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Homework Help: Projectile motion q

  1. Nov 11, 2007 #1
    hey guys imm new here
    i got a question in my book i cant figure out
    it says:
    an athlete executing a long jump leaves the ground at a 30 degree angle and travels
    7.80m (a) what was the takeoff speed? (b) if this speed were increased just 5.0 % how much longer would the jump be?

    anyone kno how to do this
    i tried using the kinematic equations but ther isnt enuf info
    please help if u know the answer.
    thanks alot !
    Last edited: Nov 11, 2007
  2. jcsd
  3. Nov 11, 2007 #2
    Start out by splitting the problem up into 2 parts: Horizontal motion & Vertical motion. From here, you will have 2 equations with 2 unknowns.
  4. Nov 11, 2007 #3
    i know to do that.. but i still dont get how u can use the equations when u only have the horizontal displacement of 7.80 m ? how do i use the 30 degree angle to find anything?
  5. Nov 11, 2007 #4
    You can calculate the time of flight.

    If you have a horizontal displacement of 7.8 metres you can use symmetry to find where the highest point is - this is also half way through the flight.

    The vertical velocity here is zero.

    The 30 degree angle lets you split the flight into its horizontal and vertical components.
  6. Nov 11, 2007 #5
    ok so i plugged 3.9 m and 0m/s into the displacement equation so it makes t=.892s
    and then i found out Vi for vertical is 8.74 m/s
    then i used SOH to find the hyponuse to be equal to 17.48 by dividing 8.74 by sin30
    is this the right answer?
    Last edited: Nov 11, 2007
  7. Nov 11, 2007 #6
    Why did you use 0m/s?
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