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APEX Calculus

Gregory Hartman, Ph.D., Sean Fitzpatrick, Ph.D. (Editor), Alex Jordan, Ph.D. (Editor), Carly Vollet, M.S. (Editor)

Section 11.3 The Dot Product

The previous section introduced vectors and described how to add them together and how to multiply them by scalars. This section introduces a multiplication on vectors called the dot product.
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Figure 11.3.1. Video introduction to Section 11.3
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Definition 11.3.2. Dot Product.

  1. Let u→=⟨u1,u2⟩ and v→=⟨v1,v2⟩ in R2. The dot product of u→ and v→, denoted u→⋅v→, is
    u→⋅v→=u1v1+u2v2.
  2. Let u→=⟨u1,u2,u3⟩ and v→=⟨v1,v2,v3⟩ in R3. The dot product of u→ and v→, denoted u→⋅v→, is
    u→⋅v→=u1v1+u2v2+u3v3.
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Note how this product of vectors returns a scalar, not another vector. We practice evaluating a dot product in the following example, then we will discuss why this product is useful.
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Example 11.3.3. Evaluating dot products.

  1. Let u→=⟨1,2⟩, v→=⟨3,−1⟩ in R2. Find u→⋅v→.
  2. Let x→=⟨2,−2,5⟩ and y→=⟨−1,0,3⟩ in R3. Find x→⋅y→.
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Solution 1.
  1. Using Definition 11.3.2, we have
    u→⋅v→=1(3)+2(−1)=1.
  2. Using the definition, we have
    x→⋅y→=2(−1)−2(0)+5(3)=13.
Solution 2. Video solution
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The dot product, as shown by the preceding example, is very simple to evaluate. It is only the sum of products. While the definition gives no hint as to why we would care about this operation, there is an amazing connection between the dot product and angles formed by the vectors. Before stating this connection, we give a theorem stating some of the properties of the dot product.
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The last statement of the theorem makes a handy connection between the magnitude of a vector and the dot product with itself. Our definition and theorem give properties of the dot product, but we are still likely wondering “What does the dot product mean?” It is helpful to understand that the dot product of a vector with itself is connected to its magnitude.
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Figure 11.3.5. Video presentation of Theorem 11.3.4
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The next theorem extends this understanding by connecting the dot product to magnitudes and angles. Given vectors u→ and v→ in the plane, an angle θ is clearly formed when u→ and v→ are drawn with the same initial point as illustrated in Figure 11.3.6.(a). (We always take θ to be the angle in [0,π] as two angles are actually created.)
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The angle formed by two vectors.
Image shows the angle θ formed by two vectors, u→ and v→. The angle is formed from u→ to v→.
(a)
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Figure 11.3.6. Illustrating the angle formed by two vectors with the same initial point
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The same is also true of 2 vectors in space: given u→ and v→ in R3 with the same initial point, there is a plane that contains both u→ and v→. (When u→ and v→ are co-linear, there are infinitely many planes that contain both vectors.) In that plane, we can again find an angle θ between them (and again, 0≤θ≤π). This is illustrated in Figure 11.3.6.(b).
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The following theorem connects this angle θ to the dot product of u→ and v→.
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Figure 11.3.8. Video presentation of Theorem 11.3.7
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Using Theorem 11.3.4, we can rewrite this theorem as
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u→‖u→‖⋅v→‖v→‖=cos⁡(θ).
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Note how on the left hand side of the equation, we are computing the dot product of two unit vectors. Recalling that unit vectors essentially only provide direction information, we can informally restate Theorem 11.3.7 as saying “The dot product of two directions gives the cosine of the angle between them.”
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When θ is an acute angle (i.e., 0≤θ<π/2), cos⁡(θ) is positive; when θ=π/2, cos⁡(θ)=0; when θ is an obtuse angle (π/2<θ≤π), cos⁡(θ) is negative. Thus the sign of the dot product gives a general indication of the angle between the vectors, illustrated in Figure 11.3.9.
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Image shows the relation between the angle between two vectors and the sign of their dot product.
Each image has the two vectors u→ and v→ along with θ shown. The dot product along with three possible cases of <, > and equal to 0 are shown. In the first image, the dot product has a positive value, the angle between the two vectors is acute. In the second image, the dot product is equal to 0, the angle is 90 degrees. In the third image, the dot product has a negative value and the angle is obtuse.
Figure 11.3.9. Illustrating the relationship between the angle between vectors and the sign of their dot product
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We can use Theorem 11.3.7 to compute the dot product, but generally this theorem is used to find the angle between known vectors (since the dot product is generally easy to compute). To this end, we rewrite the theorem’s equation as
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cos⁡(θ)=u→⋅v→‖u→‖‖v→‖⇔θ=cos−1⁡(u→⋅v→‖u→‖‖v→‖).
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We practice using this theorem in the following example.
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Example 11.3.10. Using the dot product to find angles.

