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Homework Help: Upward Acceleration of an Object

  1. Dec 15, 2009 #1
    I just have a quick question. If an object is moving upwards at constant acceleration, does it take into account the acceleration of gravity pulling downwards?

    For example if I have to find the acceleration of a rocket during its thrust. Would its acceleration include the force of gravity pulling downwards?

    How would I find the acceleration during thrust?
  2. jcsd
  3. Dec 15, 2009 #2


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    I don't think it does. Normally one would measure the resultant acceleration and use that to find the thrust.

  4. Dec 15, 2009 #3
    yes it would.

    imagine if you threw a ball in the air. it would start off faster than it would end up at its highest point before it fell. a rocket is much different because you have to factor in the thrust of the engine, but yes gravity still acts on the rocket as a downward force.

    Does this help?

    wait but wouldnt the rocket still experience the downward force of gravity?

    but to the original question, the gravity doesnt have accleration it exerts a negative force.
  5. Dec 15, 2009 #4
    k my main problem is i need to find the maximum height of the rocket i launched using kinematic equations. I'm assuming i use d=vt+0.5at^2 for the distance during thrust and then again after thrust to its maximum height. Then I would just add these two distance and get the max height. (btw i have the values for thrust time, thrust angle, max time and max angle)

    but i got stumped when i had to find the acceleration of the rocket during thrust. How would i calculate this? I'm not sure how to take into account gravity when trying to find the upward acceleration during thrust.
  6. Dec 15, 2009 #5
    whats the weight of the rocket? and gravity equals -9.81 m/s^2

    do you know the acceleration due to the thrust. i guess you could take that number and subtract the gravity (-9.81) and that would equal your upward acceleration per second

    dont trust me on this though
  7. Dec 15, 2009 #6
    I don't know the weight of the rocket its not given. and I cant use forces and all that. I have to use ONLY kinematics
  8. Dec 15, 2009 #7


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    Gravity is the only thing holding the rocket down, really, so it's obviously important since it's the force you need to overcome to get a rocket into space. If there was no gravity, you wouldn't even need to reach high velocities, so air resistance wouldn't really be a problem either.
  9. Dec 15, 2009 #8
    so how would I calculate that into when I'm trying to find the acceleration during thrust?
  10. Dec 15, 2009 #9
    hmmmi didnt know you were only using kinematics my apologies.

    yeah gravity is basically the only resisting forc ein this equation as i am assuming you dont have to worry about ar like phythagorean said....

    is your op the actual problem they gave you or did you get any numbers?
  11. Dec 15, 2009 #10
    well so assuming this is all theoretical you would assume no air resistance. However for the thrust you will need to account Fg on the object because the value put in has to be fnet=ma. So you would need mass for this or else you cant really solve for upwards acceleration thus you cant sub in a value in to the kinematic equation.

    -Thrust doesn't include fg acting on it.
  12. Dec 15, 2009 #11


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    I'm confused. Are you given the acceleration, or do you need to find it using forces?

    If I say "the rocket is accelerating upwards at 2 m/s^2", well, that's how fast it's accelerating upwards! So at t=0 v=0 m/s, at t=1 v=2 m/s, at t=2 v=4m/s, etc.
  13. Dec 15, 2009 #12
    he wants to know the net acceleration acting on the rocket i thought not only thrust.
  14. Dec 15, 2009 #13
    alright guys sorry, let me a bit more clear. I need to find the average acceleration during thrust of my rocket.
    I have these values.
    Thrust Time = 1.05 s
    Thrust Angle = 28 degrees
    Max Time = 3.6 s
    Max Angle = 49 degrees

    This is what i have so far. We stood 50 m away horizontally from the rockets and measured it as it went up using some sort of tool to measure the angle as the rocket went up.

    So I have a = v/t = d/t /t = d/t^2. Distance would be 50 tan(theta) so.. 50 tan(theta) / t^2

    Now I'm confused here. Obviously the rocket is accelerating upwards during thrust. But doesn't that mean that gravity is also reducing the acceleration since its pulling it downwards? Does this take into factor of the average acceleration during thrust? If so how would I go about calculating it in?
  15. Dec 15, 2009 #14
    and no air resistance is not considered. Oh and I must find this using ONLY kinematics and no forces.
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