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Forces and vectors direction problem

  1. Sep 26, 2013 #1
    1. The problem statement, all variables and given/known data

    Two forces, vector F 1 = (4i+ 6j) N and vector F 2 = (4i+ 8j) N, act on a particle of mass 1.90 kg that is initially at rest at coordinates (+1.95 m, -3.95 m).

    a)What are the components of the particle's velocity at t = 10.3 s?

    b) In what direction is the particle moving at t = 10.3 s?

    (c) What displacement does the particle undergo during the first 10.3 s?

    (d) What are the coordinates of the particle at t = 10.3 s?

    2. Relevant equations

    3. The attempt at a solution
    For part a I tried dividing by the mass of the object to get Newtons to cancel to m/s^2
    I also tried adding the two forces together, and adding -F2 to +F1 because in my textbook it says that once force is basically the opposite of the other
    All of those were wrong
    For part b, I took the arctan of 1.95 and -3.95 to get 63.7° but webassign said I was around 10% off on my answer
    I havent been able to do c or d yet
    Any help or understanding as to what I did wrong would be greatly appreciated
  2. jcsd
  3. Sep 26, 2013 #2
    The net force is always the sum of all forces acting on a body, you don't have to worry about the directions vectors will take care of that themselves.
    Calculate the net force and use that to get acceleration and then the rest is easy just resolve everything into x and y components and use the kinematic equations.:wink:
  4. Sep 26, 2013 #3
    So your saying I just need to add the two forces together to get the components of the particles velocity?
    So Final F= (4i+6j)N + (4i+8j)N and that is what would be the answer for part a?
    I did that though. That was my first attempt at an answer
    And it came out wrong
    Maybe Im not understanding calculating the net force?
  5. Sep 26, 2013 #4
    Components of force ≠ Components of velocity.
    When you add the forces you get the net force which when you divide by mass you get the acceleration. Now you already know initial velocity is zero and you now have the acceleration too. So just use the kinematic equations.
  6. Sep 26, 2013 #5
    Ohhhh got it
    Duh haha
    Lets see if I can get it
  7. Sep 26, 2013 #6
    No :/
    Ok so I found the Final F to be (8.00i+16.00j)N
    Acceleration is [(8i+16j)N]/1.90kg
    Or [(8i+16j)m/s^2]/1.9

    Then I used the kinematic equation where Vf=Vi+at
    Because the Vi is 0 and I know the acceleration and time
    But its still wrong
  8. Sep 26, 2013 #7
    Check your net force.
  9. Sep 26, 2013 #8
    Please show us the details of your calculations to get the components of the velocity at 10.3 seconds. In your equation for the acceleration, why didn't you divide the 8 and the 16 by 1.9? Anyhow, the 16 should be a 14.
  10. Sep 27, 2013 #9
    Yeah I messed up with the addition, so its (8.00i+14.00j) right?
  11. Sep 27, 2013 #10
    In my calculations I did divide by 1.9
    And yeah i fixed the 16 to a 14
  12. Sep 27, 2013 #11
    Oh nevermind I got it
    Thanks for helping guys :)
  13. Sep 27, 2013 #12
    Actually quick question
    For c and d

    so c is asking for the displacement the particle undergoes during the first 10.3s?
    Ive tried using the answer from part a for this part but every variation i use is wrong
    The answer for a was (43.3i+75.9j) m/s
    What am i doing wrong?
  14. Sep 27, 2013 #13
    You can't use the velocity after 10.3 seconds to get the displacement after 10.3 seconds. That is because the velocity is changing during those 10.3 seconds. The velocity at time zero was zero. Show us the equations you are using to get the velocity and displacement. Show us your calculations in detail.

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