Let u→=⟨3,1⟩, v→=⟨−2,6⟩ and w→=⟨−4,3⟩, as shown in Figure 11.3.11. Find the angles α, β and θ.
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Graph shows three vectors and the angles between them.
The x axis is drawn from −4 to 4 and the y axis is drawn from 0 to 6. The u→, v→ and w→ are shown.
The u→ starts at the origin and ends at point (3,1). The v→ also starts at the origin and ends at point (−2,6) and w→ is also drawn from the origin and ends at point (−4,3).
The angle α is drawn between u→ and v→, angle β is drawn between v→ and w→ and angle θ is drawn between u→ and w→.
Figure 11.3.11. Vectors used in Example 11.3.10
Solution 1.
We start by computing the magnitude of each vector.
‖u→‖=10;‖v→‖=210;‖w→‖=5.
We now apply Theorem 11.3.7 to find the angles.
α=cos−1⁡(u→⋅v→(10)(210))=cos−1⁡(0)=π2=90∘.
β=cos−1⁡(v→⋅w→(210)(5))=cos−1⁡(261010)≈0.6055≈34.7∘.θ=cos−1⁡(u→⋅w→(10)(5))=cos−1⁡(−9510)≈2.1763≈124.7∘
Solution 2. Video solution
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We see from our computation that α+β=θ, as indicated by Figure 11.3.11. While we knew this should be the case, it is nice to see that this non-intuitive formula indeed returns the results we expected.
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We do a similar example next in the context of vectors in space.
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Example 11.3.12. Using the dot product to find angles.

Let u→=⟨1,1,1⟩, v→=⟨−1,3,−2⟩ and w→=⟨−5,1,4⟩, as illustrated in Figure 11.3.13. Find the angle between each pair of vectors.
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Figure 11.3.13. Vectors used in Example 11.3.12
Solution.
  1. Between u→ and v→:
    θ=cos−1⁡(u→⋅v→‖u→‖‖v→‖)=cos−1⁡(0314)=π2.
  2. Between u→ and w→:
    θ=cos−1⁡(u→⋅w→‖u→‖‖w→‖)=cos−1⁡(0342)=π2.
  3. Between v→ and w→:
    θ=cos−1⁡(v→⋅w→‖v→‖‖w→‖)=cos−1⁡(01442)=π2.
While our work shows that each angle is π/2, i.e., 90∘, none of these angles looks to be a right angle in Figure 11.3.13. Such is the case when drawing three-dimensional objects on the page.
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All three angles between these vectors was π/2, or 90∘. We know from geometry and everyday life that 90∘ angles are “nice” for a variety of reasons, so it should seem significant that these angles are all π/2. Notice the common feature in each calculation (and also the calculation of α in Example 11.3.10): the dot products of each pair of angles was 0. We use this as a basis for a definition of the term orthogonal, which is essentially synonymous to perpendicular.
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Definition 11.3.14. Orthogonal.

Nonzero vectors u→ and v→ are orthogonal if their dot product is 0.
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Example 11.3.15. Finding orthogonal vectors.

