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Finding Velocity with Kinetic Energy.

  1. Apr 21, 2013 #1
    1. The problem statement, all variables and given/known data

    I have to use energy conservation to solve this. So i cannot solve with kinematics

    "On a windless day, the engine of a 250 g toy rocket exerts 30N of thrust straight upward until it burns out at a height of 165 m"

    How fast is the rocket moving at 165m?

    2. Relevant equations

    Ek=mv^2 / 2
    Em=eg+ek
    Eg=mgh


    3. The attempt at a solution

    First i found my acceleration.

    A= f/m
    A= (30- 9.8 x .25) / 0.25
    =110.2

    Then subbed into

    Vf^2 = 0 + 2(110.2)x165
    190.1

    But my teacher told us to use the law of conservation....
     
  2. jcsd
  3. Apr 21, 2013 #2

    rock.freak667

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    Well the work done in moving the rocket 165 m will be equal to the kinetic energy of the rocket at 165 m.

    How would you find the work done by the force of 30 N in moving 165 m?
     
  4. Apr 21, 2013 #3
    Mutliply 30 by 165 which is 4950

    oh.... Then solve for v
     
  5. Apr 21, 2013 #4

    vela

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    You need to take into account gravity.
     
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