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Homework Help: Finding Magnitude of Velocity with Variables

  1. Oct 1, 2007 #1
    1. The problem statement, all variables and given/known data
    Romeo tosses a pebble at Juliet's window. Before crashing through the window the pebble is moving horizontally, having traveled a horizontal distance x and a vertical distance y. Find the magnitude and direction of the pebble's velocity as it leaves Romeo's hand.

    2. Relevant equations
    I know that Vx=VCosA and Vy=SinA.
    Also, t=VSinA/g

    3. The attempt at a solution
    The attempts are pretty much any relevent equations I can find. There's no real math, except I know that the initial Vy is 0. I also figured if the projectile was graphed it would be the right side of the vertex of the parabola. This is why t=VSinA/g. I do not know where to go from here though. I was thinking of using y=VsinA*t-.5g(t)^2. but I really don't know
  2. jcsd
  3. Oct 1, 2007 #2


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    Gold Member

    You know this -

    time to hit window, t = x/Vx where Vx is the x velocity

    Vy = gt, because Vy=0 at time t ( the sign may be wrong here, Vy is in the opposite direction to g)

    y = 1/2 gt^2, distance traveled vertically in time t.

    Some manipulation should give what you're looking for.
    Last edited: Oct 1, 2007
  4. May 7, 2008 #3
    I don't understand this. How can we figure out what t is. I do understand that y = .5gt^2. Just not the rest of it. Any help here would be appreciated!
  5. May 8, 2008 #4


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    Homework Helper

    Hi RedRumriots,

    Why do you think the Vy=0 initially? You do know that Vy is zero right before the stone hits the window; what does that tell you about that point of the trajectory?

    If Vy=0 initially, then this stone would never move upwards.
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