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Homework Help: Find the velocity as it reaches the water

  1. Sep 25, 2004 #1

    Q. A boy stands at the edge of a bridge 20.0 m above a river
    and throws a stone straight down with a speed of 12.0 m/s.
    He throws another pebble straight upward with the same
    speed so that it misses the edge of the bridge on the way
    back down and falls into the river. For each stone find (a)
    the velocity as it reaches the water and (b) the average
    velocity while it is in flight.

    Note: Ignore the affects of air resistance.

    Can't figure it out, please help this is due monday
  2. jcsd
  3. Sep 25, 2004 #2


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    First, please show us what you have done.
  4. Sep 25, 2004 #3
    SORRY , yeah i shud have written that i attempted it, cant figure out which formula to use , does it mean the initial velocity for both pebbles is 20 m/s and the distance is 20m
  5. Sep 25, 2004 #4
    a) Some hints: for the pebble thrown straight down, try to find a kinematic formula that relates initial velocity, final velocity, distance, and acceleration. For the stone thrown up, remember that when you throw something up vertically, assuming no air resistance, it returns to the ground with the same speed it was released with.
    b) This is uniformily accelerated motion: what formula is used for [itex]v_{av}[/itex]?
  6. Sep 25, 2004 #5
    okay thanks, i cant find a formula so do i take more than one equation to solve for unknowns? i dun get it
  7. Sep 26, 2004 #6
    for part b the asnwer id -12 m/s right negative cuz of the direction?
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