Chapter 3 Describing Movement in Two Dimensions, 2-D Linear Kinematics
Chapter 3 Describing Movement in Two Dimensions, 2-D Linear Kinematics
3.3 Adding and Subtracting Vectors: Algebra
Authors: Paul Peter Urone, Roger Hinrichs
Adapted by: Rob Pryce, Alix Blacklin
Learning Objectives
By the end of this section, you will be able to:
- Understand the rules of vector addition and subtraction using analytical methods.
- Apply analytical methods to determine vertical and horizontal component vectors.
- Apply analytical methods to determine the magnitude and direction of a resultant vector.
Analytical methods of vector addition and subtraction employ geometry and simple trigonometry rather than the ruler and protractor of graphical methods. Part of the graphical technique is retained, because vectors are still represented by arrows for easy visualization. However, analytical methods are more concise, accurate, and precise than graphical methods, which are limited by the accuracy with which a drawing can be made. Analytical methods are limited only by the accuracy and precision with which physical quantities are known.
Resolving a Vector into Its Components
Analytical techniques and right triangles go hand-in-hand in biomechanics because (among other things) motions along perpendicular directions are independent. We very often need to separate a vector into its perpendicular components. For example, given a vector like [latex]\vec{A}[/latex] in Figure 3.24, we may wish to find which two perpendicular vectors, [latex]\vec{A_x}[/latex]) and [latex]\vec{A_y}[/latex], add to produce it.
[latex]\vec{A_x}[/latex] and [latex]\vec{A_y}[/latex] are defined to be the components of [latex]\vec{A}[/latex] along the x– and y-axes. The three vectors [latex]\vec{A}[/latex], [latex]\vec{A_x}[/latex], and [latex]\vec{A_y}[/latex] form a right triangle:
[latex]\vec{A_x} + \vec{A_y} = \vec{A}[/latex]
Note that this relationship between components of the vector and the resultant holds only for vector quantities (which include both magnitude and direction). The relationship does not apply for the magnitudes alone. For example, if [latex]\vec{A_x}=\text{3 m east}[/latex], [latex]\vec{A_y}=\text{4 m north}[/latex], and [latex]\vec{A}=\text{5 m north-east}[/latex], then it is true that the vectors [latex]\vec{A_x} + \vec{A_y} = \vec{A}[/latex]. However, it is not true that the sum of the magnitudes of the vectors is also equal. That is,
Thus,
[latex]\textit{A}_x + \textit{A}_y \ne \textit{A}[/latex]
If the vector [latex]\vec{A}[/latex] is known, then its magnitude [latex]\textit{A}[/latex] (its length) and its angle [latex]\theta[/latex] (its direction) are known. To find [latex]\vec{A_x}[/latex] and [latex]\vec{A_y}[/latex], its x- and y-components, we use the following relationships for a right triangle.
[latex]\vec{A_x}=A\phantom{\rule{0.25em}{0ex}}\text{cos}\phantom{\rule{0.25em}{0ex}}\theta[/latex]
and
Suppose, for example, that [latex]\vec{A}[/latex] is the vector representing the total displacement of the person walking in a city considere