Angle between a vector and the positive X axis

Hi all,

I wish to find the angle between a vector that has the equation y=m*x+c and the positive X axis. Is there a way in MATLAB that I can do it. I get the values for m and c using polyfit. The slope m doesnt always give the angle with reference to the positive X axis. I was wondering if there is an easier way in MATLAB to do this.

Your help is appreciated.

Answer 1

[Steve Eddins]

Try this:

atan(m)

If you need the answer in degrees, then:

atan(m) * 180 / pi

[Neena]

Hi Steve,

The reason why I want to know this is that I want to rotate the vector by the angle so that its direction is along the positive X axis. I use atan(m) for the analysis. But sometimes it gives me errorenous results. Take for instance this example. (I have given my code as well)

X=[0 -0.1705 -0.1630 -0.0060 -0.0308];
Y=[0 -1.0382 -2.2907 -3.2725 -3.6321];
 
x1=[X;Y];
poly=polyfit(x1(1,1:4),x1(2,1:4),1); %the vector is the line that fits the first four points
theta=atan(poly(1));
%this is the way I find the correct angle of rotation. 
sumx=sum(x1(1,:));
if (sumx>0)
theta=-theta;
else
theta=-(pi+theta);
end
%computation of rotation angle
rot=[cos(theta) -sin(theta); sin(theta) cos(theta)];
x2=rot*x1;
plot(x1(1,:),x1(2,:))
hold on
plot(x2(1,:),x2(2,:),'*r')

even though the vector points along the negative X direction, atan gives an angle estimate of about 20 degrees. Can you please tell me what I am doing wrong. I want x1 to point along the positive X axis after rotation.

Answer 2

[oblivious]

you might want to consider all possible combinations i.e. 1st and 2nd quadrant.

b=atan(m);

if b<0

ang=pi+b;

else ang=b;
end

[oblivious]

answer is in radian

[Neena]

See my comment to the above answer. Thanks for the help

Answer 3

[Walter Roberson]

Is this actually the same question as http://www.mathworks.co.uk/matlabcentral/answers/40497-error-with-atan ?

Answer 4

[Ryan seader]

You decide to find a resultant of more than just two forces by resolving each individual force into its rectangular components and then combining them to find the rectangular components of the resultant force. You do not want to use the triangle rule with the law of sines and cosines, because you would need to do it first for the first two vectors and then again to combine the third vector with the resultant of the first two. The following steps can be arranged to allow for the determination of the magnitude and direction of the resultant force of multiple forces using their rectangular components. Give the correct order in which to implement each step, assuming that you are going to calculate the magnitude of the resultant first before you calculate its direction.