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Homework Help: Air Friction

  1. Jan 23, 2009 #1
    1. The problem statement, all variables and given/known data

    Two identical small balls are shown in picture above. A ball is thrown from the ground and another ball is dropped without any initial velocity.
    First, consider no air friction. In order to make the balls collide each other, the ball on ground has to make an angle α from the ground, where tan(α)=h/d.
    Then consider a more real case, there's air friction which is proportional to ball's velocity, F=-kv. In order to make the balls collide, determine whether angle of the ball has to be greater than α, less than α, or equals to α!

    (Problem's source and image: http://collectionofphysicsproblems.blogspot.com" [Broken])

    2. Relevant equations

    Ffriction = -kv
    Fgravity = mg

    3. The attempt at a solution

    I think it has to be greater than α, is it correct?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jan 23, 2009 #2


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    Welcome to PF.

    F = -kv affects which ball the most at t = 0?
  4. Jan 24, 2009 #3
    The thrown ball, because it has initial velocity, and the other ball doesn't.
    How's my answer?
  5. Jan 24, 2009 #4


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    Consider both of the components of the velocity at the angle α then of the projectile. If the frictional retarding forces are slowing the projectile in both x,y what will happen if α is left the same? Will it undershoot or over shoot?
  6. Jan 24, 2009 #5
    Can we consider the initial condition only?
    There's also gravitational force and air friction on both balls.
    It's hard to imagine...
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