Vectors
From Lm
Newtonian Physics by Benjamin Crowell
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Chapter 7 - Vectors
a / Vectors are used in aerial navigation.
Vectors
a / The x an y components of a vector can be thought of as the shadows it casts onto the x and y axes.
b / Self-check B.
Contents |
Vector Notation
The idea of components freed us from the confines of one-dimensional physics, but the component notation can be unwieldy, since every one-dimensional equation has to be written as a set of three separate equations in the three-dimensional case. Newton was stuck with the component notation until the day he died, but eventually someone sufficiently lazy and clever figured out a way of abbreviating three equations as one.
| (a) | overrightarrowFtextA on BoverrightarrowFtextB on A | stands for | beginmatrix FtextA on Bx FtextB on Ax FtextA on By FtextB on Ay FtextA on Bz FtextB on Az endmatrix |
| (b) | overrightarrowFtexttotaloverrightarrowF1overrightarrowF2ldots | stands for | beginmatrix Ftexttotalx F1xF2xldots Ftexttotaly F1yF2yldots Ftexttotalz F1zF2zldots endmatrix |
| (c) | overrightarrowafracDelta overrightarrowvDelta t | stands for | beginmatrix ax Delta vx Delta t ay Delta vy Delta t az Delta vz Delta t endmatrix |
Example (a) shows both ways of writing Newton's third law. Which would you rather write?
The idea is that each of the algebra symbols with an arrow
written on top, called a vector, is
actually an abbreviation for three different numbers, the
x, y, and z components. The three components are referred
to as the components of the vector, e.g., Fx is the x
component of the vector
. The notation with an arrow on top
is good for handwritten equations, but is unattractive in a
printed book, so books use boldface, F, to represent
vectors. After this point, I'll use boldface for vectors
throughout this book.
In general, the vector notation is useful for any quantity that has both an amount and a direction in space. Even when you are not going to write any actual vector notation, the concept itself is a useful one. We say that force and velocity, for example, are vectors. A quantity that has no direction in space, such as mass or time, is called a scalar. The amount of a vector quantity is called its magnitude. The notation for the magnitude of a vector A is |A|, like the absolute value sign used with scalars.
Often, as in example (b), we wish to use the vector notation to represent adding up all the x components to get a total x component, etc. The plus sign is used between two vectors to indicate this type of component-by-component addition. Of course, vectors are really triplets of numbers, not numbers, so this is not the same as the use of the plus sign with individual numbers. But since we don't want to have to invent new words and symbols for this operation on vectors, we use the same old plus sign, and the same old addition-related words like “add,” “sum,” and “total.” Combining vectors this way is called vector addition.
Similarly, the minus sign in example (a) was used to indicate negating each of the vector's three components individually. The equals sign is used to mean that all three components of the vector on the left side of an equation are the same as the corresponding components on the right.
Example (c) shows how we abuse the division symbol in a similar manner. When we write the vector Δ v divided by the scalar Δt, we mean the new vector formed by dividing each one of the velocity components by Δ t.
It's not hard to imagine a variety of operations that would combine vectors with vectors or vectors with scalars, but only four of them are required in order to express Newton's laws:
| operation | definition |
| texttextbfvectortexttextbfvector | Add component by component to make a new set of three numbers. |
| texttextbfvectortexttextbfvector | Subtract component by component to make a new set of three numbers. |
| texttextbfvectorcdottextscalar | Multiply each component of the vector by the scalar. |
| texttextbfvectortextscalar | Divide each component of the vector by the scalar. |
As an example of an operation that is not useful for physics, there just aren't any useful physics applications for dividing a vector by another vector component by component. In optional section 7.5, we discuss in more detail the fundamental reasons why some vector operations are useful and others useless.
We can do algebra with vectors, or with a mixture of vectors and scalars in the same equation. Basically all the normal rules of algebra apply, but if you're not sure if a certain step is valid, you should simply translate it into three component-based equations and see if it works.
Example 1: Order of addition
◊ If we are adding two force vectors, F+G, is it valid to assume as in ordinary algebra that F+G is the same as G+F?
◊ To tell if this algebra rule also applies to vectors, we simply translate the vector notation into ordinary algebra notation. In terms of ordinary numbers, the components of the vector F+G would be Fx+Gx, Fy+Gy, and Fz+Gz, which are certainly the same three numbers as Gx+Fx, Gy+Fy, and Gz+Fz. Yes, F+G is the same as G+F.
