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Homework Help: Maximum altitude of a rocket in 2D motion and free body motion

  1. Feb 25, 2010 #1
    1. The problem statement, all variables and given/known data

    A rocket is launched at an angle of 53 above the horizontal with an initial speed of 75 m/s. It moves for 25s along it's initial line of motion with an overall acceleration of 25 m/22. At this time its engines fail and the rocket proceeds to move as a freebody.

    a) What is the rocket's maximum altitude?

    2. Relevant equations

    displacement = V1time + (1/2)a(time)2
    V2 = V1 + 2adisplacement

    3. The attempt at a solution

    I've googled this question after attempting it, and have seen several different methods from my own.

    For part a, I'm interpreting it as the maximum vertical displacement. I set up a vector diagram, where the angle between the average velocity vector and horizontal velocity vector is 53. I use sin ratio to solve for the opposite, which is sinTHETA x hyp. This gives me 60m/s. I do the same process to find the vertical acceleration, which is 20.0 m/s2. I plug them into the displacement equation, solve to get 7750m.

    Now for the free body motion. Velocity will equal 0 when it gets to it's top. I use the second equation to rearrange for displacement. V2 is 0, V1 is -602, acceleration is -9.81. Essentially, displacement is a negative value. Meaning the max. altitude is still 7750m.

    Makes sense to me, but I applied the same approach to a different question (identical) with diff. values. The answer was way off. (I did this to see if my approach was right).

    What am I doing wrong?
  2. jcsd
  3. Feb 25, 2010 #2
    What will be the rocket's velocity when it runs out of fuel?

    V1 isn't 60m/s because the rocket has been accelerating.
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