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Homework Help: Motion in a Plane (Graphing Projectile Motion)

  1. Oct 26, 2009 #1
    1. The problem statement, all variables and given/known data

    Suppose you hit a baseball with an initial velocity of 35m/s at an angle of 52'.

    a. Find the direction and magnitude of its velocity 2.5s after you hit the ball.
    b. Find the postition vector for the position of the ball 2.5s after it was hit.

    2. Relevant equations

    The equations of motion, gravity(-9.81)

    3. The attempt at a solution

    I'm not sure whether i have to find the vertical velocity component or the horizontal velocity component. I think you have to use sin and cos to find either one but not really sure.
  2. jcsd
  3. Oct 26, 2009 #2


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    Staff: Mentor

    Welcome to the PF. In this type of kinematics problem (very common, BTW), you will write two sets of equations. One set of equations is for the horizontal motion, where there is no accelertion assuming no air resistance, so the horizontal component of velocity is constant. In the vertical direction, you have the acceleration of gravity pointing down, creating a force on the projectile. So you use the kinematic equations of motion, with the value of g as the acceleration.

    Does that help? Start writing out the equations, and if you have trouble with the trig functions, just consult your textbook or www.wikipedia.org
  4. Oct 26, 2009 #3
    I believe you are in the wrong forum for this, this is introductory physics. But...

    You need to find both components, vertical and horizontal. You will need to use sine and cosine, but that depends on where the angle is being measured (just remember SOH CAH TOA[tex]\rightarrow[/tex]Sine Opposite over Hypotenuse, Cosine Adjacent over Hypotenuse, Tangent Opposite over Adjacent)
  5. Oct 26, 2009 #4


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    Staff: Mentor

    (Thread moved to Intro Physics)
  6. Oct 27, 2009 #5

    you MUST find the components of the velocity... 'cause that's what projectile motions are all about! :)

    so it should be:

    vertical component = 35sin52
    horizontal component = 35cos52

    trick: draw out the picture and you'll see why you should use sin and cos. =)
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