It is useful to define a symbol r for the vector whose components are x, y, and z, and a symbol Δr made out of Δ x, Δ y, and Δ z.
Although this may all seem a little formidable, keep in mind that it amounts to nothing more than a way of abbreviating equations! Also, to keep things from getting too confusing the remainder of this chapter focuses mainly on the Δ r vector, which is relatively easy to visualize.
self-check: Translate the equations vx=Δ x/Δ t, vy=Δ y/Δ t, and vz=Δ z/Δ t for motion with constant velocity into a single equation in vector notation. (answer in the back of the PDF version of the book)
Drawing vectors as arrows
A vector in two dimensions can be easily visualized by drawing an arrow whose length represents its magnitude and whose direction represents its direction. The x component of a vector can then be visualized as the length of the shadow it would cast in a beam of light projected onto the x axis, and similarly for the y component. Shadows with arrowheads pointing back against the direction of the positive axis correspond to negative components.
In this type of diagram, the negative of a vector is the vector with the same magnitude but in the opposite direction. Multiplying a vector by a scalar is represented by lengthening the arrow by that factor, and similarly for division.
self-check: Given vector Q represented by an arrow in figure c, draw arrows representing the vectors 1.5Q and -Q. (answer in the back of the PDF version of the book)
”Discussion Questions”
◊ Would it make sense to define a zero vector? Discuss what the zero vector's components, magnitude, and direction would be; are there any issues here? If you wanted to disqualify such a thing from being a vector, consider whether the system of vectors would be complete. For comparison, can you think of a simple arithmetic problem with ordinary numbers where you need zero as the result? Does the same reasoning apply to vectors, or not?
◊ You drive to your friend's house. How does the magnitude of your Δr vector compare with the distance you've added to the car's odometer?
c / Example 2.
d / Example 4.
Calculations With Magnitude and Direction
If you ask someone where Las Vegas is compared to Los Angeles, they are unlikely to say that the Δ x is 290 km and the Δ y is 230 km, in a coordinate system where the positive x axis is east and the y axis points north. They will probably say instead that it's 370 km to the northeast. If they were being precise, they might specify the direction as 38° counterclockwise from east. In two dimensions, we can always specify a vector's direction like this, using a single angle. A magnitude plus an angle suffice to specify everything about the vector. The following two examples show how we use trigonometry and the Pythagorean theorem to go back and forth between the x-y and magnitude-angle descriptions of vectors.
Example 2: Finding magnitude and angle from components
◊ Given that the Δ r vector from LA to Las Vegas has Δ x=290 km and Δ y=230 km, how would we find the magnitude and direction of Δr?
◊ We find the magnitude of Δ r from the Pythagorean theorem:
Failed to parse (unknown function\vc): \begin{align} |\Delta \vc{r}| &= \sqrt{\Delta x^2 + \Delta y^2} \\ &= 370\ text{km} \end{align}
We know all three sides of the triangle, so the angle θ can be found using any of the inverse trig functions. For example, we know the opposite and adjacent sides, so
Failed to parse (unknown function\degunit): \begin{align} \theta &= \tan^{-1}\frac{\Delta y}{\Delta x} \\ &= 38\degunit \qquad . \end{align}
Example 3: Finding components from magnitude and angle
◊ Given that the straight-line distance from Los Angeles to Las Vegas is 370 km, and that the angle θ in the figure is 38°, how can the xand y components of the Δ r vector be found?
◊ The sine and cosine of θ relate the given information to the information we wish to find:
Failed to parse (unknown function\vc): \begin{align} \cos \theta &= \frac{\Delta x}{|\Delta\vc{r}|} \\ \sin \theta &= \frac{\Delta y}{|\Delta\vc{r}|} \end{align}
Solving for the unknowns gives
Failed to parse (unknown function\vc): \begin{align} \Delta x &= |\Delta\vc{r}|\cos\theta \\ &= 290\ text{km} \qquad and\\ \Delta y &= |\Delta\vc{r}|\sin\theta \\ &= 230\ text{km} \qquad . \end{align}
The following example shows the correct handling of the plus
and minus signs, which is usually the main cause of mistakes.
Example 4: Negative components
◊ San Diego is 120 km east and 150 km south of Los Angeles. An airplane pilot is setting course from San Diego to Los Angeles. At what angle should she set her course, measured counterclockwise from east, as shown in the figure?
