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Homework Help: Easiest Physics Problem Ever Why Can't I Solve

  1. Nov 15, 2011 #1
    1. The problem statement, all variables and given/known data
    A diver performs a handstand dive from the 12m tower. Compute the following.
    A. Time to hit the water
    B. The divers velocity at 6m
    C. The divers velocity at impact
    D. What was the divers mean acceleration acting on the diver

    2. Relevant equations
    d=Vit + 1/2 a t^2
    vf^2 = vi^2 + 2 a d
    vf = vi + a t
    a average = Change in velocity/Change in time

    3. The attempt at a solution
    A. i tried to solve for time, using equation 1 and got 1.56 seconds
    B. then i tried to solve for velocity at 6m, used the same equation but used half the time and got like -3.87m/s but could I use the equation 3 and plug in half the time and the Vi as 0 and got -7.65 m/s
    C. I just doubled both of those values?? to get the velocity at impact
    D. I would take the final - initial divided by the time
  2. jcsd
  3. Nov 15, 2011 #2
    That's because you forgot a key equation:

    mgh = 1/2 mv2

    And life is simplified.
  4. Nov 15, 2011 #3
    i wasnt given a mass....
  5. Nov 15, 2011 #4
    You don't need the mass. Mass cancels from both sides of the equation. You could have derived it from your second equation though.

    vf2 = vi2 + 2ad

    You know that vi=0. Therefore,

    vf2 = 2ad

    1/2 vf2 = ad

    d is just h, the height. a is just g, the acceleration due to gravity.

    1/2 vf2 = gh

    1/2 mvf2 = mgh

    (This last equation is just a statement of the conservation of energy.)
  6. Nov 15, 2011 #5
    youre a genius :)
  7. Nov 15, 2011 #6
    but what is time? did I do that correctly?
  8. Nov 16, 2011 #7
    Why do you need the time? The acceleration was constant throughout the entire process.
  9. Nov 16, 2011 #8
    the time it took for the diver to hit the water. am i overlooking something. i feel like this was soooo easy, i feel dumb
  10. Nov 16, 2011 #9
    Oh sorry. That was my mistake. I didn't see that your question called for that. Use the last equation.

    vf = vi + at

    You know that vi = 0 and a = g.
  11. Nov 16, 2011 #10
    thanks :)
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