 Physics Help Forum What is the correct formula to calculate the angle of vector A = 0i + 1j
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 General Physics General Physics Help Forum Aug 1st 2019, 06:25 PM #1 Junior Member   Join Date: Aug 2019 Posts: 3 What is the correct formula to calculate the angle of vector A = 0i + 1j I know the angle is 90 degrees. and the formula to calculate the angle of vector A is inverse tan of (Ay / Ax), but Excel returns an error message because Ax is zero. Thanks.   Aug 1st 2019, 06:52 PM   #2

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 Originally Posted by Joseph3 I know the angle is 90 degrees. and the formula to calculate the angle of vector A is inverse tan of (Ay / Ax), but Excel returns an error message because Ax is zero. Thanks.
It has to do with, as you say, the 90 degree angle.

Try this:
$\displaystyle \lim_{A_x \to 0^-} tan^{-1} \left ( \dfrac{A_y}{A_x} \right )$

and
$\displaystyle \lim_{A_x \to 0^+} tan^{-1} \left ( \dfrac{A_y}{A_x} \right )$

What do these two limits tell you?

-Dan
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See the forum rules here.   Aug 2nd 2019, 12:43 AM #3 Junior Member   Join Date: Aug 2019 Posts: 3 Ok, I got it figured out with a little help from Microsoft. For a vertical vector, Ax = 0, you can't use ATAN(Ay/Ax) to calculate the inverse tan, you have to use ATAN2(Ax, Ay) - the arguments are different. Thanks for responding.   Aug 2nd 2019, 03:04 AM #4 Senior Member   Join Date: Jun 2016 Location: England Posts: 1,069 ATAN vs ATAN2 Plotting the TAN function shows why there is a problem with ATAN. However the nature of TAN=SIN/COS means that practical problems often come in the form Q=ATAN(X/Y). By looking at the relative signs of X and Y (and watching for zeros at 90deg) and using lots of IFs A full 360deg solution can be derived. Happily the ATAN2 function does all this for you... __________________ ~\o/~   Aug 2nd 2019, 07:20 PM #5 Junior Member   Join Date: Aug 2019 Posts: 3 I've found another way to go all around from -360 to 360 without using the Excel function ATAN2. Instead of ATAN2(x,y), I use ATAN(y / (sqrt(x^2+y^2) + x)) and multiply by 2 to get theta. This avoids the infinities. Last edited by Joseph3; Aug 2nd 2019 at 07:23 PM.  Tags angle, calculate, correct, formula, vector Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post hbadgeress Kinematics and Dynamics 5 Oct 4th 2018 02:47 PM Ram1590 New Users 2 Apr 18th 2018 04:48 AM epson Theoretical Physics 0 Apr 14th 2013 07:12 AM SpeedBirdNine Kinematics and Dynamics 3 Sep 16th 2011 08:23 AM Paul46 Kinematics and Dynamics 6 Oct 31st 2009 12:11 PM