a / Vectors are used in aerial navigation.

Chapter 7. Vectors

7.1 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.

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., F_x 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.

Quantities can be classified as vectors or scalars. In a phrase like “a \_\_\_\_\_ to the northeast,” it makes sense to fill in the blank with “force” or “velocity,” which are vectors, but not with “mass” or “time,” which are scalars. Any nonzero vector has both a direction and an amount. The amount is called its magnitude. The notation for the magnitude of a vector A is , 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 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:

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.

It is useful to define a symbol r for the vector whose components are x, y, and z, and a symbol 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 vector, which is relatively easy to visualize.

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.

This leads to a way of defining vectors and scalars that reflects how physicists think in general about these things:

For example, a force reverses itself under a 180-degree rotation, but a mass doesn't. We could have defined a vector as something that had both a magnitude and a direction, but that would have left out zero vectors, which don't have a direction. A zero vector is a legitimate vector, because it behaves the same way under rotations as a zero-length arrow, which is simply a dot.

A remark for those who enjoy brain-teasers: not everything is a vector or a scalar. An American football is distorted compared to a sphere, and we can measure the orientation and amount of that distortion quantitatively. The distortion is not a vector, since a 180-degree rotation brings it back to its original state. Something similar happens with playing cards, figure d. For some subatomic particles, such as electrons, 360 degrees isn't even enough; a 720-degree rotation is needed to put them back the way they were!

Discussion Questions

7.2 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 give 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 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 from the Pythagorean theorem:

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

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 x and y components of the vector be found?

◊ The sine and cosine of θ relate the given information to the information we wish to find:

Solving for the unknowns gives

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 example 2, we get

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:

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.

Example 5: A shortcut

◊ A split second after nine o'clock, the hour hand on a clock dial has moved clockwise past the nine-o'clock position by some imperceptibly small angle φ. Let positive x be to the right and positive y up. If the hand, with length ℓ, is represented by a vector going from the dial's center to the tip of the hand, find this vector's Δ x.

◊ The following shortcut is the easiest way to work out examples like these, in which a vector's direction is known relative to one of the axes. We can tell that will have a large, negative x component and a small, positive y. Since Δ x<0, there are really only two logical possibilities: either Δ x = -ℓ cosφ, or Δ x = -ℓ sinφ. Because φ is small, cosφ is large and sinφ is small. We conclude that Δ x = -ℓ cosφ.

A typical application of this technique to force vectors is given in example 6 on p. 216.

Discussion Question

7.3 Techniques for adding vectors

Vector addition is one of the three essential mathematical skills, summarized on pp.526-527, that you need for success in this course.

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 6: 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. In our San Diego-Los Angeles-Las Vegas example, we can simply string together the preceding examples; this is done on p. 527.

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. (It has nothing to do with x-y graphs or graph paper. “Graphical” here simply means drawing. It comes from the Greek verb “grapho,” to write, like related English words including “graphic.”)

Example 7: LA to Vegas, graphically

◊ Given the magnitudes and angles of the vectors from San Diego to Los Angeles and from Los Angeles to Las Vegas, find the magnitude and angle of the vector from San Diego to Las Vegas.

◊ Using a protractor and a ruler, we make a careful scale drawing, as shown in figure i. The protractor can be conveniently aligned with the blue rules on the notebook paper. A scale of 1 mm→ 2 km was chosen for this solution because it was as big as possible (for accuracy) without being so big that the drawing wouldn't fit on the page. 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°.

h / Example 7.

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 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.

Discussion Questions

7.4 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:

A more compact notation is to write

where the vectors , , and , 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 , , and :

7.5 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 j/1. If we compute the result of our new vector operation using the coordinate system in j/2, we find:

The x component is zero because P_x=0, the y component is zero because Q_y=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 j/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

Homework Problems

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.