Calculate angles correctly between two vectors using the dot product. (2024)

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LH on 4 Jan 2024

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Answered: Torsten on 4 Jan 2024

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Hi all,

As shown below in the figure, I have two vectors, Calculate angles correctly between two vectors using the dot product. (2)and Calculate angles correctly between two vectors using the dot product. (3), and I want to calculate the angle with the norm,Calculate angles correctly between two vectors using the dot product. (4) andCalculate angles correctly between two vectors using the dot product. (5), between each of these vectors with respect to vector L using the dot product.

Calculate angles correctly between two vectors using the dot product. (6)

Here is a simple code to calculate these angles:

close all;

clear all;

%define the reference vector

L = [-0.5 -0.5];

%v1

v1 = [-0.6788 0.3214];

%theta1 with the x axis

theta1 = acos((v1(1)*L(1)+v1(2)*L(2))/(sqrt(v1(1)^2+v1(2)^2)*sqrt(L(1)^2+L(2)^2)));

%theta1 with the norm

theta1norm = pi/2 - theta1;

%v2

v2 = [0.3214 -0.6788];

%theta2 with the x axis

theta2 = acos((v2(1)*L(1)+v2(2)*L(2))/(sqrt(v2(1)^2+v2(2)^2)*sqrt(L(1)^2+L(2)^2)));

%theta1 with the norm

theta2norm = pi/2 - theta2;

My question here is that the product produces both angles to be equal, i.e., Calculate angles correctly between two vectors using the dot product. (7). However, and as shwon in the figure, it is clear that Calculate angles correctly between two vectors using the dot product. (8), i.e., Calculate angles correctly between two vectors using the dot product. (9)and Calculate angles correctly between two vectors using the dot product. (10). How can I caulcate these angles correctly using the dot product?

Thanks.

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Dyuman Joshi on 4 Jan 2024

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"However, and as shwon in the figure, it is clear that Theta2n = Theta1N + pi/2"

How exactly is that clear or shown?

"How can I caulcate these angles correctly using the dot product?"

Utilize these functions - dot, norm

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Answers (3)

Hassaan on 4 Jan 2024

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% Close all figures and clear variables

close all;

clear all;

% Define the reference vector

L = [-0.5 -0.5];

% Define the first vector v1 and calculate the angle with L

v1 = [-0.6788 0.3214];

% Dot product of v1 and L

dot_v1_L = dot(v1, L);

% Norms of v1 and L

norm_v1 = norm(v1);

norm_L = norm(L);

% Angle theta1 with the x axis

theta1 = acos(dot_v1_L / (norm_v1 * norm_L));

% Angle theta1 with the norm

theta1norm = pi/2 - theta1;

% Define the second vector v2 and calculate the angle with L

% Dot product of v2 and L

dot_v2_L = dot(v2, L);

% Norms of v2

norm_v2 = norm(v2);

% Angle theta2 with the x axis

theta2 = acos(dot_v2_L / (norm_v2 * norm_L));

% Angle theta2 with the norm

theta2norm = pi/2 - theta2;

% Convert angles to degrees

theta1_degree = radtodeg(theta1);

theta2_degree = radtodeg(theta2);

theta1norm_degree = radtodeg(theta1norm);

theta2norm_degree = radtodeg(theta2norm);

% Display the results

disp(['Theta1: ', num2str(theta1_degree), ' degrees']);

Theta1: 70.3367 degrees

disp(['Theta2: ', num2str(theta2_degree), ' degrees']);

Theta2: 70.3367 degrees

disp(['Theta1 from the norm: ', num2str(theta1norm_degree), ' degrees']);

Theta1 from the norm: 19.6633 degrees

disp(['Theta2 from the norm: ', num2str(theta2norm_degree), ' degrees']);

Theta2 from the norm: 19.6633 degrees

The functions dot and norm are used to calculate the dot product and the magnitude of the vectors, respectively, and acos computes the arccosine of the given value to obtain the angle in radians. The function radtodeg converts the angle from radians to degrees.