Let u→=⟨3,5⟩ and v→=⟨1,2,3⟩.
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  1. Find two vectors in R2 that are orthogonal to u→.
  2. Find two non-parallel vectors in R3 that are orthogonal to v→.
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Solution 1.
  1. Recall that a line perpendicular to a line with slope m has slope −1/m, the “opposite reciprocal slope.” We can think of the slope of u→ as 5/3, its “rise over run.” A vector orthogonal to u→ will have slope −3/5. There are many such choices, though all parallel:
    ⟨−5,3⟩ or ⟨5,−3⟩ or ⟨−10,6⟩ or ⟨15,−9⟩, etc. 
  2. There are infinitely many directions in space orthogonal to any given direction, so there are an infinite number of non-parallel vectors orthogonal to v→. Since there are so many, we have great leeway in finding some. One way is to arbitrarily pick values for the first two components, leaving the third unknown. For instance, let v→1=⟨2,7,z⟩. If v→1 is to be orthogonal to v→, then v→1⋅v→=0, so
    2+14+3z=0⇒z=−163.
    So v→1=⟨2,7,−16/3⟩ is orthogonal to v→. We can apply a similar technique by leaving the first or second component unknown. Another method of finding a vector orthogonal to v→ mirrors what we did in part 1. Let v→2=⟨−2,1,0⟩. Here we switched the first two components of v→, changing the sign of one of them (similar to the “opposite reciprocal” concept before). Letting the third component be 0 effectively ignores the third component of v→, and it is easy to see that
    v→2⋅v→=⟨−2,1,0⟩⋅⟨1,2,3⟩=0.
    Clearly v→1 and v→2 are not parallel.
Solution 2. Video solution
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An important construction is illustrated in Figure 11.3.16, where vectors u→ and v→ are sketched. In Figure 11.3.16.(a), a dotted line is drawn from the tip of u→ to the line containing v→, where the dotted line is orthogonal to v→. In Figure 11.3.16.(b), the dotted line is replaced with the vector z→ and w→ is formed, parallel to v→. It is clear by the diagram that u→=w→+z→. What is important about this construction is this: u→ is decomposed as the sum of two vectors, one of which is parallel to v→ and one that is perpendicular to v→. It is hard to overstate the importance of this construction (as we’ll see in upcoming examples).
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The vectors w→, z→ and u→ as shown in Figure 11.3.16.(b) form a right triangle, where the angle between v→ and u→ is labeled θ. We can find w→ in terms of v→ and u→.
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Using trigonometry, we can state that
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(11.3.1)‖w→‖=‖u→‖cos⁡(θ).
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Diagram shows two vectors and the angle between them.
Two vectors u→ and v→ are shown that start from the same position, the angle between them is marked θ, the vector u→ is shorter. From the end of u→ a dashed perpendicular is drawn on v→.
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Diagram shows two vectors and the angle between them.
Two vectors u→ and v→ are shown that start from the same position, the angle between them is marked θ, the vector u→ is shorter. From the end of u→ a dashed perpendicular is drawn on v→. The vector w→ and z→ are also added. From the initial point the vector w→ is drawn in the same direction as v→ but it ends at the start of the perpendicular. The perpendicular starts at the tip of w→ and ends at the tip of u→ and is labeled z→.
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Figure 11.3.16. Developing the construction of the orthogonal projection
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We also know that w→ is parallel to to v→; that is, the direction of w→ is the direction of v→, described by the unit vector v→/‖v→‖. The vector w→ is the vector in the direction v→/‖v→‖ with magnitude ‖u→‖cos⁡(θ):
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w→=(‖u→‖cos⁡(θ))1‖v→‖v→=(‖u→‖u→⋅v→‖u→‖‖v→‖)1‖v→‖v→ (Replacing cos⁡(θ) using Theorem 11.3.7)=u→⋅v→‖v→‖2v→=u→⋅v→v→⋅v→v→ (Applying Theorem 11.3.4).
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Since this construction is so important, it is given a special name.
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Definition 11.3.17. Orthogonal Projection.

Let nonzero vectors u→ and v→ be given. The orthogonal projection of u→ onto v→, denoted projv→u→, is
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projv→u→=u→⋅v→v→⋅v→v→.
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Figure 11.3.18. Video presentation of Definition 11.3.17
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Example 11.3.19. Computing the orthogonal projection.

