [tex]Ft = m\Delta v[/tex]​

    The total change in velocity is 0.447 m/s. Multiply that by the mass of your 40 billion ton rock (3.629×10^13 kg) to get the change in momentum:

    [tex]Ft = 1.622×10^{13} N s[/tex]​

    From here, you can either pick the amount of force to use and solve for t, or choose an arbitrary time in seconds and solve for F which is measured in Newtons.

    In terms of energy, you'll need 3.624×10^12 Joules to change the velocity by .447 m/s (1 mile/hour).

    [tex]E = \frac{I^2}{2m}[/tex]​
  5. Jul 5, 2012 #4
    Thank You
    I did a bit of fast math and that would come out about a 150000 pounds of h and the o2 to burn it minus boiler and electrical loss for each mile an hour of change sound right?
  6. Jul 5, 2012 #5


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    Science Advisor

    How did you come up with that figure?

    You have postulated a reactionless drive. A reactionless drive need not require any energy at all. The figure of 3.624×10^12 Joules would imply that the reaction mass is infinitely large and is at rest in your chosen reference frame.

    Choose a different referrence frame and the energy delta changes. That's one reason why reactionless drives do not fit well with Newton's laws.
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