Author Archives: Engineering Math

College Physics by Openstax Chapter 3 Problem 20

A new landowner has a triangular piece of flat land she wishes to fence. Starting at the west corner, she measures the first side to be 80.0 m long and the next to be 105 m. These sides are represented as displacement vectors A from B in Figure 3.59. She then correctly calculates the length and orientation of the third side C. What is her result?

Figure 3.59

Solution:

Consider the illustration shown.

We need to solve for an interior angle of the triangle. So, we need to solve for the value of α first. This can be done by simply subtracting the sum of 21 and 11 degrees from 90 degrees.

α=90∘−(21∘+11∘)α=58∘\begin{align*} \alpha & = 90 ^ \circ -\left( 21^\circ +11^\circ \right) \\ \alpha & = 58^\circ \end{align*}

Then, using the cosine law, we can now solve for the magnitude of vector C. That is

C2=A2+B2−2ABcos⁡αC2=(80 m)2+(105 m)2−2(80 m)(105 m)cos⁡58∘C2=8522.3564C=8522.3564C=92.3 m  (Answer)\begin{align*} C^2 & = A^2 + B^2 - 2AB \cos \alpha \\ C^2 & = \left( 80\ \text{m} \right)^2+\left( 105\ \text{m} \right)^2-2\left( 80\ \text{m} \right)\left( 105\ \text{m} \right) \cos 58^\circ \\ C^2 & = 8522.3564 \\ C & = \sqrt{8522.3564} \\ C & = 92.3 \ \text{m} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

Before we can solve for the value of θ, we need to know the value of β first. This can be done by using the sine law.

sin⁡β80 m=sin⁡58∘92.3 msin⁡β=(80 m)sin⁡58∘92.3 mβ=arcsin⁡[(80 m)sin⁡58∘92.3 m]β=47.3∘\begin{align*} \frac{\sin \beta}{80\ \text{m}} & = \frac{\sin 58^\circ }{92.3 \ \text{m}} \\ \sin \beta & = \frac{\left( 80 \ \text{m} \right)\sin 58^\circ }{92.3 \ \text{m}} \\ \beta & = \arcsin \left[ \frac{\left( 80 \ \text{m} \right)\sin 58^\circ }{92.3 \ \text{m}} \right] \\ \beta & = 47.3^\circ \end{align*}

Finally, we can solve for θ.

θ=(90∘+11∘)−βθ=(90∘+11∘)−47.3∘θ=53.7∘  (Answer)\begin{align*} \theta & = \left( 90 ^\circ +11^\circ \right) - \beta \\ \theta & = \left( 90 ^\circ +11^\circ \right) - 47.3^\circ \\ \theta & = 53.7^\circ \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}
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College Physics by Openstax Chapter 3 Problem 18

You drive 7.50 km in a straight line in a direction 15º east of north. (a) Find the distances you would have to drive straight east and then straight north to arrive at the same point. (This determination is equivalent to find the components of the displacement along the east and north directions.) (b) Show that you still arrive at the same point if the east and north legs are reversed in order.

Solution:

Part A

Consider the illustration shown.

Let DE be the east component of the distance, and DN be the north component of the distance.

DE=7.50 sin⁡15∘DE=1.9411 kmDE=1.94 km  (Answer)\begin{align*} D_E & = 7.50 \ \sin 15^\circ \\ D_E & = 1.9411 \ \text{km} \\ D_E & = 1.94 \ \text{km} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}
DN=7.50 cos⁡15∘DN=7.2444 kmDN=7.24 km  (Answer)\begin{align*} D_N & = 7.50 \ \cos 15^\circ \\ D_N & = 7.2444\ \text{km} \\ D_N & = 7.24 \ \text{km} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

Part B

It can be obviously seen from the figure below that you still arrive at the same point if the east and north legs are reversed in order.

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College Physics by Openstax Chapter 3 Problem 17

Repeat Exercise 3.16 using analytical techniques, but reverse the order of the two legs of the walk and show that you get the same final result. (This problem shows that adding them in reverse order gives the same result—that is, B + A = A + B .) Discuss how taking another path to reach the same point might help to overcome an obstacle blocking your other path.

Figure 3.58 The two displacements A and B add to give a total displacement R having magnitude R and direction θ.

Solution:

Considering the right triangle formed by the vectors A, B, and R. We can solve for the magnitude of R using the Pythagorean Theorem. That is

R=A2+B2=(18.0m)2+(25.0m)2=30.806m≈30.8m(Answer)\begin{align*} R & = \sqrt{A^2+B^2} \\ & = \sqrt{\left( 18.0 \text{m} \right)^2+\left( 25.0 \text{m} \right)^2} \\ & =30.806 \text{m} \\ & \approx 30.8 \text{m} \qquad {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

Then we solve for the compass direction by solving the value θ using the same right triangle.

