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Homework Help: 1D Kinematics Again

  1. Oct 7, 2006 #1
    A model rocket is launched straight upward with an initial speed of 70.0 m/s. It accelerates with a constant upward acceleration of 1.00 m/s2 until its engines stop at an altitude of 190 m.

    (a) What is the maximum height reached by the rocket?
    m

    I don't know what I am doing wrong here. I have two pages full of screw ups, haha.

    I tried to find out the final velocity it hits right before it runs out of fuel and I'm stuck

    Using:
    [tex]x = x_0 + v_0 t + (1/2) a t^2[/tex]

    I did 190 = 0 + (70)t + 1/2(1)(t^2)

    I get down to 190 = 70t + .5t^2

    I'm just lost so just some guidance would be nice, I know the steps I'm just making a simple error somewhere or something and I've tried for like 60 minutes, so I figured I'd post.
     
  2. jcsd
  3. Oct 7, 2006 #2
    Last edited: Oct 7, 2006
  4. Oct 7, 2006 #3

    Astronuc

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    Science Advisor

    The rocket starts with an initial velocity 70.0 m/s and accelerates at a rate of 1 m/s2 over a distance of 190 m to a greater velocity at which point it stops accelerating (because the motor cuts out = no more thrust) and it starts decelerating with gravity -9.8 m/s2.

    So there are two parts to the problem.

    1. The acceleration phase to 190 m. So solve for a constant acceleration problem over 190 m with vo = 70 m/s, and find the velocity at 190 m.

    2. With the velocity determined in 1) determine the altitude the rocket achieves with that initial velocity at 190 m.

    http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html#mot1
     
  5. Oct 10, 2006 #4
    Thank you, I got the answers using your help.

    A model rocket is launched straight upward with an initial speed of 70.0 m/s. It accelerates with a constant upward acceleration of 1.00 m/s2 until its engines stop at an altitude of 190 m.

    (a) What is the maximum height reached by the rocket?
    455.68 m
    (b) How long after lift-off does the rocket reach its maximum height?
    10.06 s
    (c) How long is the rocket in the air?
    19.876 s
     
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