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Homework Help: Rocket fires its engine, force/acceleration

  1. Sep 24, 2011 #1
    1. The problem statement, all variables and given/known data
    A small space probe of mass 235 kg is traveling at 85.0 m/s. It fires its main engine at an angle of 63.00 to its original direction of travel. The engine produces a thrust of 12,000 N and there is enough fuel on board for a 10.0 s burn. What distance will it have traveled 3.00 hrs after the engine shuts down? Assume no mass loss due to firing the engine.

    In which direction is the rocket travelling?

    2. Relevant equations
    I'm having trouble understanding what the free body diagram for this would look like.

    3. The attempt at a solution
    http://img851.imageshack.us/img851/1162/66673407.png [Broken]
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 24, 2011 #2


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    One thing I think you can ignore if the force of gravity - this is a space probe, so will be out away from any significant body in space.

    The force of the engine is the only force acting, and given the time of the burn, you can find the impulse imparted - that will give the change in momentum, and thus the change in velocity.
    That change in velocity will be added to the original velocity to get the new velocity. Mainain that for 3 hours and you know how far the probe will have gone.
    Last edited by a moderator: May 5, 2017
  4. Sep 24, 2011 #3
    I haven't learned momentum yet, can't solve the problem using that method.
    Also, what direction is the rocket travelling in?

    Fey or Fe?
  5. Sep 24, 2011 #4


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    The direction the probe is travelling in is irrelevant. It is only important that the force from the rocket motor is at an angle of 63 degrees to the direction. If it will make you more comfortable, make the original direction straight up the page.

    Impulse - momentum is only a neat summary of V = Vo + at and F = ma.

    Use Newton's second law [ F = ma] to find the acceleration, then use V = Vo + at to find the change in velocity.

    For that change, assume Vo = 0 as you are only calculating the change in velocity, to be later added to the original velocity.
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