◊ If we make the traditional choice of coordinate axes, with x pointing to the right and y pointing up on the map, then her Δ x is negative, because her final x value is less than her initial x value. Her Δ y is positive, so we have
If we work by analogy with the previous example, we get
Failed to parse (unknown function\degunit): \begin{align} \theta &= \tan^{-1}\frac{\Delta y}{\Delta x} \\ &= \tan^{-1}(-1.25) \\ &= -51\degunit \qquad . \end{align}
According to the usual way of defining angles in trigonometry, a negative result means an angle that lies clockwise from the x axis, which would have her heading for the Baja California. What went wrong? The answer is that when you ask your calculator to take the arctangent of a number, there are always two valid possibilities differing by 180°. That is, there are two possible angles whose tangents equal -1.25:
Failed to parse (unknown function\degunit): \begin{align} \tan 129\degunit &= -1.25 \\ \tan -51\degunit &= -1.25 \end{align}
You calculator doesn't know which is the correct one, so it
just picks one. In this case, the one it picked was the
wrong one, and it was up to you to add 180° to it to
find the right answer.
”Discussion Question”
◊ In the example above, we dealt with components that were negative. Does it make sense to talk about positive and negative vectors?
e / Example 5.
f / Vectors can be added graphically by placing them tip to tail, and then drawing a vector from the tail of the first vector to the tip of the second vector.
Techniques for Adding Vectors
Addition of vectors given their components
The easiest type of vector addition is when you are in possession of the components, and want to find the components of their sum.
Example 5: Adding components
◊ Given the Δ x and Δ y values from the previous examples, find the Δ x and Δ y from San Diego to Las Vegas.
◊
Note how the signs of the x components take care of the
westward and eastward motions, which partially cancel.
Addition of vectors given their magnitudes and directions
In this case, you must first translate the magnitudes and directions into components, and the add the components.
Graphical addition of vectors
Often the easiest way to add vectors is by making a scale drawing on a piece of paper. This is known as graphical addition, as opposed to the analytic techniques discussed previously.
Example 6: LA to Vegas, graphically
◊ Given the magnitudes and angles of the Δ r vectors from San Diego to Los Angeles and from Los Angeles to Las Vegas, find the magnitude and angle of the Δ r vector from San Diego to Las Vegas.
◊ Using a protractor and a ruler, we make a careful scale drawing, as shown in figure h. A scale of 1 mm→ 2 km was chosen for this solution. With a ruler, we measure the distance from San Diego to Las Vegas to be 206 mm, which corresponds to 412 km. With a protractor, we measure the angle θ to be 65°.
Even when we don't intend to do an actual graphical calculation with a ruler and protractor, it can be convenient to diagram the addition of vectors in this way. With Δ r vectors, it intuitively makes sense to lay the vectors tip-to-tail and draw the sum vector from the tail of the first vector to the tip of the second vector. We can do the same when adding other vectors such as force vectors.
g / Example 6.
self-check:
How would you subtract vectors graphically?
(answer in the back of the PDF version of the book)
”Discussion Questions”
◊ If you're doing graphical addition of vectors, does it matter which vector you start with and which vector you start from the other vector's tip?
◊ If you add a vector with magnitude 1 to a vector of magnitude 2, what magnitudes are possible for the vector sum?
◊ Which of these examples of vector addition are correct, and which are incorrect?
-
Unit Vector Notation (optional)
When we want to specify a vector by its components, it can be cumbersome to have to write the algebra symbol for each component: Δ x= 290 km, Δ y=230 km A more compact notation is to write
Failed to parse (unknown function\vc): \Delta \vc{r} = (290\ text{km})\hat{\vc{x}} + (230\ text{km})\hat{\vc{y}} \qquad ,
where the vectors Failed to parse (unknown function\vc): \hat{\vc{x}} , Failed to parse (unknown function\vc): \hat{\vc{y}} , and Failed to parse (unknown function\vc): \hat{\vc{z}} , called the unit vectors, are defined as the vectors that have magnitude equal to 1 and directions lying along the x, y, and z axes. In speech, they are referred to as “x-hat” and so on.
A slightly different, and harder to remember, version of this notation is unfortunately more prevalent. In this version, the unit vectors are called Failed to parse (unknown function\vc): \hat{\vc{i}} , Failed to parse (unknown function\vc): \hat{\vc{j}} , and Failed to parse (unknown function\vc): \hat{\vc{k}}
Failed to parse (unknown function\vc): \Delta \vc{r} = (290\ text{km})\hat{\vc{i}} + (230\ text{km})\hat{\vc{j}} \qquad .
h / Component-by-component multiplication of the vectors in 1 would produce different vectors in coordinate systems 2 and 3.