θ=arctan⁡(BA)=arctan⁡(25.0m18.0m)=54.246∘≈54.2∘\begin{align*} \theta & = \arctan \left( \frac{B}{A} \right) \\ & = \arctan \left( \frac{25.0 \text{m}}{18.0 \text{m}} \right) \\ & = 54.246 ^\circ \\ & \approx 54.2 ^ \circ \\ \end{align*}

Therefore, the compass direction of the resultant is 54.2° North of West.

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College Physics by Openstax Chapter 3 Problem 16

Solve the following problem using analytical techniques: Suppose you walk 18.0 m straight west and then 25.0 m straight north. How far are you from your starting point, and what is the compass direction of a line connecting your starting point to your final position? (If you represent the two legs of the walk as vector displacements A and B, as in Figure 3.58, then this problem asks you to find their sum R=A+B.)

Figure 3.58 The two displacements A and B add to give a total displacement R having magnitude R and direction θ.

Solution:

Considering the right triangle formed by the vectors A, B, and R. We can solve for the magnitude of R using the Pythagorean Theorem. That is

R=A2+B2=(18.0 m)2+(25.0 m)2=30.806 m≈30.8 m  (Answer)\begin{align*} R & = \sqrt{A^2+B^2} \\ & = \sqrt{\left( 18.0\ \text{m} \right)^2+\left( 25.0\ \text{m} \right)^2} \\ & =30.806 \ \text{m} \\ & \approx 30.8 \ \text{m} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

Then we solve for the compass direction by solving the value θ using the same right triangle.

θ=arctan⁡(BA)=arctan⁡(25.0 m18.0 m)=54.246∘≈54.2∘\begin{align*} \theta & = \arctan\left( \frac{B}{A} \right) \\ & = \arctan\left( \frac{25.0\ \text{m}}{18.0\ \text{m}} \right) \\ & = 54.246 ^\circ \\ & \approx 54.2 ^ \circ \end{align*}

Therefore, the compass direction of the resultant is 54.2° North of West.

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College Physics by Openstax Chapter 3 Problem 15

Find the north and east components of the displacement from San Francisco to Sacramento shown in Figure 3.57.

Figure 3.57

Solution:

Consider the following figure.

Using the right triangle formed, we can solve for the east component and the north component. Let SE be the east component and SN be the north component of S.

SE=Scos⁡45∘=(123 km)cos⁡45∘=86.974 km≈87.0 km  (Answer)\begin{align*} S_E & = S \cos 45^\circ \\ & = \left( 123 \ \text{km} \right) \cos45^\circ \\ & = 86.974 \ \text{km} \\ & \approx 87.0 \ \text{km} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}
SN=Ssin⁡45∘=(123 km)sin⁡45∘=86.974 km≈87.0 km  (Answer)\begin{align*} S_N & = S \sin 45^\circ \\ & = \left( 123 \ \text{km} \right) \sin 45^\circ \\ & = 86.974 \ \text{km} \\ & \approx 87.0 \ \text{km} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

Therefore, the east and north components are equal at about 87.0 km.

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College Physics by Openstax Chapter 3 Problem 14

Find the following for path D in Figure 3.56: (a) the total distance traveled and (b) the magnitude and direction of the displacement from start to finish. In this part of the problem, explicitly show how you follow the steps of the analytical method of vector addition.

Figure 3.56 The various lines represent paths taken by different people walking in a city. All blocks are 120 m on a side.

Solution:

Part A

Looking at path D, we can see that it moves 2 blocks downward, 6 blocks to the right, 4 blocks upward, and 1 block to the left. Thus, the total distance of path D is

distance=(2×120 m)+(6×120 m)+(4×120 m)+(1×120 m)=1 560 m=1.56×103 m  (Answer)\begin{align*} \text{distance} & = \left( 2\times 120\ \text{m} \right)+\left( 6\times 120\ \text{m} \right)+\left( 4\times 120\ \text{m} \right)+\left( 1\times 120\ \text{m} \right) \\ & = 1\ 560 \ \text{m} \\ & = 1.56 \times 10^{3} \ \text{m} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

Part B

Looking at the initial and final position of path D, the final position is 5 blocks to the right or 600 meters to the right of the initial position, and 2 blocks or 240 meters upward from the initial position. Refer to the figure below.

Using the right triangle, we can solve for the displacement using the Pythagorean Theorem.

displacement=(600 m)2+(240 m)2=646.2198 m≈646 m  (Answer)\begin{align*} \text{displacement} & = \sqrt{\left( 600\ \text{m} \right)^2+\left( 240\ \text{m} \right)^2} \\ & = 646.2198 \ \text{m} \\ & \approx 646 \ \text{m} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}
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College Physics by Openstax Chapter 3 Problem 13

Find the following for path C in Figure 3.56: (a) the total distance traveled and (b) the magnitude and direction of the displacement from start to finish. In this part of the problem, explicitly show how you follow the steps of the analytical method of vector addition.

