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Homework Help: Newton's second law with vectors

  1. Oct 8, 2014 #1
    1. The problem statement, all variables and given/known data
    A 9 kg object is subjected to two forces, [PLAIN]http://www.webassign.net/images/Farrowbolditalic.gif1 [Broken] = 7 N i - 4 N j and [PLAIN]http://www.webassign.net/images/Farrowbolditalic.gif2 [Broken] = 7 N i - 9 N j. The object is at rest at the origin at time t = 0.

    (a) What is the object's acceleration?

    (b) What is its velocity at time t = 14 s?

    (c) Where is the object at time t = 14 s?

    (All answers in unit vector notation)

    2. Relevant equations
    (a) Fnet=ma

    (b) V = V0 + at

    (c) X = X0 + V0t + 1/2at2

    3. The attempt at a solution
    (a) Fnet = F1 + F2 = 14i - 13j

    Fnet = ma -> a = (14i-13j)/9
    a = 1.56i - 1.44j (MARKED INCORRECT)

    Why is this incorrect? If a = F/m then all I should have to do is add up the forces to get Fnet.

    (b and c) I've used the equations above and got the answers wrong. Overall, I won't be able to get these answers correct unless I get the correct acceleration.
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Oct 8, 2014 #2


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    Probably because ##1.56\ne14/9## and ##-1.44\ne -13/9##. They are just decimal approximations to the fractions, which are the correct answers.
    Last edited by a moderator: May 7, 2017
  4. Oct 8, 2014 #3
    So you think that I was correct to add F1 and F2? If that is true I should just leave Fnet as 14/9i - 13/9j?
  5. Oct 8, 2014 #4


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    Yes. You could always ask your teacher, or if it's an online problem, try typing in the fractions. In that case be sure to use parentheses as in (14/9)i
  6. Oct 8, 2014 #5
    Alright, I will try that. Thank you very much. Are my equations for (b) and (c) okay? I just want to check because I've already submitted this question a few times.

    EDIT: Hey, I got it right! =D Fractions for life, haha. Thanks a lot!
    Last edited: Oct 8, 2014
  7. Oct 8, 2014 #6


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    Those equations (b and c) are valid only if you have constant acceleration. Do you have constant acceleration? If so, you can use them.
  8. Oct 8, 2014 #7
    Yes, I had constant acceleration. Thank you.
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