Rotational Invariance (optional)
Let's take a closer look at why certain vector operations are useful and others are not. Consider the operation of multiplying two vectors component by component to produce a third vector:
As a simple example, we choose vectors P and Q to have
length 1, and make them perpendicular to each other, as
shown in figure i/1. If we compute the result of our new
vector operation using the coordinate system in i/2, we find:
The x component is zero because Px=0, the y component
is zero because Qy=0, and the z component is of course
zero because both vectors are in the x-y plane. However,
if we carry out the same operations in coordinate system
i/3, rotated 45 degrees with respect to the previous one, we find
The operation's result depends on what coordinate system we
use, and since the two versions of R have different
lengths (one being zero and the other nonzero), they don't
just represent the same answer expressed in two different
coordinate systems. Such an operation will never be useful
in physics, because experiments show physics works the same
regardless of which way we orient the laboratory building!
The useful vector operations, such as addition and
scalar multiplication, are rotationally invariant, i.e., come
out the same regardless of the orientation of the coordinate system.
Summary
Vocabulary
vector — a quantity that has both an amount (magnitude) and a direction in space
magnitude — the “amount” associated with a vector
scalar — a quantity that has no direction in space, only an amount
Notation
A — a vector with components Ax, Ay, and Az
— handwritten notation for a vector
|A| — the magnitude of vector A
r — the vector whose components are x, y, and z
Δr — the vector whose components are Δ x, Δ y, and Δ z
Failed to parse (unknown function\vc): \hat{\vc{x}} , Failed to parse (unknown function\vc): \hat{\vc{y}} , Failed to parse (unknown function\vc): \hat{\vc{z}}
— (optional topic) unit vectors; the vectors with magnitude 1 lying along the x, y, and z axes
Failed to parse (unknown function\vc): \hat{\vc{i}} , Failed to parse (unknown function\vc): \hat{\vc{j}} , Failed to parse (unknown function\vc): \hat{\vc{k}}
— a harder to remember notation for the unit vectors
Other Notation
displacement vector — a name for the symbol Δ r speed — the magnitude of the velocity vector, i.e., the velocity stripped of any information about its direction
Summary
A vector is a quantity that has both a magnitude (amount) and a direction in space, as opposed to a scalar, which has no direction. The vector notation amounts simply to an abbreviation for writing the vector's three components.
In two dimensions, a vector can be represented either by its two components or by its magnitude and direction. The two ways of describing a vector can be related by trigonometry.
The two main operations on vectors are addition of a vector to a vector, and multiplication of a vector by a scalar.
Vector addition means adding the components of two vectors to form the components of a new vector. In graphical terms, this corresponds to drawing the vectors as two arrows laid tip-to-tail and drawing the sum vector from the tail of the first vector to the tip of the second one. Vector subtraction is performed by negating the vector to be subtracted and then adding.
Multiplying a vector by a scalar means multiplying each of its components by the scalar to create a new vector. Division by a scalar is defined similarly.
i / Problem 1.
j / Problem 4.
Homework Problems
1. The figure shows vectors A and B. Graphically calculate the following:
A+B, A-B, B-A, -2B, A-2B
No numbers are involved.
2. Phnom Penh is 470 km east and 250 km south of Bangkok.
Hanoi is 60 km east and 1030 km north of Phnom Penh.
(a)
Choose a coordinate system, and translate these data into
Δ x and Δ y values with the proper plus and
minus signs.
(b) Find the components of the Δ r
vector pointing from Bangkok to Hanoi.(answer check available at lightandmatter.com)
3. If you walk 35 km at an angle 25° counterclockwise
from east, and then 22 km at 230° counterclockwise from
east, find the distance and direction from your starting
point to your destination.(answer check available at lightandmatter.com)
4. A machinist is drilling holes in a piece of aluminum
according to the plan shown in the figure. She starts with
the top hole, then moves to the one on the left, and then to
the one on the right. Since this is a high-precision job,
she finishes by moving in the direction and at the angle
that should take her back to the top hole, and checks that
she ends up in the same place. What are the distance and
direction from the right-hand hole to the top one?(answer check available at lightandmatter.com)
5. Suppose someone proposes a new operation in which a vector A and a scalar B are added together to make a new vector C like this:
Prove that this operation won't be useful in physics, because it's not
rotationally invariant.
