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Kinematics - projectile motion - time to maximum height?

  1. Feb 11, 2014 #1
    Kinematics -- projectile motion -- time to maximum height?

    1. The problem statement, all variables and given/known data
    The nozzle of a fire hose discharges water at a speed of 10 m/s. The nozzle is aimed straight up. How long does it take for a water drop to reach its maximum height?
    Start with translating the question:
    vi= 10 m/s
    vf= 0 m/s
    t=? s
    a= -9.8 m/s^2
    delta−x= ?

    2. Relevant equations



    3. The attempt at a solution

    OK, so I'm assuming that solving this takes 2 steps. One entails finding the maximum height, then using a kinematic formula to find time. Is that logic sound?
     
  2. jcsd
  3. Feb 11, 2014 #2

    ehild

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    What are the relevant equations?

    ehild
     
  4. Feb 12, 2014 #3
    I tried this:
    Vfy=Viy+Ay*t
    0 m/s=10 m/s-9.8 m/s^2*t
    -10 m/s=-9.8 m/s^2 *t
    t=10 m/s / -9.8 m/s^2
    t=1.02 s
     
  5. Feb 12, 2014 #4

    ehild

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    It is correct. You see, the maximum height was not needed.


    ehild
     
  6. Feb 12, 2014 #5
    1.02 s is not an option for this multiple choice problem, the closest is 1.0 s. Is that likely the answer then?

    Edit: The question asks how long it takes to reach maximum height, so because it is a parabola, it stands to reason that 1.02s/2=0.51s is how long it takes to reach the maximum height. Right?
     
  7. Feb 12, 2014 #6

    ehild

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    The data in the problem are given with two significant digits. Round of your result to t=1.0 s. .

    What is parabola?

    When does a body projected straight upward reach maximum height? What is its velocity at the apex?
    What is the maximum height?

    What would be the velocity at t=0.5 s? Would the water drop move upward or downward then? Does it reach the maximum height at t=0.5 s?

    ehild
     
  8. Feb 12, 2014 #7
    I don't know. I remember someone in class saying that projectile motion was a parabola...

    t=1.0 s means what? The time it takes for the motion to finish? or the time at which the water reaches its maximum height?
     
  9. Feb 12, 2014 #8

    ehild

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    The motion is not parabola. A graph of a function can be parabola.

    You applied the equation Vfy=Viy+Ay*t. What do the letters mean? You substituted Vfy=0. If you throw up a pebble, will it reach the ground with zero velocity? Where is the velocity zero?
    Just throw up something and see...

    ehild
     
  10. Feb 12, 2014 #9

    CWatters

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    The problem statement says "The nozzle is aimed straight up".
     
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