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Projectile Motion Please Help

  1. Jan 29, 2008 #1

    GSV

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    Projectile Motion!! Please Help!!

    1. The problem statement, all variables and given/known data

    The acceleration due to gravity can be measured by firing a projectile upward and
    measuring the time it takes to pass two given points in both directions. The figure below
    is a plot of height versus time and shows that the time the projectile takes to pass a
    horizontal line A in both directions is TA, and the time it takes to pass a horizontal line B
    in both directions is TB. Derive an expression for g, the acceleration due to gravity. Your
    expression should involve only h, TA, and TB. State any assumptions you make.

    [​IMG]


    3. The attempt at a solution

    Dont even know where or how to start!! Please Help!! thanks
     
  2. jcsd
  3. Jan 29, 2008 #2

    mjsd

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    Homework Helper

    this is a constant acceleration problem.
    need standard kinematics equations for constant accelerations
    relate relevant variables based on the plot (ie. what you know) and then solve those resultant simultaneous equations
     
  4. Jan 29, 2008 #3

    GSV

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    Could you guide me through them!
    I tried but i couldn't make it to work.
     
  5. Jan 30, 2008 #4

    mjsd

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  6. Aug 18, 2009 #5
    Re: Projectile Motion!! Please Help!!

    I'm working this problem right now. Where do you start? I mean do you go from when it passes A the first time to when it passes A the second time, similiarly with B? Do you go from ground to A, to A to B, to B to B again on the way down, to B to the ground?

    If you measure from A to A and B to B I get:

    0=(V1)*(Ta)-.5*g*(Ta)^2
    and
    0=(V2)*(Tb)-.5*g*(Tb)^2

    But how are you supposed to related them? And How do you get rid of the V's?

    If I do it the second Way I get times that arn't right like (t1-t2). :confused:
     
  7. Aug 18, 2009 #6
    Re: Projectile Motion!! Please Help!!

    What are you defining V1 and V2 as?
     
  8. Aug 18, 2009 #7
    Re: Projectile Motion!! Please Help!!

    I'm Defining V1 as the velocity of the object as it passes the A line, and V2 as the velocity as the object passes the B line. Is this correct?
     
  9. Aug 18, 2009 #8
    Re: Projectile Motion!! Please Help!!

    Ok, can someone at least tell me how they would start this freaking problem?
     
  10. Aug 18, 2009 #9

    rl.bhat

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    Re: Projectile Motion!! Please Help!!

    If u is the velocity of projection, at t1 the velocity will be
    vA = u - g*t1. ........(1)
    At t4 the velocity will be
    v2 = -vA = u - g*t4....(2). From equ. 1 and 2 we get
    2vA = g*(t2 - t1 ) or vA = g/2*(t4 - t1) .....(3)
    Similarly vB = g/2*( t3 - t2).....(4)
    Now use the vB^2 - vA^2 = 2g*h find g.
     
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