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Energy Needed to Maintain Velocity

  1. Oct 27, 2009 #1
    I know how to figure out how much energy is needed to acclerate to a certain speed. What I don't know is how much energy is needed to maintain that speed.

    For example. I was watching the first Trek movie the other day, and Kirk orders the Enterprise to go to .5 light speed. So the Enterperise goes from 0 to 150M meters per second in like 5 seconds. I wondered how much force would be needed to do that, so I calculated:

    F = M * A
    The ship has a mass of 200,000 metric tons, so 200,000,000 kg * acceleration, which going from 0 to .5 light in 5 seconds would be 30,000,000 meters per second per second (i hope that's right).

    That gave me 6*10^15 Newtons (damn!)

    I then tried to see what this would be in joules, so I next calculated the distance traveled, using:

    Distance = .5*Acceleration*Time^2

    That gave me 375,000,000 meters.

    To then calculate joules, I used W(joules) = Force * Distance
    6*10^15 * 375,000,000

    That gave me 2.25*10^24 joules of energy needed to accelerate a 200,000 metric ton object to .5 light speed in 5 seconds (please correct me if I'm wrong).

    My question, how many joules per second is needed to maintain that speed?

    This is not homework. I'm just a nerd.
  2. jcsd
  3. Oct 27, 2009 #2
    0 Joules/sec are necessary, if you are traveling in a vacuum(space) see newtons first law. Also, the energy required to get up the speed is a little more than you calculated because when you increase your speed, your mass increases as well, see special relativity / einstein. But, your method of calculations are right if you ignore special relativity ( I didnt actually do your calculations). You don't need to calculate the energy from force times d, you can use 1/2 m v^2 too. (would have been quicker, but same answer.)
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