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Classical Mechanics Accelerated Frame/Rotation Problem

  1. Apr 4, 2010 #1
    1. The problem statement, all variables and given/known data

    Sally the physics student conducts the following experiment: There is a popular playground ride which is just a horizontal wooden disk free to rotate around a vertical axis. Sally hops onto the disk (spinning counter clock wise with angular velocity [tex]\omega[/tex] ) with a bunch of tennis balls. With practice, Sally discovers that she can throw a tennis ball such that the path of the ball passes through the axis and returns to her.

    Throughout this problem neglect air resistance and neglect the vertical component of the ball's velocity (ie pretend the ball's path is limited to a horizontal plane). You may also neglect any effects due to the rotation of the earth.

    A. Sketch a path of the ball r (t) in earth frame. Choose the coordinate system to coincide with axis of ride.

    (I think this is just a straight chord)

    B. Find an expression for r(t) in earth frame. Give an answer in terms of Sally's distance from the axis R and speed of the ball v. (Hint check that v(t) is constant)

    (I got just r(t) = R + vt) Too simple?

    C. Find the speed of the ball v in terms of R and [tex]\omega[/tex] assuming that the ball passes through the axis and returns to Sally.


    D. Transform the coordinates to find x'(t) and y'(t), the path of the ball in the rotating frame centered on the axis and co-spinning with the ride. Sketch the path of the ball in the rotating frame.

    (need the prior answer)

    E. Take the time derivative of r'(t) to find the components of the ball's velocity in the rotating frame. Check that v(t)=v'(t) + [tex]\omega[/tex] x r' (t).

    (need prior answer)

    2. Relevant equations

    a = a' + (omegadot) x r' + 2(omega) x v' + (omega) x (omega x r')
    v = v' + (omega) x r' + v0
  2. jcsd
  3. Apr 4, 2010 #2
    Find the initial velocity components of the tennis ball with respect to the earth's frame keeping in mind an initial tangential velocity component exists due to rotation.
  4. Apr 4, 2010 #3
    thanks! I'll try working it again.
  5. Apr 4, 2010 #4
    Okay so I got r_(t) = R + vt + Rwt
    which is good, because v_(t) = v+Rw

    Now how can I find the value for v in part C?
  6. Apr 4, 2010 #5
    Express the ball position in terms of unit vector components


    where A and B are the magnitudes, and let r(t) = 0 for the ball passing through the axis of rotation with respect to the earth's reference frame. This is one equation with two unknowns. Also, express Sally's position in terms of vector components. Then equate Sally's position to the position of the ball. This will give two equations with two unknowns. Think about how Sally has to release the ball. Does she aim directly at the center of rotation in order for the ball to pass through this center?
  7. Apr 4, 2010 #6
    Thanks for the help - I'll get back to that problem
  8. Apr 5, 2010 #7
    cool - i got v = 2rw, which makes enough sense to me.
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