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    ok mechanics is like the WORST subject for me! i actually HATE it with a passion

    ok this question seems so easy but i just can't do it! please help me out guys!

    its exercise 3b in the edexcel heinemann book...

    A stone is dropped from the top of a tower. one second later another stone is thrown vertically downwards from the same point with a velocity of 14ms-1. If they hit the ground together find the height of the tower.

    so i was thinking, do i have to use simultaneous equations? i tried doing that with the second stone being (t+1) coz its thrown 1 sec later...i dont get the same answer as the back of the book though! how do i simultaneously solve this guys? or am i going about it the wrong way?

    any help would be soo appreciated! thank you
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    ok: (we will take down to be -ve)
    for the first stone:
    S = ut + ½at²
    S = g/2 t²

    for the second stone: it took it one second less to reach the ground... but it moved the same height... so:
    S = u(t-1) + ½a(t-1)²
    S = 14(t-1) + g/2 (t-1)²

    so:
    g/2 t² = 14(t-1) + g/2 (t-1)²
    solve and find t, and then find S ( which is the height)
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    arghhhh thanks sooooo much! im so stupid! i got the two equations that you wrote but i didn't make them equal to each other....god i think im brain dead or something!

    thankss!
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    (Original post by rozzy)
    A stone is dropped from the top of a tower. one second later another stone is thrown vertically downwards from the same point with a velocity of 14ms-1. If they hit the ground together find the height of the tower.
    For stone 1
    s = h
    t = t
    u = 0
    a = 9.8
    v = ?

    s = 0.5at^2 = 4.9t^2


    For stone 2
    s = h
    t = t-1 (since it is in the air for 1 second less than stone 1)
    u = 14
    a = 9.8
    v = ?

    s = 14(t-1) + 0.5(9.8)(t-1)^2
    => s = 14(t-1) + 4.9(t-1)^2

    At bottom of tower
    4.9t^2 = 14t -14 + 4.9(t-1)^2
    => 4.9t^2 = 14t -14 + 4.9(t^2-2t+1)
    => 4.9t^2 = 14t - 14 + 4.9t^2 - 9.8t + 4.9
    => 0 = 14t - 14 - 9.8t + 4.9
    => 4.2t = 9.1
    => t = 9.1/4.2 = 2.17s

    s = 4.9t^2 = 4.9(9.1/4.2)^2 = 23m
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    oh wait! lol i didnt get the same equations as you...i got (t+1) for the second stone...oh well close enough!

    thanks again...you've cleared it up for me
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    (Original post by rozzy)
    oh wait! lol i didnt get the same equations as you...i got (t+1) for the second stone...oh well close enough!

    thanks again...you've cleared it up for me
    Nah it's definitely t-1
    t+1 gives you an answer that is way off.
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    ooh thanks too windowmaker
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    lol yeah i know...im so silly! its friday afternoon lol!
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    (Original post by rozzy)
    ooh thanks too windowmaker
    hehe, I love it when people call me that.
 
 
 
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