  1. Let u→=⟨−2,1⟩ and v→=⟨3,1⟩. Find projv→u→, and sketch all three vectors with initial points at the origin.
  2. Let w→=⟨2,1,3⟩ and x→=⟨1,1,1⟩. Find projx→w→, and sketch all three vectors with initial points at the origin.
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Solution 1.
  1. Applying Definition 11.3.17, we have
    projv→u→=u→⋅v→v→⋅v→v→=−510⟨3,1⟩=⟨−32,−12⟩.
    Vectors u→, v→ and projv→u→ are sketched in Figure 11.3.20. Note how the projection is parallel to v→; that is, it lies on the same line through the origin as v→, although it points in the opposite direction. That is because the angle between u→ and v→ is obtuse (i.e., greater than 90∘).
    Graph shows two vectors u and v, and the projection of u on v.
    The x axis is drawn from −2 to 3 and the y axis is drawn from −2 to 2. Two vectors u→ and v→ are shown, both start at the origin. The vector u ends in point (−2,1) and lies in the second quadrant and v→ ends in point (3,1) and lies in the first quadrant. The projection of u→ on v→is shown and it lies in the third quadrant.
    Figure 11.3.20. Sketching the three vectors in Part 1 of Example 11.3.19
  2. Apply the definition:
    projx→w→=w→⋅x→x→⋅x→x→=63⟨1,1,1⟩=⟨2,2,2⟩.
    These vectors are sketched in Figure 11.3.21.(a), and again in Figure 11.3.21.(b) from a different perspective. Because of the nature of graphing these vectors, the sketch in Figure 11.3.21.(a) makes it difficult to recognize that the drawn projection has the geometric properties it should. The graph shown in Figure 11.3.21.(b) illustrates these properties better.
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    Figure 11.3.21. Sketching the three vectors in Part 2 of Example 11.3.19
Solution 2. Video solution
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We can use the properties of the dot product found in Theorem 11.3.4 to rearrange the formula found in Definition 11.3.17:
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projv→u→=u→⋅v→v→⋅v→v→=u→⋅v→‖v→‖2v→=(u→⋅v→‖v→‖)v→‖v→‖.
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The above formula shows that the orthogonal projection of u→ onto v→ is only concerned with the direction of v→, as both instances of v→ in the formula come in the form v→/‖v→‖, the unit vector in the direction of v→.
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A special case of orthogonal projection occurs when v→ is a unit vector. In this situation, the formula for the orthogonal projection of a vector u→ onto v→ reduces to just projv→u→=(u→⋅v→)v→, as v→⋅v→=1.
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This gives us a new understanding of the dot product. When v→ is a unit vector, essentially providing only direction information, the dot product of u→ and v→ gives “how much of u→ is in the direction of v→.” This use of the dot product will be very useful in future sections.
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Diagram shows two vectors and the angle between them.
Two vectors u→ and v→ are shown that start from the same position, the angle between them is marked θ, the vector u is shorter. From the end of u→ a dashed perpendicular is drawn on v→. The projection of u→ and z→ are also added. The projection of u→ is in the same direction as v→ but it ends at the start of the perpendicular. The perpendicular starts at the tip of the projection of u→ and ends at the tip of u→ and is labeled z→.
Figure 11.3.22. Illustrating the orthogonal projection
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Now consider Figure 11.3.22 where the concept of the orthogonal projection is again illustrated. It is clear that
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(11.3.2)u→=projv→u→+z→.
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As we know what u→ and projv→u→ are, we can solve for z→ and state that
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z→=u→−projv→u→.
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This leads us to rewrite Equation (11.3.2) in a seemingly silly way:
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u→=projv→u→+(u→−projv→u→).
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This is not nonsense, as pointed out in the following Key Idea. (Notation note: the expression “∥y→” means “is parallel to y→.” We can use this notation to state “x→∥y→” which means “x→ is parallel to y→.” The expression “⊥y→” means “is orthogonal to y→,” and is used similarly.)
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Key Idea 11.3.23. Orthogonal Decomposition of Vectors.

