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Homework Help: Magnitude and direction of the net force vector

  1. Sep 16, 2010 #1
    1. The problem statement, all variables and given/known data
    The position r as a function of time of a 15.0 kg mass, moving
    in the x-y plane, is given by r(t) = (6.00t^3 x + 1.20t^4 y) meters where x and y are unit vectors. At t = 5.00 s, find the magnitude and direction of the net force vector
    which is causing this acceleration


    2. Relevant equations
    a=(Ax)x + (Ay)y +(Az)z
    and each variable after the () has a hat on them




    3. The attempt at a solution
    I dont know how to start the problem : /
     
  2. jcsd
  3. Sep 16, 2010 #2

    PhanthomJay

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    Are you familiar with calculus and acceleration as a function of displacement?
     
  4. Sep 16, 2010 #3
    Yes, this is going to sound strange but im really good at calculus but whhen it comes to physics im having troubles setting things up into the right format and i have no examples at all to follow. If i could just see a graph of what im looking at all would make sense
     
  5. Sep 16, 2010 #4

    PhanthomJay

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    In the Physics motion equations, the instantaneous velocity is the first derivative of the displacement vector with respect to time, and the acceleration vector is the 1st derivative of the velocity vector with respect to time. But then you need to know and apply which law to solve for the net force causing the acceleration?
     
  6. Sep 16, 2010 #5
    Im sorry i am really trying, I see the formula i know how to solve that..i just dont understand what variables go where. r(t) = (6.00t^3 x + 1.20t^4 y) leaves me very confused. i think its the powers that are throwing me off i cant find any examples like this
     
  7. Sep 16, 2010 #6

    PhanthomJay

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    The derivative of a sum is the sum of the derivatives of each term. Look at the x direction and y direction, separately. Those x and y's in the equation are confusing. they indicate the component directions of the displacemnt. I'll start you off with dr/dt in the x direction: d/dt(6t^3) =18t^2. Continue....
     
  8. Sep 16, 2010 #7
    (thank you so much for being patient)
    dr/dt in y direction d/dt(1.20t^4)=4.8t^3
    so now i have the derivative of the x direction and y direction. Where can i go from here? when will the mass and seconds come in play?
     
  9. Sep 16, 2010 #8

    PhanthomJay

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    That's the first derivative....now differentiate again to find the acceleration components....and their magnitude and direction at t=5 seconds...
     
  10. Sep 16, 2010 #9
    Okay! so now i have 14.4t^2 +36t has my acceleration equation! so my acceleration at 5 seconds is 540 m/s ?!? now how can i use this to find the magnitude and direction! finally im getting a breakthough(hopefully haha)
     
  11. Sep 16, 2010 #10

    PhanthomJay

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    Not quite. the acceleration is 14.4t^2 in the x direction and 36t in the ydirection. You have to look in each direction separately. That's what the 'hat' indicated in the original equation...x_hat is the displacement in the x direction, and y_hat is the displacement in the y direction. So you have to plug in t=5 for each component. You can't just add them because they are perpendicular. So what is the x component of acceleration and what is the y component of acceleration at t=5? And by the way, the acceleration has units of m/sec^2.
     
  12. Sep 16, 2010 #11
    Awesome(by the way go red soxs!) anyways haha so i have a 180 m/s^2 for the y axis and 360 m/s^2 for the x-axis! now do i do R=sqrt(Rx^2 + Ry^2) to find the magnitude and then tan(theta)=Ry/Rx for the direction or is this completely wrong? By the way thankyou so much your making a huge difference
     
  13. Sep 16, 2010 #12

    PhanthomJay

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    Yes, that is exactly right! But now once you get that result for the acceleration, you then need to find the net force causing that acceleration. The net force is always in the direction of the acceleration. What famous law would you use to calculate the Force, once you know the mass and acceleration? (Note: I figure if the Sox win their remaining 16 games and the Yanks or Rays lose half of their remaining games , we're in!).
     
  14. Sep 16, 2010 #13
    Newtons second law is net force= mass x acceleration... so i can use that to find the net force for x and y
    Fx=(10)(360)=3600
    Fy=(10)(180)=1800
    hmmm now im getting kinda lost
     
  15. Sep 16, 2010 #14

    PhanthomJay

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    Where'd you get the 10..the mass is given as 15 kg. Once you make that correction, you will end up with correct value of the x and y components of the net force (in units of Newtons), and then to get the resultant net force and direction, you can do that square root and tan theta thing you did for the acceleration. OR, you can calculate the net acceleration and direction as per your prior post, and then apply Newton 2 directly to that result to get the force and direction, which must be in the same direction as the acceleration. Either way is OK.
     
  16. Sep 16, 2010 #15
    oh wow haha i have no idea where i got the 10 i wrote the given wrong on my paper.
    so now i have
    Fx=5400
    Fy=2700
    SO!!!
    Magnitude of force=sqrt((5400)^2 + (2700)^2)=6037.38 N (ill do sig figs later!)

    Direction of force=Tan^-1(2700/5400)=26.56
     
  17. Sep 16, 2010 #16

    PhanthomJay

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    Looks good, nice effort!
     
  18. Sep 16, 2010 #17
    Seriously that was awesome. Thankyou so much! im going to work on a bunch of other problems! THANKYOU THANKYOU!!!!!!
     
  19. Sep 16, 2010 #18

    PhanthomJay

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    It was my pleasure. Wecome to PF! There are experts on this Forum in every area of science and math you can imagine, who are more than willing to assist as long as you show your relevant equations and an attempt at a solution. Come on back now, you hear?:smile:
     
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