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Homework Help: Find maximum thrust of a rocket blasting off

  1. Oct 16, 2015 #1
    1. The problem statement, all variables and given/known data

    A 2.49×104-kg rocket blasts off vertically from the earth's surface with a constant acceleration. During the motion considered in the problem, assume thatg remains constant. Inside the rocket, a 14.5-N instrument hangs from a wire that can support a maximum tension of 37.3 N .

    I found that :

    Acceleration = 15.42 m/s^2

    Find the minimum time for this rocket to reach the sound barrier (330m/s) without breaking the inside wire.

    The answer to this was 21.4 .

    How far is the rocket above the earth's surface when it breaks the sound barrier?
    3530 m.

    Now I have to find the maximum vertical thrust of the rocket engines under these conditions.

    2. Relevant equations
    I tried using F=ma and some basic kinematics.

    3. The attempt at a solution

    Using F=ma...
    i tried F=(2.49x10^4) * 15.42 . This wasn't correct.
  2. jcsd
  3. Oct 16, 2015 #2


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    Tension acts on the instrument and on the rocket. (Downwards on rocket and upwards on the instrument) Let acceleration of rocket be ##a##. Since the string is inextensible, the instrument will also have same acceleration. Write newtons law equation for both rocket and instrument.
  4. Oct 16, 2015 #3

    Mister T

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    Is this the net force on the rocket or the thrust force?
  5. Oct 16, 2015 #4
    Well it's a vertical launch..So wouldn't it be the same thing? the only component is in the y direction.. I don't really understand thrust that well. I think it's something like thrust comes out the bottom of the rocket and propels it upwards so the thrust acts upwards? I really don't know
  6. Oct 16, 2015 #5
    What do you mean?
    So for the rocket it would be F=(2.49x10^4) * 15.42
    For the instrument.. F=(14.5N/9.81) * 15.42 ? Now what?
  7. Oct 16, 2015 #6

    Mister T

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    Thrust is the exhaust gas pushing upward on the rocket. Gravitational force (weight) is Earth pulling downward on the rocket.
  8. Oct 16, 2015 #7
    So weight would be the mass times 9.8. But i did mass * acceleration..Okay.. so should I find weight of the rocket and subtract that from the upward thrust?
  9. Oct 18, 2015 #8


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    To get the net force, yes.
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