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So I was given the following homework problem:

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 found a solution on answers.yahoo.com, but I still don't really understand it as well as I would like.

I drew a picture and know that after 15m it takes 2secs to fall back to 15m and then an additional 5secs until it hits the ground. I can't assume that the velocity is uniform for entire trip can I? (e.g. 6secs travel time going both up and down/quadratic curve, for total 12secs) I have the 4 equations of kinematics and usually I just look where I can plug in values and simply get the answer, but obviously that's not going to help me to solve slightly more advanced problems such as this!

Could someone please give me advice on how to approach this problem (and/or physics problems in general). Anything that can help me conceptualize these better. I just want to be able to solve these without staring at it for 5hrs...Thanks in advanced.

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Presumably you are familiar with the usual kinematic expressions for constant acceleration, no? At a guess I would look for two expressions one relating to the windowsill-to-windowsill period and one relating to the downward track from the sill to the ground. These should form a solvable system. You will also want to think about the symmetry of upward and downward velocity in the absence of air resistance (which is necessary for this to be a constant acceleration problem). –  dmckee Sep 1 '11 at 0:48
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That said our FAQ and policy on homework discourages particular exercises in favor of questions about the underling principles. (Sorry about the scattered meta links, that's just how things worked out.) –  dmckee Sep 1 '11 at 0:52
    
Hi Mr_CryptoPrime, welcome to Physics Stack Exchange! This is actually a site for questions about specific physics concepts, not general homework help, and your question as it stands is not the kind of thing we're really equipped to deal with. It would make a better question if you quote the solution you found on Yahoo! Answers and ask about the specific concept in that answer that you are having trouble understanding. I'd suggest that you head to Physics Chat where I would be happy to help you fix up your question and perhaps even explain the problem. –  David Z Sep 1 '11 at 0:54
    
ah, looks like we cross-posted, @dmckee ;-) –  David Z Sep 1 '11 at 0:55
    
I'm voting to reopen because the question asks some particular question, but then does generalize it to ask about the underlying principles. It's difficult to ask a question only about the underlying principles when you don't yet have a good understanding of what those principles are. –  Mark Eichenlaub Sep 1 '11 at 0:55
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closed as too localized by dmckee Sep 1 '11 at 0:53

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