Vector addition angle calculation error

[Josh Eiland]

Homework Statement

In photo:
https://gyazo.com/6348b5ffe368852ee9868a2ba0dd7e38

Homework Equations

In image

The Attempt at a Solution

In the boxes

Where did I go wrong with the angle calculation?

Thanks

 

[paisiello2]

I don't know, you didn't show the Calc.

 

[Josh Eiland]

I don't know, you didn't show the Calc.

https://gyazo.com/efc009de2080feedea420d6b2a5edab4

Triangle on the right shows how I calculated -10° angle by using tan-1(-3.2/17.4), -3.2 being the change in y and 17.4 change in x after I added the vectors.

 

[paisiello2]

Can't follow your work. Your original post gives the answer as 350. How did you come up with this number?

 

[Josh Eiland]

Can't follow your work. Your original post gives the answer as 350. How v did you come up with this number?

I calculated the angle as -10°, but since the question asked for the angle counter-clockwise from the x axis, I thought it would be correctly expressed as 350.

 

[paisiello2]

-10 degrees measured from what?

 

[Josh Eiland]

-10 degrees measured from what?

From the positive x axis.

 

[Feodalherren]

Okay where did you get the -10 degrees from?

You can find the coordinate of the player's position through your vectors. After that you can simply use geometry to determine the ccw angle, after that, 360 - positive angle = clockwise angle. The program may not want to accept negative angles, try the positive equivalent between 0 and 360.

 

[Josh Eiland]

Okay where did you get the -10 degrees from?

You can find the coordinate of the player's position through your vectors. After that you can simply use geometry to determine the ccw angle, after that, 360 - positive angle = clockwise angle. The program may not want to accept negative angles, try the positive equivalent between 0 and 360.

https://gyazo.com/efc009de2080feedea420d6b2a5edab4

Triangle on the right shows how I calculated -10° angle by using tan-1(-3.2/17.4), -3.2 being the change in y and 17.4 change in x after I added the vectors.

 

[Josh Eiland]

https://gyazo.com/efc009de2080feedea420d6b2a5edab4

Triangle on the right shows how I calculated -10° angle by using tan-1(-3.2/17.4), -3.2 being the change in y and 17.4 change in x after I added the vectors.

And yes, that's how I got 350° counter-clockwise. By doing 360 minus the clockwise angle (360-10).

 

[paisiello2]

From the positive x axis.

I think if you draw yourself a picture you will see that it can't be -10 degrees from the +ve x-axis.

 

[Josh Eiland]

I think if you draw yourself a picture you will see that it can't be -10 degrees from the +ve x-axis.

I got -10 from the X-axis counter-clockwise when drawing a picture, as I've explained. Do you have a better answer or suggestion?

 

[paisiello2]

I could not understand your drawings. So my suggestion is to redraw a simple picture that summarizes the vectors and shows the -10 degrees.

 

[Feodalherren]

From this image, find the vector P.

 

[Josh Eiland]

From this image, find the vector P.

That wouldn't make sense, as the angle from initial to final points would be from the top of the red line on the y-axis to the final spot, not from the origin.

 

[Josh Eiland]

I could not understand your drawings. So my suggestion is to redraw a simple picture that summarizes the vectors and shows the -10 degrees.

I calculated the final point (17.4, -3.2) and drew a line from the origin (0,0) to this point, creating a triangle with the x and y segments. Using Pythagorean theorem I calculated the magnitude of this vector 17.7
I then used inverse tangent to find the angle -10°

 

[paisiello2]

Ok, I follow you now. I think you got it right. Maybe a small rounding error. Try and see if 349.6 works.

 

[Josh Eiland]

Ok, I follow you now. I think you got it right. Maybe a small rounding error. Try and see if 349.6 works.

Tried it, didn't work. I think the teacher probably just put the answer in wrong accidentally. Will ask tomorrow. Thanks for help.

 

[Feodalherren]

That is not the answer that I am getting at all.

I am getting a positive angle of 24.213 degrees counter-clockwise.

Again, if you're not showing your calculations it's impossible for us to tell you where you're going wrong.

Your y-coordinate is wrong. The point is in the first quadrant.

Bx=11.3137
By=11.3137
Cx=6.0622
Cy=3.5

Therefore the resultant vector is
r=[Bx+Cx,By-Cy]

 

[paisiello2]

I got the same answer as the OP.

 

[Feodalherren]

OP calculated his final Y-coordinate as a negative, it doesn't make sense from the given picture assuming that the Y-axis goes along vector A and the X-axis goes through the bottom corner.