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Homework Help: Simple newton law problem

  1. Oct 4, 2009 #1
    1. The problem statement, all variables and given/known data
    An arrow, starting from rest, leaves the bow with a speed of 29.5 m/s. If the average force exerted on the arrow by the bow were doubled, all else remaining the same, with what speed would the arrow leave the bow?

    2. Relevant equations

    use a= v-vo/t and f=ma

    3. The attempt at a solution

    i tried to just multiply the speed by 2 but that's wrong... and the same speed is wrong too..
  2. jcsd
  3. Oct 4, 2009 #2
    F = ma
    If the force doubled, then the acceleration must double, because the mass definitely can't double xD

    So, that means the new acceleration is twice the initial acceleration. This doesn't mean the new velocity is twice the old velocity.

    First, let's find the initial acceleration.
    vf = vi + at
    29.5 = 0 + at
    ai = 59/t

    The new acceleration is twice the initial acceleration, so
    anew = (2*59)/t = 118/t
    anew = = 118/t

    Now, plugging that back into the equation:
    vf = vi + at
    vf = 29.5 + (118/t)*t ===> Note how the t cancels out
    vf = 29.5 + 118
    vf = 147.5 m/s
  4. Oct 4, 2009 #3
    thanks so much man
  5. Oct 5, 2009 #4


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Firecloak! Welcome to PF! :smile:

    Sorry, but your method is completely wrong …

    vi is still zero, not 29.5; and it isn't the same t. :redface:

    Hi ethrust2! :wink:

    The distance will be the same (exactly the same if it's a crossbow, and almost exactly if it's a longbow) …

    so … using work done and conservation of energy … how does v depend on F? :smile:
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