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Physics 11 vector question ?

  1. Feb 14, 2009 #1
    1. The problem statement, all variables and given/known data

    A stone is thrown at 30 m/s. what is the widest river it can be thrown across? (the stones ends at the same vertical height from where it was thrown)

    2. Relevant equations

    x=vxot
    y=vyot + 0.5gt2

    3. The attempt at a solution
    i know i need to find t first. using the second equation i got t as 5.2s. when i multiplied 5.2s by vxo (30m/s) i got 156.3 m but the answer should be 92m. wat did i do wrong?
     
  2. jcsd
  3. Feb 14, 2009 #2

    LowlyPion

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    You can't just use the V as Vx.

    Vo is constrained to be 30 m/s.

    I rather think they want you to derive the Range Equation and determine where range reaches its maximum.

    Using your equations then, and

    Vox = Vo*Cosθ

    and

    Voy = Vo*Sinθ

    derive an expression for X, total distance, and then determine at what angle of launch you get the maximum.

    Along the way you might find this useful ... 2*Cosθ*Sinθ = sin(2θ)
     
  4. Feb 14, 2009 #3
    well when i used x=vxot
    i put in (30cos45)(5.2s) but i got 82m instead of 92 m
     
  5. Feb 14, 2009 #4

    LowlyPion

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    You maybe want to recalculate.

    I get a different time which is less and the answer given - after rounding.
     
  6. Feb 15, 2009 #5
    how did u get ur time?
    i cant seem to get any other answer than what i got..
     
  7. Feb 15, 2009 #6

    LowlyPion

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    v = g*t

    t = v/g

    That's time to max. Double it to come back down.

    v = 30*sin45 = 30*√2/2

    30*√2/2*2/g = 30*√2/g = ... ?
     
    Last edited: Feb 15, 2009
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