Physics Help Forum Angle of space motion and radial-velocity vector

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Oct 2nd 2018, 03:54 PM #1 Junior Member   Join Date: Sep 2018 Posts: 7 Angle of space motion and radial-velocity vector Hello So I am trying to figure out the angle of a space motion vector with its radial velocity vector. I have to sample equations, but despite plugging in the numbers I am not getting the same answers. So I have: tan-1((4.74d(u")/(vr)) With: d = 5.14 pc u" = 0.66092" vr = -26.3km s-1 so: tan-1((4.74x5.14(0.66092)/-26.3) = 1.1779 But the answer is supposed to -31.5 I tried convering parsecs and kilometers to meters, but still didn't get the right answer. What am I missing? Also, I am not entirely sure what the 4.74 number is, I can't find it in my text book. Thankyou in advance!
 Oct 3rd 2018, 12:50 PM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,347 It looks like you expressed your answer in radians, whereas the "correct " answer is in degrees.
 Oct 4th 2018, 10:33 AM #3 Junior Member   Join Date: Sep 2018 Posts: 7 Converting the anser to degrees would make it 67.488, Not -31.5
 Oct 4th 2018, 11:08 AM #4 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,347 4.74 x 5.14 x 0.66092 /( -26.3) = -0.612 tan-1(-0.612)= -31.5 degrees
 Oct 4th 2018, 02:28 PM #5 Junior Member   Join Date: Sep 2018 Posts: 7 So I have caught up with getting to -0.612 but when I do: tan-1(-0.612) The answer I am getting is +0.01068 When I convert that to degrees I get: +0.611 I am still not sure what I am doing wrong? Last edited by hbadgeress; Oct 4th 2018 at 02:31 PM.
Oct 4th 2018, 02:47 PM   #6
Physics Team

Join Date: Jun 2010
Location: Morristown, NJ USA
Posts: 2,347
 Originally Posted by hbadgeress but when I do: tan-1(-0.612) The answer I am getting is +0.01068
Remember that the notation

$\tan^{-1} (x)$

means arctangent of x. What you have apparently been calculating is the tangent of 0.612 degrees, but what you need to do instead is calculate the arctangent of 0.612, and express the answer in degrees.

 Tags angle, motion, radialvelocity, space, vector

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