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Homework Help: Projectle motion

  1. Jun 13, 2013 #1

    How do I find total time of free flight for lettered images on this photo if time equal zero at point A? The time between flashes is .010s. Note the number of significant figures. The strobe specification say the scale is accurate to +- %1.
    I was also given this diagram for this problem.

    I absolutely have no idea how to proceed on this. :(
  2. jcsd
  3. Jun 13, 2013 #2
    The launched object will be at B 0.010 s after passing point A, 0.020 s later it would be at C ... That is the camera see it in 0.010 s intervals so each position is 0.010 s later in time.
  4. Jun 13, 2013 #3
    so I just add up numbers? from point to point? it's 0 at a so at B it's 0.010s, at C it's 0.020s, at D same, at E 0.030s and at F 0.040s so total flight time is 0.040s +- 1%?
  5. Jun 13, 2013 #4
    There are some more points where the camera saw the launched object between points C and D. You should include these in your time reconing. Your problem statement also are not clear. The time of flight will be the total time it spend in its trajectory before hitting the surface. In this case that seems to be where the y-coordinate is zero for which there is no recorded image sou you would need to guess a bit.
  6. Jun 13, 2013 #5
    My teacher told me use first 3 points and last 3 points as I marked on pictures. He also said horizontal distances between points are always equal.
  7. Jun 13, 2013 #6
    Yes, the horizontal distance covered between each successive point should be constant. So do your teacher want you to use only the labeled 6 points to somehow determine the time of free flight?
  8. Jun 13, 2013 #7
    I believe so. When we took pictures at lab, he told us use first 3 and last 3 points. I am reading through lab problems and there aren't any other problems saying that requires all points so I believe we are only using 6 points marked on pic. to be honest, instruction seems to vague to me and this class uses no book and only thing he does at class is solving problems like math class.
  9. Jun 13, 2013 #8
    Well for starters you could calculate the component of g in the plane of the slanted board.
  10. Jun 13, 2013 #9
    hmm.. well.. d = Vit+.5(-9.8)t^2 since it's launched... so Vi = 0..
    I get -43.8 = .5(-9.8)t^2 resulting t = 2.9 second and uncertainty is +- 1% so answer is 3 second +- 1%.
    did I get this wrong? :(
  11. Jun 13, 2013 #10
    You can get the angle of the board from the inches dimensions.
  12. Jun 13, 2013 #11
    which angle should I get? one between 91 and 43.8 or one between 91 and 29.5?
    I got theta = 9 degree by using h = 91 , o = 14.3, a = 90.
  13. Jun 13, 2013 #12
    You want to calculate the component of gravitational acceleration along the slanted plane of the board. This will determine the acceleration of the launched object in this plane.
  14. Jun 13, 2013 #13
    I am now more confused.. what does angle have to do with find total flgiht time here? and I don't understand when you say calculate the component of gravitation acceleration along the slanted plane of the board. You totally lost me. :(
  15. Jun 13, 2013 #14
    I am trying to use the given information to solve the problem. This might be one way of doing it.

    Attached Files:

  16. Jun 13, 2013 #15
    ok then.. how would calculate vertical distance from A to C?
    I am thinking D = .5at^2
    so D = .5(9.8)(.2^2)
    D = .2 inches
  17. Jun 13, 2013 #16
    Sorry, let's drop this approach and rather go for the ticker tape approach (I think that is what your teacher wants you to do).
    The acceleration, a, can be calculated from two average velocities, v1a and v2a, in two intervals following each other, eg interval AB and BC.

    The average velocities are calculated from

    va = Δy/Δt

    so that the acceleration will then be

    a = (v2a - v1a) / Δt

    The Δy you get from the photo between the points and Δt = 0.010 s
  18. Jun 13, 2013 #17


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    Staff: Mentor

    Number ALL of the projectile images from 0 through 13 beginning with A being assigned 0. The time period between flashes is 0.010s ± Δt, where Δt determined by the accuracy of the strobe. Then the time of the nth flash is n times the flash period. You'll want to include the error calculation, since the time is ##T_n = n \times (0.010s ± Δt)##.

    As Basic Physics has stated, that will let you work out a value for the effective acceleration due to gravity acting in the plane of the table surface. Start by finding the angle the surface makes with the horizontal (that's the angle of the slope).

    Were you given the measurements of the dot grid (spacing) on the table surface? If not you won't have a direct way to relate image locations to real-world measurements, and you'll have to "invent" a distance unit to work with.
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