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James Tursa on 4 Jan 2024

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Edited: James Tursa on 4 Jan 2024

Also see this related link for a robust method using atan2 that can recover small angles:

https://www.mathworks.com/matlabcentral/answers/101590-how-can-i-determine-the-angle-between-two-vectors-in-matlab?s_tid=srchtitle

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Torsten on 4 Jan 2024

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I think an intuitive way is to compute the angles between the positive x-axis and the respective vector counterclockwise first (the result will be between 0 and 360) and then make the necessary subtractions.

theta1 = cart2pol(L(1)/norm(L),L(2)/norm(L))*180/pi

if theta1 <=0

theta1 = 360 + theta1;

end

theta2 = cart2pol(v1(1)/norm(v1),v1(2)/norm(v1))*180/pi

if theta2 <=0

theta2 = 360 + theta2;

end

theta3 = cart2pol(v2(1)/norm(v2),v2(2)/norm(v2))*180/pi

if theta3 <=0

theta3 = 360 + theta3;

end

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Calculate angles correctly between two vectors using the dot product. (15)

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Calculate angles correctly between two vectors using the dot product. (2024)

FAQs

How do you find the distance between two vectors using the dot product? ›

d=∥∥∥∥PQ ∥∥∥∥cosθ. Now, multiply both the numerator and the denominator of the right hand side of the equation by the magnitude of the normal vector ⃗ n : ⃗ ∥ ∥ n ⃗ ⁡ θ ∥ n ⃗ d=\frac { \left\| \vec { PQ } \right\| \left\| \vec { n } \right\| \cos\theta }{ \left\| \vec { n } \right\| }.

What is the dot product of two vectors at right angles? ›

When two vectors are at right angles to each other the dot product is zero.

What is the answer when you take dot product between two vectors? ›

The dot product of two vectors is the product of the magnitude of each vector and the cosine of the angle between them: ⇀u⋅⇀v=‖⇀u‖‖⇀v‖cosθ.

What is the formula for the dot product of two vectors? ›

A dot product is the product of the magnitude of the vectors and the cos of the angle between them. a . b = |a| |b| cosθ. A vector product is the product of the magnitude of the vectors and the sine of the angle between them.

At what angle between the two vectors will the dot product be a maximum? ›

One way to calculate the dot product of two vectors is shown below. Now, it is easy to see that if the two vectors have identical directions, then the angle between the vectors is 0, so the dot product will be a maximum.

What is the rule for dot product? ›

Dot Product of Vectors

The scalar product of two vectors a and b of magnitude |a| and |b| is given as |a||b| cos θ, where θ represents the angle between the vectors a and b taken in the direction of the vectors.

What is the dot product of two vectors coordinates? ›

To take the dot product of two vectors a and b, we multiply the vectors' like coordinates and then add the products together. In other words, we multiply the x coordinates of the two vectors, then add this to the product of the y coordinates.

What does the dot product of two vectors measure? ›

The dot product, also called scalar product, is a measure of how closely two vectors align, in terms of the directions they point.

What is the angle between two vectors given that the dot product of the two vectors is zero? ›

⇒ θ = 90∘

How to find the angle between two vectors? ›

How to Find Angle Between Two Vectors? To find the angle between two vectors a and b, we can use the dot product formula: a. b = |a| |b| cos θ. If we solve this for θ, we get θ = cos-1 [ (a.

What does the dot product say about the angle between two vectors? ›

The dot product and orthogonality

cos(θ)>0 if θ is an acute angle,cos(θ)=0 if θ is a right angle, andcos(θ)<0 if θ is an obtuse angle, we see that for nonzero vectors u and v, u⋅v>0 if θ is an acute angle,u⋅v=0 if θ is a right angle, andu⋅v<0 if θ is an obtuse angle.

How do you show two vectors are parallel using dot product? ›

Two vectors a and b are said to be parallel if their cross product is a zero vector. i.e., a × b = 0. For any two parallel vectors a and b, their dot product is equal to the product of their magnitudes. i.e., a · b = |a| |b|.

What is the angle of the dot product of unit vectors? ›

The dot product of two unit vectors is the cosine of the angle between them. If they point in the same direction (i.e., they're the same vector), then that angle is 0, and their dot product is 1. If they're orthogonal (i.e., perpendicular), then that angle is 90°, and their dot product is 0.

What is the angle between two vectors if the ratio of their dot product? ›

The angle between the vectors is 30°.

What is the angle between axb and bxa? ›

(b) (A×B)and(B×A) are parallel and opposite to each other . So the angle will be π.

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