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Homework Help: Finding Force with T and V given

  1. Jun 20, 2011 #1
    1. The problem statement, all variables and given/known data
    A child pushes a block of wood with a mass of 0.72kg across a smooth table. The block starts from a position of rest and after 2.0s it has a velocity of 1.6m/s [forward]. The coefficient of kinetic friction is 0.64.

    I have already determined that
    F[itex]_{net}[/itex]= 7.06N
    F[itex]_{f}[/itex]= 4.52 N

    Find the force with which the child actually pushes on the block of wood

    2. Relevant equations
    I have no clue what formula to use here in order to calculate this. Any sort of guidance be great! -Thanks

    3. The attempt at a solution
    I know that this has something to do with the velocity and it being 1.6m/s after 2.0 but I don't know even know where to being to find which formula to use!
  2. jcsd
  3. Jun 20, 2011 #2

    I like Serena

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    Hi mom2maxncoop! Welcome to PF! :smile:

    You have the force of friction right! :smile:

    Do you know the following formulas?
    v = v0 + a t
    Fresultant = m a

    Btw, what you have named Fnet is not the net force, but the force of gravity on the block, also called weight.

    From the first formula you can calculate the acceleration a.
    From the second formula you can calculate the resultant force on the block.
    And finally you can calculate the force that the child exerts.
  4. Jul 14, 2011 #3
    Hi, I'm new here. I just thought I'd hijack mom2maxncoop's thread and revive it because I'm having a little difficulty with the same question.

    Using the formula:

    F resultant = m a
    F resultant = (0.72kg) (0.8m/s^2)
    F resultant = 0.576N

    So, is this the net horizontal force exerted on the block of wood? If so, then I figured that the frictional force is:

    F friction = u Fn
    F friction = (0.64) (7.06N)
    F friction = 4.52N

    And so the applied force must be 4.52N + 0.58N = 5.1N

    Correct? Thanks in advance!
  5. Jul 14, 2011 #4

    I like Serena

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    Welcome to PF too, Seinfeld4! :smile:

    Yes, you have it entirely correct.
    Nice to see this thread had some use after all, since the OP never came back.
  6. Jul 14, 2011 #5
    Thanks for the quick reply, and thanks for the welcome!

    I have a feeling I'm going to be asking a lot more questions around here in the future. I feel a little bad though, because I'm just starting to learn about physics, so I'll be asking for help WAY more than I'll be dishing it out. Oh well, thanks again!
  7. Jul 14, 2011 #6

    I like Serena

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    Don't worry about it. :)
    The homework helpers don't help because of any expected return favor.
    Last edited: Jul 14, 2011
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