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Homework Help: Conceptual - Work

  1. Feb 27, 2010 #1
    Conceptual ---- Work

    1. The problem statement, all variables and given/known data

    In general, the work done by a force F_vec is written as

    [itex] W=\int_{\rm i}^{\rm f} \vec{F}(\vec{r})\cdot d\vec{r}. [/itex]

    Now, consider whether the following statements are true or false:


    1) The dot product assures that the integrand is always nonnegative.
    2) The dot product indicates that only the component of the force perpendicular to the
    path contributes to the integral.
    3) The dot product indicates that only the component of the force parallel to the path
    contributes to the integral.


    2. Relevant equations

    n/a

    3. The attempt at a solution

    To be honest i have no idea about these. My professor doesnt teach, my book (which i just looked through for about 15 mins) doesnt talk about this anywhere, and yet it is on my homework :(

    This is what i was thinking but i am really again not sure.

    1) true
    2) false
    3) true

    Thanks for any help.
     
  2. jcsd
  3. Feb 27, 2010 #2
    Re: Conceptual ---- Work

    Write (Provide) vector definition of the dot product.
     
  4. Feb 27, 2010 #3

    PhanthomJay

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    Re: Conceptual ---- Work

    By guessing or otherwise, you got two of them right, and one of them wrong. If a particle moves to the right and the force acting on it is to the right, is the work done by the force positive or negative? If the particle moves to the right and the force acting on it is to the left, is the work done by the force positive or negative?
     
  5. Feb 27, 2010 #4
    Re: Conceptual ---- Work

    This is what i found in my book.

    Scalar product -> A . B = ABcos(theta)
     
  6. Feb 27, 2010 #5
    Re: Conceptual ---- Work

    moves to the right and force acting on it is to the right then work is POSITIVE.

    moves to the right and force acting on it is to the left then work is NEGATIVE

    so..... is this saying that 1) is actually false because it can be negative?
     
  7. Feb 28, 2010 #6

    PhanthomJay

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    Re: Conceptual ---- Work

    yes....
    yes....
    I am hesitating to give answers to true-false questions, so it is up to you to decide....:smile:
     
  8. Feb 28, 2010 #7
    Re: Conceptual ---- Work

    Thank you :)
     
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