Figure 3.56 The various lines represent paths taken by different people walking in a city. All blocks are 120 m on a side.

Solution:

Part A

Looking at path C, it moves 1 block upward, 5 blocks to the right, 2 blocks downward, 1 block to the left, 1 block upward, and 3 blocks to the left. So, the total distance is

distance = (1×120 m)+(5×120 m)+(2×120 m)+(1×120 m)+(1×120 m)+(3×120 m)= 120 m+600 m+240 m+120 m+120 m+360 m= 1560 m  (Answer)\begin{align*} \text{distance} \ = \ &\left( 1\times 120 \ \text{m} \right)+\left( 5\times 120\ \text{m} \right)+\left( 2\times 120\ \text{m} \right)+\left( 1\times 120\ \text{m} \right) \\ & +\left( 1\times 120\ \text{m} \right)+\left( 3\times 120\ \text{m} \right) \\ = \ & 120\ \text{m}+600 \ \text{m}+240 \ \text{m} + 120 \ \text{m}+ 120 \ \text{m}+360\ \text{m} \\ = \ & 1560 \ \text{m} \ \qquad \ {\color{DarkOrange}\left( \text{Answer} \right) } \end{align*}

Part B

It can be seen from the figure that the end of path C is just one block to the right from the starting point. Therefore, the magnitude of the displacement is

displacement=120 m  (Answer)\begin{align*} \text{displacement} = 120\ \text{m} \ \qquad \ {\color{DarkOrange}\left( \text{Answer} \right) } \end{align*}

The direction is to the right or is equivalent to 0° measured from the positive x-axis.

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College Physics by Openstax Chapter 3 Problem 7

(a) Repeat the problem two problems prior, but for the second leg you walk 20.0 m in a direction 40.0º north of east (which is equivalent to subtracting B from A —that is, to finding R’=A−B ). (b) Repeat the problem two problems prior, but now you first walk 20.0 m in a direction 40.0º south of west and then 12.0 m in a direction 20.0º east of south (which is equivalent to subtracting A from B —that is, to finding R”=B−A=−R’ ). Show that this is the case.

Solution:

Refer to this problem.

Part A

Consider Figure 3-7A

Figure 3-7A

First, we solve for the value of α using a simple geometry.

α=40∘+(90∘−20∘)=110∘\alpha = 40^\circ+\left( 90^\circ-20^\circ \right) = 110^\circ

We can solve for R using the cosine law.

R2=A2+B2−2ABcos⁡αR=A2+B2−2ABcos⁡αR=(12.0 m)2+(20.0 m)2−2(12.0 m)(20.0 m)cos⁡110∘R=26.6115 mR=26.6 m  (Answer)\begin{align*} R^2 & = A^2 +B^2-2AB \cos \alpha \\ R & = \sqrt{A^2 +B^2-2AB \cos \alpha} \\ R & = \sqrt{\left( 12.0\ \text{m} \right)^2+\left( 20.0\ \text{m} \right)^2-2\left( 12.0\ \text{m} \right)\left( 20.0\ \text{m} \right) \cos 110^\circ} \\ R & = 26.6115 \ \text{m} \\ R & = 26.6 \ \text{m} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

Before we can solve for θ, we need to solve for 𝛽 first using the sine law.

sin⁡βB=sin⁡αRsin⁡β=B sin⁡αRβ=sin⁡−1(B sin⁡αR)β=sin⁡−1(20.0 m sin⁡110∘26.6115 m)β=44.9290∘\begin{align*} \frac{\sin \beta}{B} & = \frac{\sin \alpha }{R} \\ \sin \beta & = \frac{B \ \sin \alpha}{R} \\ \beta & = \sin ^{-1} \left( \frac{B \ \sin \alpha}{R} \right) \\ \beta & = \sin ^{-1} \left( \frac{20.0 \ \text{m}\ \sin 110^{\circ}}{26.6115 \ \text{m}} \right) \\ \beta & = 44.9290^\circ \end{align*}

Now, we can solve for θ.

θ=(90∘+20∘)−βθ=110∘−44.9290∘θ=65.071θ=65.1∘\begin{align*} \theta & = \left( 90^\circ + 20^\circ \right)-\beta \\ \theta & = 110^\circ - 44.9290^\circ \\ \theta & = 65.071 \\ \theta & = 65.1 ^\circ \end{align*}

Therefore, the compass reading is

65.1∘,North of East  (Answer)65.1^\circ, \text{North of East} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)}

Part B

Refer to Figure 3-7B

Figure 3-7B

First, we solve for the value of α using a simple geometry.