Let nonzero vectors u→ and v→ be given. Then u→ can be written as the sum of two vectors, one of which is parallel to v→, and one of which is orthogonal to v→:
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u→=projv→u→⏟∥ v→ + (u→−projv→u→⏟⊥ v→).
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We illustrate the use of this equality in the following example.
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Example 11.3.24. Orthogonal decomposition of vectors.

  1. Let u→=⟨−2,1⟩ and v→=⟨3,1⟩ as in Example 11.3.19. Decompose u→ as the sum of a vector parallel to v→ and a vector orthogonal to v→.
  2. Let w→=⟨2,1,3⟩ and x→=⟨1,1,1⟩ as in Example 11.3.19. Decompose w→ as the sum of a vector parallel to x→ and a vector orthogonal to x→.
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Solution 1.
  1. In Example 11.3.19, we found that projv→u→=⟨−1.5,−0.5⟩. Let
    z→=u→−projv→u→=⟨−2,1⟩−⟨−1.5,−0.5⟩=⟨−0.5,1.5⟩.
    Is z→ orthogonal to v→? (i.e., is z→⊥v→ ?) We check for orthogonality with the dot product:
    z→⋅v→=⟨−0.5,1.5⟩⋅⟨3,1⟩=0.
    Since the dot product is 0, we know z→⊥v→. Thus:
    u→=projv→u→ + (u→−projv→u→)⟨−2,1⟩=⟨−1.5,−0.5⟩⏟∥ v→ + ⟨−0.5,1.5⟩⏟⊥ v→.
  2. We found in Example 11.3.19 that projx→w→=⟨2,2,2⟩. Applying the Key Idea, we have:
    z→=w→−projx→w→=⟨2,1,3⟩−⟨2,2,2⟩=⟨0,−1,1⟩.
    We check to see if z→⊥x→:
    z→⋅x→=⟨0,−1,1⟩⋅⟨1,1,1⟩=0.
    Since the dot product is 0, we know the two vectors are orthogonal. We now write w→ as the sum of two vectors, one parallel and one orthogonal to x→:
    w→=projx→w→ + (w→−projx→w→)⟨2,1,3⟩=⟨2,2,2⟩⏟∥ x→ + ⟨0,−1,1⟩⏟⊥ x→
Solution 2. Video solution
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We give an example of where this decomposition is useful.
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Example 11.3.25. Orthogonally decomposing a force vector.

Consider Figure 11.3.26.(a), showing a box weighing 50lb on a ramp that rises 5ft over a span of 20ft. Find the components of force, and their magnitudes, acting on the box (as sketched in Figure 11.3.26.(b)):
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Diagram showing a box on a ramp.
Image of a box being placed on an inclined plane. The downward force for gravity is marked as g→. The surface of the ramp is labeled as r→ base of the ramp is labeled 20 and the height is marked as 5.
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Diagram showing a box on a ramp.
Image of a box being placed on an inclined plane. The downward force for gravity is marked as g→. The surface of the ramp is labeled as r→ base of the ramp is labeled 20 and the height is marked as 5. The projection of g→ on r→ is shown.
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Figure 11.3.26. Sketching the ramp and box in Example 11.3.25. Note: The vectors are not drawn to scale.
  1. in the direction of the ramp, and
  2. orthogonal to the ramp.
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Solution 1.
As the ramp rises 5ft over a horizontal distance of 20ft, we can represent the direction of the ramp with the vector r→=⟨20,5⟩. Gravity pulls down with a force of 50lb, which we represent with g→=⟨0,−50⟩.
  1. To find the force of gravity in the direction of the ramp, we compute projr→g→:
    projr→g→=g→⋅r→r→⋅r→r→=−250425⟨20,5⟩=⟨−20017,−5017⟩≈⟨−11.76,−2.94⟩.
    The magnitude of projr→g→ is ‖projr→g→‖=50/17≈12.13 lb . Though the box weighs 50lb, a force of about 12lb is enough to keep the box from sliding down the ramp.
  2. To find the component z→ of gravity orthogonal to the ramp, we use Key Idea 11.3.23.
    z→=g→−projr→g→=⟨20017,−80017⟩≈⟨11.76,−47.06⟩.
    The magnitude of this force is ‖z→‖≈48.51lb. In physics and engineering, knowing this force is important when computing things like static frictional force. (For instance, we could easily compute if the static frictional force alone was enough to keep the box from sliding down the ramp.)
Solution 2. Video solution
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Subsection 11.3.1 Application to Work

