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Homework Help: Another pre-cal problem

  1. Aug 16, 2008 #1
    Two stones are launched at the same time from the top of a building, 850 feet high. The first stone is simply dropped over the edge, while the second stone is thrown horizontally at 45 feet/second.
    a) which stone will reach the ground first? Explain your answer.
    b) About how far apart will the stones be when they land?

    If someone gave me the formulas, I could maybe solve the problem. I looked it up my old physics notes, but the formulas there really don't help me...

    2. Relevant equations

    3. The attempt at a solution
    a) The first stone is traveling with 30 feet/second, i suppose. So the second stone is faster.
    (even though it is thrown horizontally and has to travel a longer distance?:confused:)
    Well, at least its an attempt at a solution...:blushing:
    Last edited: Aug 16, 2008
  2. jcsd
  3. Aug 16, 2008 #2


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    Welcome to PF!

    Hi Lindeza! Welcome to PF! :smile:

    hmm … how to help you without actually giving you the answer … ?

    Well … this is physics, not philosophy, so any explanation has to be based on an equation. :wink:

    (and remember, the second stone is thrown horizontally)

    What equations can you think of that govern the vertical position of a projectile? :smile:
  4. Aug 16, 2008 #3


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    AI would be very surprised if the textbook that had these problems did not have the formulas in the same chapter!
  5. Aug 16, 2008 #4
    Hopefully this should help you with the first question.

    Gravity affects all objects' velocity in the vertical direction equally (disregarding air resistance).
  6. Aug 18, 2008 #5
    Thanks for helping me!!!
    Isn't the vertical position of the projectile
    so here it would be:
    y= t*45*sin(alpha)+0.5 (-32)*t²+850
    but what is alpha?
    and how is this supposed to help me?
  7. Aug 18, 2008 #6
    They arrive at the same time, I think, right?
    (I did a little experiment :D )
    Don't know how to do that with formulas or something...but...well...
    since the first stone is dropped, it should arrive at x=0, right? so I Just have to find the x value of the other stone??
  8. Aug 18, 2008 #7


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    Hi Lindeza! :smile:

    alpha is zero (because the stone is thrown horizontally).

    Does that help? :smile:
  9. Aug 18, 2008 #8
    that really does help!!!!
    but it's too late now, I just sent of the homework assignement and just left out this problem (and the others :D) (I am doing this for summer school, over the internet)
    But I don't really think it matters that much...I will still try to solve the problem, because of the final test and because I want to know the answer!!