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How do I go about this [projectile motion]

  1. Sep 29, 2005 #1
    A rocket is launched with an initial velocity of 75.0 m/s [53 degreess above the horizontal]. It goes for 25.0 s along its initial line of motion with an overall acceleration of 25 m/s^2. At this time the rockets fail and the rocket follows a projectile path. Find the total time, dy, and dx, in no particular order.

    so would the vx and vy be:

    vx=700cos53 m/s
    vy=700sin53 m/s

    (700 is the vf of the rocket after 25.0 s, before getting into a projectile path; so is it correct to assume that 700 is the vi for the projectile?)

    And so far I've come up with dy = tan θ + ½ a (dx/ Vicosθ)^2... I really don't know what to do next... :confused:
  2. jcsd
  3. Sep 30, 2005 #2


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    Homework Helper

    I'm not sure exactly what work you're showing or where you got the answers you give, so let me give you some general advice here.

    Notice that the conditions of the problem change partway through. That means you're pretty much going to have to solve the problem in two parts.

    During the first part, the rocket starts with the initial velocities you have worked out. It also has an "overall acceleration" of 25 m/s^2. I presume that means the net acceleration, and that gravity has been taken into account, so you'll need to split the acceleration into its horizontal and vertical components and figure out from that the horizontal and vertical distances travelled as well as the final horizontal and vertical speeds.

    When the rocket engines cut off, the second part of the problem begins. Now you'll use the final velocities you worked out in the first part as the initial velocities in a free fall problem. The rocket will be rising at thus and such a rate - how long until it hits the ground? Obviously, to get that you'll need to know the initial height, which will be the vertical distance you determined in the first part.

    Does this help?

    Oh - and for the record, I suspect you'd have to take into account the curvature of the Earth to get an accurate answer to this question, but I think I also wouldn't worry about that if I were you. I'd just use it to tease my teacher. :)
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