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Homework Help: Momentum Principle question dealing with a spacecraft's velocity

  1. Sep 11, 2012 #1
    1. The problem statement, all variables and given/known data

    You are navigating a spacecraft in space. The mass of the spacecraft is 4.0 x 10^4 kg. The engines are off, and you're coasting with a constant velocity of ‹ 0, 28, 0 › km/s. As you pass the location ‹ 5, 6, 0 › km you briefly fire side thruster rockets, so that your spacecraft experiences a net force of ‹ 5.0 105, 0, 0 › N for 22 s. The gases have a mass that is small compared to the mass of the spacecraft. You then continue coasting with the engines turned off. Where are you an hour later?

    2. Relevant equations

    Instructions say to use the step one of the Momentum Principle (p = Fnet(Δt)), then step one of the position update equation. I'm not sure if I am using the right equation, I think it's →rf = →ri + Vavg(Δt). I have also used pf = pi + Fnet(Δt), rf = ri + viΔt + 1/2(F/m)(Δt^2).

    3. The attempt at a solution

    Well, when I first attempted the question, I used the Momentum Principle to find the initial momentum. After finding the initial momentum, I found the final momentum using the equation pf = pi + Fnet(Δt). I then found the final velocity by rearranging p = mv... then got stuck. I attempted to use rf = ri + viΔt + 1/2(F/m)(Δt^2), but that didn't result in a correct answer. I'm not sure if I'm misunderstanding the question, or the instructions.
    Help is very much appreciated! Thank you so much.
  2. jcsd
  3. Sep 12, 2012 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    How did you get stuck with p=mv?

    It looks like you are trying to apply equations without understanding what is happening.
    I notice that your force vector appears to have four components... but if I'm guessing correctly, the entire motion occurs in the x-y plane?

    I'm not sure how you used the equation ... here are a number of ways that would be wrong ... i.e. Δt would only be for the time that the force was applied - 22s. Once the rockets stop firing you are back to constant velocity.
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