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Homework Help: I need help with physics webassign dealing with vectors and force with spring.

  1. Feb 12, 2007 #1
    1. The problem statement, all variables and given/known data
    1) Calculate the angle between the following vectors.
    A = 6.1 i + 4.6 j + 6.3 k and B = 8.2 i + 2.3 j - 7.0 k.

    2) What should be the spring constant k of a spring designed to bring a 1320 kg car to rest from a speed of 116 km/h so that the occupants undergo a maximum acceleration of 5.0 g?


    2. Relevant equations

    v^2=vo^2+2a(x-xo)
    F=-kx=ma

    3. The attempt at a solution

    1) For this one I don't know how to approach this so I found the force by multiplying it (6.1*8.2)i + (4.6*2.3)j+(6.3*-7.0)k = 16.5 N and I think I'm approaching this wrong.

    2) I converted the speed 116 km/h = 31 m/s and 5.0g=49.05 m/s^2.
    than used v^2=vo^2+2a(x-xo) to find the distance. x=(v^2-vo^2)/2a
    x=31^2/(2*49.05)=9.7961. Next, used F=-kx=ma so k=ma/x, which is 1320*49.05/9.7961=6609.36 N/m and I don't know why this isn't working.
     
  2. jcsd
  3. Feb 12, 2007 #2
    1) Would multiplying two vectors necessarily result in a force? This problem can be assigned to any two vectors, regardless of what they represent - i'd suppose that you may have access to a formula in your notes that relates the angle between vectors and the components you have at your disposal. (Well, related to operations you perform on those components: the dot product)

    For the second one, the equation involves the square initial and final velocities - you seem to have forgotten to square the 49ms^-2.
     
  4. Feb 12, 2007 #3
    1) I did 16.5/sqrt((6.1^2+4.6^2+6.3^2)(8.2^2+2.3^2+(-7^2))=712.93
    arccos(16.5/712.93)= 83.67 is that right??
    2) 49.05 is the acceleration and the final velocity will be 0 so i squared the initial velocity which is 31
     
  5. Feb 12, 2007 #4
    bump for help
     
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