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Homework Help: Speed Time Estimation from a video

  1. Feb 28, 2015 #1
    1. The problem statement, all variables and given/known data

    Watch the crash test video , and determine the following:

    1. How fast is one of the crash test dummies thrown forward (the crash at :34 seconds) ? Pick either one, and be clear how you estimate this.

    2. Assuming the speed you got from part (1), how high would a different dummy, specifically the one thrown nearly straight up (and out of sight), go if thrown at that speed?

    3. Find a way of estimating the actual height achieved by this crash test dummy (the one thrown vertically, out of sight). Since the only scene where this one lands is in slow-motion ( :34) you will have to find a way to calibrate the timing with other scenes shown in real time.

    2. Relevant equations

    3. The attempt at a solution
    I'm lost! I think I should make a speed time plot but how should I calibrate for slow motion in the crash at 34 seconds? Help Please!
  2. jcsd
  3. Feb 28, 2015 #2

    Simon Bridge

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    In order to estimate distance and time from a record, you need a clock and a ruler in the video.
    The top of the screen has a clock, sometimes. You'll need to find something you know the length of in the video to get a distance scale.
  4. Mar 1, 2015 #3
    Ive got the slow motion part figured out.
    How do I go about this speed/time plot?
  5. Mar 1, 2015 #4

    Simon Bridge

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    use the definition of speed.
    you can also do it from a displacement time plot.
  6. Mar 1, 2015 #5
    Okay perfect, so I looked up the width of that model of car and estimated the crash test dummy is about 1.5 car widths in the air before he falls.

    The scene is in slow motion so I timed the dummy being ejected to when he falls (about 2 seconds). I'm having a a hard time figuring out how to convert this time into "real time". The clock at the top of the screen isn't very reliable, so whats another way I could go about this?
  7. Mar 2, 2015 #6

    Simon Bridge

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    The clock at the top of the screen is the only reliable one. Its hard to read but you dont need actual numbers, just count how many times the seconds display changes in a set stopwatch time.
    You could hope that air resistance is negligible and calculate the time to fall from max height to the ground.

    For your report, get screen captures of the important bits.
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