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Homework Help: Projectile questions~ really hard >_<

  1. Nov 19, 2007 #1

    A kangaroo was seen to jump to a vertical height of 2.8m. for how long was it in the air?

    Attempt on the solution

    not possible? what do you guys think?


    A ball was photographed stroboscopically as it was released and fell freely. eight images appear on the phto which is reduced in scale by a factor of five. use the fact that the acceleration of the ball is 9.8ms(-2) to discover the time interval between the flashes of the stroboscope.

    Attempt on the solution

    part 1. calculating the distance between the first and second image, then mutiply by 5 (due to the scale factor).

    since we are taking the first and second image, it would have a overall time of 1s.

    Part 2.
    take the calculated length as 0.04m:


    v= 0.04/1
    = 0.04 ms(-1)

    then i got stuck on this step....
  2. jcsd
  3. Nov 20, 2007 #2
    If you drop an object from 2.8m, how log will it take before it hits the ground?

    If you can find that, then double it, because the time it took for the kangaroo to reach 2.8m is the same as the time it takes for something to fall that distance, assuming air resistance is neglected.
  4. Nov 20, 2007 #3

    I don't know how did you find 1s.If the distance between them is 1s,then it must be the time interval between slashes of stroboscope.

    If d=0.04 m is true,

    the equation of motion is


    so the distance between slashes should be

    [tex]\Delta y=\Delta vt+v\Delta t-gt\Delta t[/tex]

    [tex]\Delta v=g\Delta t[/tex]

    [tex]\Delta y=gt\Delta t[/tex]

    No need to calculate velocity.Note that velocity is not uniform.
  5. Nov 20, 2007 #4
    @Bill Foster
    oh now i get it, so i have to use of the SUVAT equations to find the time, am i right? (^^)


    i think i had a mistake to take the time as 1s. and also, wouldn't i be taking the initial velocity (u) as 0 ms(-1) because the starting velocity is 0??.

    and also, did i mistake the question to find the time interval between the flashes of the stroboscope to be a question on the time taken to reach from 1 image to another?

    or is it surpose to mean that i have to find the displacement?
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