In physics, the application of a force F to move an object in a straight line a distance d produces work; the amount of work W is W=Fd, (where F is in the direction of travel). The orthogonal projection allows us to compute work when the force is not in the direction of travel.
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Diagram showing the two vectors for force and displacement used to find work.
A box is shown that is being pushed to the right by a force. The displacement is shown as a horizontal vector and is labeled d→. Force is drawn from the middle of the box at an angle and is labeled as F→. The projection of the F→ is also shown, it is parallel to the displacement vector.
Figure 11.3.27. Finding work when the force and direction of travel are given as vectors
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Consider Figure 11.3.27, where a force F→ is being applied to an object moving in the direction of d→. (The distance the object travels is the magnitude of d→.) The work done is the amount of force in the direction of d→, ‖projd→F→‖, times ‖d→‖:
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‖projd→F→‖⋅‖d→‖=‖F→⋅d→d→⋅d→d→‖⋅‖d→‖=|F→⋅d→‖d→‖2|⋅‖d→‖⋅‖d→‖=|F→⋅d→|‖d→‖2‖d→‖2=|F→⋅d→|.
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The expression F→⋅d→ will be positive if the angle between F→ and d→ is acute; when the angle is obtuse (hence F→⋅d→ is negative), the force is causing motion in the opposite direction of d→, resulting in “negative work.” We want to capture this sign, so we drop the absolute value and find that W=F→⋅d→.
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Definition 11.3.28. Work.

Let F→ be a constant force that moves an object in a straight line from point P to point Q. Let d→=PQ→. The work W done by F→ along d→ is W=F→⋅d→.
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Example 11.3.29. Computing work.

A man slides a box along a ramp that rises 3ft over a distance of 15ft by applying 50lb of force as shown in Figure 11.3.30. Compute the work done.
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Diagram showing a box on a ramp.
Image of a box being placed on an inclined plane or ramp. The base of the ramp is labeled 15 and the height is marked as 3. Force is drawn from the middle of the box at an angle of 30 from the horizontal and is labeled as F→.
Figure 11.3.30. Computing work when sliding a box up a ramp in Example 11.3.29
Solution.
The figure indicates that the force applied makes a 30∘ angle with the horizontal, so F→=50⟨cos⁡(30∘),sin⁡(30∘)⟩≈⟨43.3,25⟩. The ramp is represented by d→=⟨15,3⟩. The work done is simply
F→⋅d→=50⟨cos⁡(30∘),sin⁡(30∘)⟩⋅⟨15,3⟩≈724.5 ft--lb .
Note how we did not actually compute the distance the object traveled, nor the magnitude of the force in the direction of travel; this is all inherently computed by the dot product!
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The dot product is a powerful way of evaluating computations that depend on angles without actually using angles. The next section explores another “product” on vectors, the cross product. Once again, angles play an important role, though in a much different way.
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Exercises 11.3.2 Exercises

Terms and Concepts

1.
The dot product of two vectors is a , not a vector.
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2.
How are the concepts of the dot product and vector magnitude related?
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3.
How can one quickly tell if the angle between two vectors is acute or obtuse?
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4.
Give a synonym for “orthogonal.”
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Problems

