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Homework Help: Projectile Motion marble Problem

  1. Sep 24, 2009 #1
    1. The problem statement, all variables and given/known data
    A marble rolls off a table with a velocity of 1.93 m/s [horiz]. The tabletop is 76.5 cm above the floor. If air resistance is negligible, determine the velocity at impact.
    -[tex]\Delta[/tex]t has been found in a previous question; =0.395 s
    -I know the answer is 4.33 m/s [63.5 degrees below the horizontal]
    2. Relevant equations

    3. The attempt at a solution
    0.765 = -.5(9.8)(.395)+v2(.395)
    but I get velocity to equal 1. something m/s?
  2. jcsd
  3. Sep 24, 2009 #2
    Do you want to exclusively use kinematics, or do you just want the answer? If you just want the answer, I say go the easy route: energy. I've said this before on this forum already.

    Be sure to only consider the y-axis energy.
  4. Sep 24, 2009 #3
    i have to do it the kinematics way..
    don't confuse me with the energy just yet! haha.
  5. Sep 24, 2009 #4
    Using kinematics makes it more confusing actually. =)

    But to answer your question, if you know the time it takes to fall, and you know what acceleration it goes through, it's a simple matter of [tex]v_f(t) = v_0 + a t[/tex]. Plug in initial velocity in y direction and time, then you have the y component of the velocity vector. Air resistance is nonexistent, so the horizontal velocity stays the same. Take the magnitude of this vector.

    I plugged in the numbers myself (using WolframAlpha!) and got the right answer.
  6. Sep 24, 2009 #5
    V2 is not final velocity, it's initial velocity and in the y direction it is equal to?
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