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Sum & Angle of Three Vectors (College Physics) 
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#1
Sep1709, 08:12 PM

P: 7

1. You are given three vectors of lengths A=65.7, B=38.4, and C=43.7. The angles are theta a=29.1 degree and theta b=57.2 degree, and C points along the negative yaxis. (a) Determine the length of the vector AB+C. (b) Calculate the angle of this vector
2. (a) Rx = AxBx+Cx Ry = AyBy+Cy (b) R=square root of Rx^2+Ry^2 3. Rx=(65.7cos29.1)(38.4cos57.2)+(43.7cos270) Rx=57.40720.802+0 Rx=36.605 Ry=(65.7sin29.1)(38.4sin57.2)+(43.7sin270) Ry=31.9532.2843.7 Ry= 44.03 R=square root of 36.605^2+(44.03)^2 R=57.26 The HW site says my answers are wrong. Please help me and let me know what I did wrong. My hw is due Saturday. 


#2
Sep1709, 08:22 PM

P: 21

Your formula appears to be prematurely subtracting a force that may not necessarily be an opposing force. 


#3
Sep1809, 07:33 PM

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P: 12,071

Welcome to PF, bebe087 and Furby



#4
Sep1809, 08:06 PM

P: 7

Sum & Angle of Three Vectors (College Physics)
We're subtracting because that's the hw question as it was written by the instructor. I added this course late and do not know how to add vectors. I've ordered the textbook, but it hasn't arrived yet.



#5
Sep1809, 08:38 PM

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P: 12,071

It may be that you gave an answer with too many significant figures (the original vector lengths were known to the nearest 0.1). Also, what did you get for the vector's angle? 


#6
Sep1809, 09:11 PM

P: 7

I entered 57.3 for part a and the answer was still wrong. I got 50.3°. Perhaps my calculations are wrong...do the equations look correct to you?
This is how I calculated the angle: tanθ=Ry/Rx tan−1(inverse tan)(Ry/Rx) 


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