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fluxer5
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Homework Statement
A scalar field is given by the function: ∅ = 3x2y + 4y2
a) Find del ∅ at the point (3,5)
b) Find the component of del ∅ that makes a -60o angle with the axis at the point (3,5)
Homework Equations
del ∅ = d∅/dx + d∅/dy
The Attempt at a Solution
I completed part a:
del ∅ = (6xy+4y2)[itex]\hat{i}[/itex] + (3x2+8y)[itex]\hat{j}[/itex] = 120[itex]\hat{i}[/itex] + 67[itex]\hat{j}[/itex] (this answer is correct)
I am having trouble with part b. My gut feeling is that I need to take a dot product; project the vector from 'part a' onto the vector that makes "a -60o angle with the axis."
Assuming the equation is |a||b|cosθ,
- I think a = 120[itex]\hat{i}[/itex] + 67[itex]\hat{j}[/itex]
- I'm not sure what b is. Maybe a unit vector?
- I'm thinking θ is 60o + atan(67/120)
Also, I'm assuming the "axis" that the problem refers to is the x-axis. The answer is -13.02.
Thank you for any help.
EDIT: Oops. 120i + 67j is not correct; it's 90i + 67j. |90i + 67j|cos(60+atan(67/90)) = -13.02
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