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Homework Help: Kinematics of thrown chalk - find time to hit the floor

  1. Jul 3, 2005 #1
    Using only one equation of motion, how will you solve this:
    Suppose a the tteacher's hand is 1m from the floor, and the teacher flicked a chalk upwards at 2m/s. How long will it take for the chalk to reach the floor?

    I can't seem to use only one equation, it's always two equations or three. :cry:
  2. jcsd
  3. Jul 3, 2005 #2
    Its simple.If the teacher throws the chalk upwards with velocity 2 m/s , then on downward motion, while at the same point on its return motion , its velocity would be 2 m/s . Now apply v=u+at .

  4. Jul 3, 2005 #3


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    But to find the time at which it hit the floor you would have to use a second "equation of motion" and apparently this question requires only one.

    I would vote for d= -(1/2)gt2+ 2t+ 1 where d is the height above the floor. Set d= 0 and solve for t.
  5. Jul 16, 2005 #4


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    Homework Helper

    Take h= -1m(downward), u = +2 m/s (upward) and "g" negative(downward) and solve eq h = ut + (1/2)gt^2.
    it is quadratic in t gives two values of t, the positive value is the answer.
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