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Homework Help: What equation to use for projectile motion problems

  1. Apr 5, 2009 #1
    I was given a list of equations for projectile motion and two of the equations have the same variables but give different outputs. I don't understand when to use one equation and when to use the other. The equations are:

    ay=(vfy-viy)/(delta t)


    (vfy)^2= (viy)^2 + 2ay(delta t)

    I tried using the second equation to solve the following problem but got it wrong. In the book they used the first equation. I see how using the first equation makes sense now, but why is it wrong to use the second equation?

    A marble rolls off a table at the horizontal velocity of 1.93 m/s. The tabletop is 76.5 cm above the floor. If air resistance is negligible, determine the velocity at impact.

    I solved for (delta t) and got 0.4s. I rearranged the second equation to find (vfy) and tried to solve using (viy)=0m/s, ay= 9.8 m/s^2, and (delta t)= 0.4s . I found (vfy) to be 2.8 m/s when its supposed to be 3.9 m/s
    Last edited: Apr 5, 2009
  2. jcsd
  3. Apr 5, 2009 #2
    Recheck your second equation. Are you sure 'delta t' isn't 'delta y'?
  4. Apr 5, 2009 #3
    They have (delta t) written in the textbook
  5. Apr 5, 2009 #4
    Everything looks good except I think your second equation isn't quite right. 0.4s is correct. What textbook is this?
  6. Apr 6, 2009 #5


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    Staff: Mentor

    The second equation should definitely have [itex]\Delta y[/itex] instead of [itex]\Delta t[/itex]. If your book has [itex]\Delta y[/itex] there, it's a misprint.
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