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Homework Help: Force and distance/velocity/acceleration

  1. Oct 30, 2008 #1
    1. The problem statement, all variables and given/known data
    A 2kg fan-cart is in linear motion (straight line on a very long track); it experiences 4N forward force due to the fan. Find the following assuming it is at rest initially:


    a. The acceleration:
    b. Its velocity 3 seconds after it starts
    c. Its velocity 5 seconds after it starts
    d. Its displacement between 3s and 5s
    e. Its displacement between 0s and 5s
    f. Its average velocity using the result from 4 for displacement

    2. Relevant equations



    3. The attempt at a solution


    I know it sounds like a lot and I need A LOT of help. Our professor explained nothing and just handed this to us and said go. We don't even have a text book, so help is really really needed. I am no where near being smart in physics.
     
  2. jcsd
  3. Oct 30, 2008 #2
    Attempt
    a.

    a = F/m
    a = 4N/2kg
    a=4kg*m/ss/2kg

    a=2m/s/s

    Is that right?
     
  4. Oct 30, 2008 #3
    acceleration = net force / mass
    acceleration = (final velocity - initial velocity) / timespan

    displacement = (final V + initial V) \ 2) x timespan

    With displacement, your initial velocity for "Its displacement between 3s and 5s" will not be 0.

    *also acceleration is 2 m / s^2, and you need that to plug into all the equations i listed, and interpret what the questions r asking you to do
     
  5. Oct 30, 2008 #4
    Can someone help me with a velocity formula? I keep looking for formulas but I can't really understand it.
     
  6. Oct 30, 2008 #5

    tiny-tim

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    Welcome to PF!

    Hi jammhawk! Welcome to PF! :smile:
    Yes, that's fine. :smile:

    In words, that means that the acceleration is constant.

    You know some formulas for constant acceleration, don't you? :wink:
     
  7. Oct 30, 2008 #6
    I have no clue what I am doing! I'm confused about all of this.

    Ok.

    I got
    b. 6 m/s
    c. 10 m/s
    but i have no clue how i got it. and its probably wrong
     
  8. Oct 30, 2008 #7

    tiny-tim

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    Hi jammhawk! :smile:

    Always show us how you got it (or how you think you got it)!

    Hint: what formulas do you know that involve constant acceleration? :smile:
     
  9. Oct 30, 2008 #8
    Basically in my head i said.
    Aceleration = 2 m/s/s
    Velocity = x m/s
    so maybe V= at
     
  10. Oct 30, 2008 #9
    And if those are true...

    displacement = .5(a)(t^2) + vinitial(t) + dinitial

    D = .5(2)(2^2) + 6(2) + 0
    D = 4 + 12 + 0
    D = 16

    d. 16?
     
  11. Oct 30, 2008 #10

    tiny-tim

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    No maybe about it! :biggrin:

    General formula v = u + at (and in this case u = 0).
    Also fine … excpet that you can scrub "+ dinitial" form the first line (though personally, I'd use vinitial = 0, and do two equations). :smile:
     
  12. Oct 30, 2008 #11
    ok so all answers are good so far.

    now how do i use the displacement of answer d (16) to find the average velocity?
     
  13. Oct 30, 2008 #12

    tiny-tim

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    Definitions:

    Speed = instantaneous distance-per-time

    Average speed = total distance per total time. :smile:
     
  14. Oct 30, 2008 #13
    so displacement = 16 meters
    total time = 2 seconds

    average velocity = 16 m/2s
    8 m/s

    right?
     
  15. Oct 30, 2008 #14

    tiny-tim

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    Yup! :biggrin:

    btw:
    but your professor did tell you all the equations you've just used, didn't he? :wink:

    Moral: physics is equations … just learn the equations, and practise choosing the right one to use. :smile:
     
  16. Oct 30, 2008 #15
    actually i found all these online through yahoo. our professor gives us an assignment just sets us free. i wish he was more hands on
     
  17. Oct 30, 2008 #16

    tiny-tim

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    ah … then I sympathise :redface:

    web research is not a reliable way of learning physics. :frown:
     
  18. Oct 30, 2008 #17
    i know... and I would like to thank you for the best help available!
     
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