1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook