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Homework Help: I need to figure this out: (A×B)⋅C

  1. Jan 20, 2015 #1
    1. The problem statement, all variables and given/known data
    Calculate (A×B)⋅C for the three vectors A with magnitude A = 5.00 and angle θA = 25.1∘ measured in the sense from the +x - axis toward the +y - axis, B with B = 4.18 and θB = 62.0∘, and C with magnitude C = 5.82 and in the +z - direction. Vectors A and B are in the xy-plane.

    2. Relevant equations
    Magnitude of (AXB)=A*B*sin(theta)
    Magnitude of (A dot B)=A*B*cos(theta)
    3. The attempt at a solution
    To be quite honest, I wasn't sure where to start. I know that A and B are 36.9 degrees apart and C is perpendicular to both of them, but I don't know where to go after this...Please help me!
  2. jcsd
  3. Jan 20, 2015 #2

    Quantum Defect

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    Remember that the cross product gives you a vector, and remember to keep your left hand in your pocket!
  4. Jan 20, 2015 #3
    I think I am supposed to use trig to figure out the vector components, but I never got the hang of trigonometry...Could I get help with solving?
  5. Jan 20, 2015 #4

    Quantum Defect

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    The equation that you give for the cross product is correct for the magnitude. (theta = angle between vectors A and B). The result of a cross product is a vector, however, which has magnitude and direction.
  6. Jan 20, 2015 #5
    I'll try to figure out the vectors for each...it might take a while.
  7. Jan 20, 2015 #6


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    Staff: Mentor

    Do you know how to convert from the "polar" representation of vectors (which is what you were given) to the "rectangular" representation? You use the rectangular representation to do the cross product more easily...
  8. Jan 20, 2015 #7


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    No, that's quite unnecessary here. Much easier to stay in polar for this one.
    Right, so what is theta in that cross product equation?
    The next challenge is to figure out the direction of AxB. There are two parts to this.
    In general, if I take the cross product of two vectors to get a third, a x b = c, what is the angle between a and c?
    The second part is to get the sign right. This is tricky because a x b is equal and opposite to b x a. As Quantum Defect indicated, you need to use the 'right hand rule'. (This is because the answer is governed by a convention and is part of the definition of cross product. It could have been defined the other way and all of the mathematics would still work, but it's necessary to have an agreed definition and the right hand rule is it.)
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