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Height, time, gravity acceleration

  1. Mar 24, 2009 #1
    1. The problem statement, all variables and given/known data
    "A stone is thrown vertically upwards from the top of a building 29 metres high at an initial speed of 11 metres/sec. How long (in seconds) will it take the stone to hit the ground? The height at any time is given by

    s = 29 + 11*t – 4.9*t2.

    Give your answer correct to one rounded off decimal place (but don't round off as you work the problem), eg., 4.37 becomes 4.8."


    2. Relevant equations

    -29 = 11t - 4.9t^2
    t(11 - 4.9t) = -29 <<< Do I do this? Do I need it?

    3. The attempt at a solution
    The answer is 3.801, but how do I get this?
    Please help me
     
  2. jcsd
  3. Mar 24, 2009 #2

    LowlyPion

    User Avatar
    Homework Helper

    That's fine as far as it goes.

    But what you have is a quadratic equation.

    You can use the quadratic formula to solve the general equation:
    http://en.wikipedia.org/wiki/Quadratic_equation#Quadratic_formula

    One of the answers will give you your solution. The other root of the equation is likely a (-) number, indicating another solution of different construction that would also satisfy the same quadratic relationship.
     
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