Exercise Group.
In the following exercises, find the dot product of the given vectors.
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5.
u→=⟨2,−4⟩, v→=⟨3,7⟩
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6.
u→=⟨5,3⟩, v→=⟨6,1⟩
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7.
u→=⟨1,−1,2⟩, v→=⟨2,5,3⟩
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8.
u→=⟨3,5,−1⟩, v→=⟨4,−1,7⟩
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9.
u→=⟨1,1⟩, v→=⟨1,2,3⟩
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10.
u→=⟨1,2,3⟩, v→=⟨0,0,0⟩
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11.
Create your own vectors u→, v→ and w→ in R2 and show that u→⋅(v→+w→)=u→⋅v→+u→⋅w→.
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12.
Create your own vectors u→ and v→ in R3 and scalar c and show that c(u→⋅v→)=u→⋅(cv→).
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Exercise Group.
In the following exercises, find the measure of the angle between the two vectors in radians.
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13.
u→=⟨1,1⟩ and v→=⟨1,2⟩.
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14.
u→=⟨−2,1⟩ and v→=⟨3,5⟩.
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15.
u→=⟨8,1,−4⟩ and v→=⟨2,2,0⟩.
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16.
u→=⟨1,7,2⟩ and v→=⟨4,−2,5⟩.
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Exercise Group.
In the following exercises, a vector v→ is given. Give two vectors that are orthogonal to v→.
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17.
Find two nonzero vectors orthogonal to v→=⟨4,7⟩.
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18.
Find two nonzero vectors orthogonal to v→=⟨−3,5⟩.
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19.
Find two nonzero vectors orthogonal to v→=⟨1,1,1⟩.
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20.
Find two nonzero vectors orthogonal to v→=⟨1,−2,3⟩.
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Exercise Group.
In the following exercises, vectors u→ and v→ are given. Find projv→u→, the orthogonal projection of u→ onto v→, and sketch all three vectors with the same initial point.
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21.
u→=⟨1,2⟩ and v→=⟨−1,3⟩.
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22.
u→=⟨5,5⟩ and v→=⟨1,3⟩.
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23.
u→=⟨−3,2⟩ and v→=⟨1,1⟩
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24.
u→=⟨−3,2⟩ and v→=⟨2,3⟩.
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25.
u→=⟨1,5,1⟩ and v→=⟨1,2,3⟩.
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26.
u→=⟨3,−1,2⟩ and v→=⟨2,2,1⟩.
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Exercise Group.
In the following exercises, vectors u→ and v→ are given. Write u→ as the sum of two vectors, one of which is parallel to v→ (or is zero) and one of which is orthogonal to v→. Note: these are the same pairs of vectors as found in Exercises 21–26.
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27.
Write u→=⟨1,2⟩ as the sum of two vectors, one parallel to v→=⟨−1,3⟩ (or zero) and the other perpendicular.
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u→=+
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28.
Write u→=⟨5,5⟩ as the sum of two vectors, one parallel to v→=⟨1,3⟩ (or zero) and the other perpendicular.
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u→=+
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29.
Write u→=⟨−3,2⟩ as the sum of two vectors, one parallel to v→=⟨1,1⟩ (or zero) and the other perpendicular.
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u→=+
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30.
Write u→=⟨−3,2⟩ as the sum of two vectors, one parallel to v→=⟨2,3⟩ (or zero) and the other perpendicular.
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u→=+
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31.
Write u→=⟨1,5,1⟩ as the sum of two vectors, one parallel to v→=⟨1,2,3⟩ (or zero) and the other perpendicular.
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u→=+
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32.
Write u→=⟨3,−1,2⟩ as the sum of two vectors, one parallel to v→=⟨2,2,1⟩ (or zero) and the other perpendicular.
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u→=+
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33.
A 10lb box sits on a ramp that rises 4ft over a distance of 20ft. How much force is required to keep the box from sliding down the ramp?
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34.
A 10lb box sits on a 15ft ramp that makes a 30∘ angle with the horizontal. How much force is required to keep the box from sliding down the ramp?
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35.
How much work is performed in moving a box horizontally 10ft with a force of 20lb applied at an angle of 45∘ to the horizontal?
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36.
How much work is performed in moving a box horizontally 10ft with a force of 20lb applied at an angle of 10∘ to the horizontal?
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37.
How much work is performed in moving a box up the length of a ramp that rises 2ft over a distance of 10ft, with a force of 50lb applied horizontally?
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38.
How much work is performed in moving a box up the length of a ramp that rises 2ft over a distance of 10ft, with a force of 50lb applied at an angle of 45∘ to the horizontal?
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39.
How much work is performed in moving a box up the length of a 10ft ramp that makes a 5∘ angle with the horizontal, with 50lb of force applied in the direction of the ramp?
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