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Finding g on unknown planet given only total time and a random height

  1. Sep 2, 2011 #1
    1. The problem statement, all variables and given/known data
    Been working on this for the past hour or so and seem to be going in circles; finally decided to call in the cavalry:

    You land on an unknown planet somewhere in the universe that clearly has weaker gravity than Earth. To measure g on this planet you do the following experiment: A ball is thrown upward from the ground. It passes a windowsill 11.0 m above ground and is seen to pass by the same windowsill 2.00 s after it went by on its way up. It reaches the ground again 5.00 s after it was thrown. Calculate the magnitude of g (the acceleration due to gravity) at the surface of this planet.


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 2, 2011 #2

    gneill

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    Staff: Mentor

    The cavalry can't help you without first seeing your attempt at a solution! What formulas are you tinkering with?
     
  4. Sep 2, 2011 #3
    Sorry, my first attempt was flawed because I mistakenly equated Vavg.= (Vfinal+Vinitial)/2 and from that eventually came up with the equation 4t*deltaY*(2Vfinal-Vavg)+Vavg=Vfinal and used the reference point of t=1.5s and y=11m to solve for Vfinal. That (probably obviously) didn't work so now I've used the equation y=Vi+Vi*t+(1/2)at2 after I plugged in the same reference point I got it down to a=(11-2.5Vi)/(1.125).

    I can't seem to get past here. Thanks!
     
  5. Sep 2, 2011 #4
    I agree, this one takes a bit of thinking and some trial and error!

    My hint would be that you need to use [itex]s=ut+\frac{1}{2}at^2[/itex] (1) and [itex]s=vt-\frac{1}{2}at^2[/itex] (2). Using these, you can get an expression for u in terms of a, and then put back into (1) to finally get an answer.
     
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