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Thread: stone

  1. #1
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    stone

    a stone is dropped from the top of a building 30 m high. a second stone is dropped from half way up the same building.. find the time that should elapse between the release of the 2 stones if they are if they are to reach the ground at the same time

    stone released from top; u=0 s=30 a=g t=t+x stone half way; u=0 s=15 a=g t=t
    x stands for extra time the top stone was relased from. when i use s=ut+.5t^2 i cant work it out. equations could be wrong
    Last edited by markosheehan; Jan 6th 2017 at 03:51 AM.
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  2. #2
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    Re: stone

    Quote Originally Posted by markosheehan View Post
    a stone is dropped from the top of a building 30 m high. a second stone is dropped from half way up the same building.. find the time that should elapse between the release of the 2 stones if they are if they are to reach the ground at the same time

    stone released from top; u=0 s=30 a=g t=t+x stone half way; u=0 s=15 a=g t=t x stands for extra time the top stone was relased from. when i use s=ut+.5t^2 i cant work it out. equations could be wrong
    Find the time for the first stone to reach the ground. Then do the same for the second. Then subtract the times.
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  3. #3
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    Re: stone

    Quote Originally Posted by markosheehan View Post
    a stone is dropped from the top of a building 30 m high. a second stone is dropped from half way up the same building.. find the time that should elapse between the release of the 2 stones if they are if they are to reach the ground at the same time

    stone released from top; u=0 s=30 a=g t=t+x stone half way; u=0 s=15 a=g t=t x stands for extra time the top stone was relased from. when i use s=ut+.5t^2 i cant work it out. equations could be wrong
    u= 0 for both as you say but you left "g" out of your equation. You should be using s= .5gt^2.

    30= .5g(t+ x)^2= .5gt^2+ gtx+ .5gx^2 and 15= .5gt^2. Subtracting, gtx+ .5gx^2= 15. First solve .5gt^2= 15 to find t, then, using that value of t, solve .5gx^2+ gtx- 15= 0 for x.

    (That is effectively the same as Debsta suggested.)
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