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Homework Help: Throwing a ball off a cliff and calculating its time, speed and using its degree.HELP

  1. Nov 4, 2011 #1
    1. The problem statement, all variables and given/known data
    A stone is thrown from a cliff 100 meters high with an initial velocity v0 = 25 m / s and at an angle of 53° projection from the horizontal axis. determine:
    a) the time before it reaches the ground
    b) the maximum height
    c) its horizontal reach
    d) its speed when it hits the ground


    2. Relevant equations
    I selected these:
    vx0 = v0cos(53)
    vy0 = v0sin(53)

    Then I applied these t the following equations:
    xf= vx0t
    yf= y0 + vy0t+ (1/2)(-9,8)t2

    t= ( -b± √(b2 - 4ac) ) / 2a



    3. The attempt at a solution
    So, for part a), using the above equations I found:

    vx0 = v0cos(53)
    vx0 = 25cos(53) = -22,96
    xf= vx0t
    xf= -22,96t

    vy0 = v0sin(53)
    vy0 = 25sin(53) = 9,898
    yf= y0 + vy0t+ (1/2)(-9,8)t2
    0= 100 + 9,898t+ (1/2)(-9,8)t2

    Then, with that last equation, I used this equation: t= ( -b± √(b2 - 4ac) ) / 2a

    to find t= -3,62s
    When the answer SHOULD BE 7s........ What am i doing wrong???


    then, i tried b) using vf= 0
    using the equation(H is maxheight):

    vf2 = v0 -2(9,8)(H-100)
    0 = 252 -16,9H +1690
    H = 136,98 m
    And the right answer should be 120m...... GRR !

    As for c), I don'T quite understand how to measure it... do i find xf?

    d) Its supposed to be (15i -48,6j) m/s but i forgot how to calculate vectors.


    ANYWAY PLEASE HELP!!! Its easy for you, but for me, this is hard. ;(
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 4, 2011 #2
    Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

    These equations usually make problems much easier:

    [itex]x(t) = v_{o}cos\theta[/itex]

    [itex]y(t) = v_{o}sin\theta - \frac{1}{2}gt^{2} + h[/itex]

    vo: Initial velocity
    h: height
     
  4. Nov 4, 2011 #3

    gneill

    User Avatar

    Staff: Mentor

    Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

    Quadratic equations have two roots. Sometimes only one of them will make physical sense for a given situation.

    EDIT: Also, make sure that your calculator is set for DEGREES rather than RADIANS if your angles are given in degrees!
     
    Last edited: Nov 4, 2011
  5. Nov 4, 2011 #4
    Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

    Thanks for the replies, but i was wondering if someone could perhaps check my work? Tell me what im doing wrong, if i miscalculated? I'm still in a nut.
    Also, I have tried finding the two roots for the quadratic equation, and neither give me the proper answer.
     
  6. Nov 5, 2011 #5

    SteamKing

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    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

    Well, you could show us your calculations after you have fixed the known errors.
     
  7. Nov 8, 2011 #6
    Re: Throwing a ball off a cliff and calculating its time, speed and using its degree.

    I got it! I've been repeating the same silly mistake over and over again.
     
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