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Need help with acceleration problem

  1. Jan 25, 2008 #1
    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 15.0 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.

    I drew the picture and started to sole the problem this way but I keep getting the wrong answers. Any input would be greatly appreciated.

    to me the initial velocity after 2 seconds should be
    v=distance/time which is 15m/2sec so the initial velocity should be 7.5m/s at that point. So from here I used

    xf=1/2at^2+vit+xi

    15=1/2a(2)^2+(7.5)(2)+0

    a=-7.5

    I get a gravitational acceleration of -7.5m/s^2. I tried 7 a well thinking the magnitude would be positive. But both answers are wrong.
     
  2. jcsd
  3. Jan 25, 2008 #2

    Shooting Star

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    The particle is undergoing accn; it's not traveling at a uniform velo. What you have used is valid if speed is const. Correct it.

    EDIT:

    After another read, it's never even said anywhere that it took 2 secs to travel 15 m. It crossed the window sill at t=t' traveling up and t=t'+5 going down.

    Use the eqns for motion under unifrom accn.
     
    Last edited: Jan 25, 2008
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