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Homework Help: Help me understand!

  1. Sep 19, 2004 #1
    Why can't I understand simple vector questions? I've just started a higher physics and that's what we're learning.

    I'm stuck with this question:

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

    I have the answer at the back of the book, but I don't know how to arrive at the answer! Please help!

    Do I need to split it up into the components:

    u - initial velocity
    v - final velocity
    a - acceleration
    s - distance
    t - time

    I know the equations, but when I plug the numbers in, it doesn't come out as the right answer!

  2. jcsd
  3. Sep 19, 2004 #2
    is that all it says, must say how much Vi it has, or does it start from rest.
    If it starts from rest you find the time it takes it to get to the top and then multiply by 2 and thats the total time, using the equation x=Vit + (1/2)at^2.
    Last edited: Sep 19, 2004
  4. Sep 19, 2004 #3
    two equations to be used here,
    Lets use the third kinematic eqn....
    (using g down as positive sign so when direction of motion is upwards g is negative)
    that gives us,

    when it reaches maximum point , the final velocity becomes 0,
    this gives v=0 and that gives us,
    u^2 = 2gs
    ** we can calculate u since we know g and s

    now let's use the second kinematic equation,
    from 1 we got 'u'
    and we know a=-g and s=2.8
    rearrange the equation and u get a quadratic in t
    solve it to find t

    But this t is the time to taken to reach the top we need the overall time which is just 2*t.

    -- AI
  5. Sep 19, 2004 #4
    I guess your right, looks like I messed up :frown: . Gj though. One more thing how does the kangaroo manage to jump up and straight down?, he must land a little to the side or sometin.
  6. Sep 19, 2004 #5
    Why is that?
  7. Sep 19, 2004 #6
    nm, sometimes i dont even know what im typing but I just type it. :tongue2:
  8. Sep 19, 2004 #7
    That's all the information I'm given.

    The answer is 1.5s, and I still don't know how to reach it! :frown:
  9. Sep 19, 2004 #8
    Okay we have that
    Vf=0 m/s
    a=-9.81 m/s^2

    So in the equation

    Vf^2 - Vi^2 = 2ax
    0 - Vi^2 = 2 (-9.81)(2.8)
    -Vi^2 = -54.936
    Vi = 7.41m/s

    Now in the equation

    a = (Vi - Vf) / 2
    -9.81 = (0-7.41) / t
    -9.81t = -7.41
    t = .76s

    so thats only to the top so we multiply by 2, .76 * 2 = 1.52 s, which is the total time.
  10. Sep 19, 2004 #9
    That's what I didn't do! I now understand why I have to multiply it by two, I didn't think about that earlier! Thank you so much. :smile:
  11. Sep 19, 2004 #10
    No problem, glad I could help yout out. :cool:
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