α=40∘+(90∘−20∘)=110∘\alpha = 40^\circ+\left( 90^\circ-20^\circ \right) = 110^\circ

We can solve for R using the cosine law.

R2=A2+B2−2ABcos⁡αR=A2+B2−2ABcos⁡αR=(12.0 m)2+(20.0 m)2−2(12.0 m)(20.0 m)cos⁡110∘R=26.6115 mR=26.6 m  (Answer)\begin{align*} R^2 & = A^2 +B^2-2AB \cos \alpha \\ R & = \sqrt{A^2 +B^2-2AB \cos \alpha} \\ R & = \sqrt{\left( 12.0\ \text{m} \right)^2+\left( 20.0\ \text{m} \right)^2-2\left( 12.0\ \text{m} \right)\left( 20.0\ \text{m} \right) \cos 110^\circ} \\ R & = 26.6115 \ \text{m} \\ R & = 26.6 \ \text{m} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

Before we can solve for θ, we need to solve for 𝛽 first using the sine law.

sin⁡βA=sin⁡αRsin⁡β=A sin⁡αRβ=sin⁡−1(A sin⁡αR)β=sin⁡−1(12.0 m sin⁡110∘26.6115 m)β=25.0708∘\begin{align*} \frac{\sin \beta}{A} & = \frac{\sin \alpha }{R} \\ \sin \beta & = \frac{A \ \sin \alpha}{R} \\ \beta & = \sin ^{-1} \left( \frac{A \ \sin \alpha}{R} \right) \\ \beta & = \sin ^{-1} \left( \frac{12.0 \ \text{m}\ \sin 110^{\circ}}{26.6115 \ \text{m}} \right) \\ \beta & = 25.0708^\circ \end{align*}

Now, we can solve for θ.

θ=40∘+βθ=40∘+25.0708∘θ=65.0708∘θ=65.1∘\begin{align*} \theta & = 40^\circ + \beta \\ \theta & = 40^\circ + 25.0708^\circ\\ \theta & = 65.0708^\circ\\ \theta & = 65.1 ^\circ \end{align*}

Therefore, the compass reading is

65.1∘,South of West  (Answer)65.1^\circ, \text{South of West} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)}

This is consistent with Part A because (A-B) = -(B-A).

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College Physics by Openstax Chapter 3 Problem 6

Repeat the problem above, but reverse the order of the two legs of the walk; show that you get the same final result. That is, you first walk leg B , which is 20.0 m in a direction exactly 40º south of west, and then leg A , which is 12.0 m in a direction exactly 20º west of north. (This problem shows that A+B=B+A.)

Solution:

Consider Figure 3-6A below with B drawn first before A.

Figure 3-6A

Compute for the value of angle β by adding 20° and the complement of 40°. This is by simple geometry.

β=20∘+(90∘−40∘)β=70∘\begin{align*} \beta & = 20^\circ +\left( 90^\circ -40^\circ \right) \\ \beta & = 70^\circ \\ \end{align*}

Solve for the magnitude of R using cosine law.

R2=A2+B2−2ABcos⁡βR=A2+B2−2ABcos⁡βR=(12.0 m)2+(20.0 m)2−2(12.0 m)(20.0 m)cos⁡70∘R=19.4892 mR=19.5 m  (Answer)\begin{align*} R^2 & = A^2 +B^2 - 2 A B \cos \beta \\ R & = \sqrt{A^2 +B^2 - 2 A B \cos \beta} \\ R & = \sqrt{\left( 12.0 \ \text{m} \right)^2+\left( 20.0 \ \text{m} \right)^2-2\left( 12.0 \ \text{m} \right)\left( 20.0 \ \text{m} \right) \cos 70^\circ} \\ R & = 19.4892 \ \text{m}\\ R & = 19.5 \ \text{m} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

To solve for θ, we need to solve angle α first. This can be done using the sine law.

sin⁡αA=sin⁡βRsin⁡α12.0 m=sin⁡70∘19.4892 msin⁡α=12.0 m sin⁡70∘19.4892 mα=sin⁡−1(12.0 m sin⁡70∘19.4892 m)α=35.3516∘\begin{align*} \frac{\sin \alpha}{A} & = \frac{\sin \beta }{R} \\ \frac{\sin \alpha}{12.0 \ \text{m}} & = \frac{\sin 70^\circ }{19.4892 \ \text{m}} \\ \sin \alpha & = \frac{12.0 \ \text{m}\ \sin 70^\circ}{19.4892 \ \text{m}} \\ \alpha & = \sin ^{-1} \left( \frac{12.0 \ \text{m}\ \sin 70^\circ}{19.4892 \ \text{m}} \right) \\ \alpha & = 35.3516 ^ \circ \end{align*}

Finally, we can solve for θ.

θ=40∘−35.3516∘=4.6484∘=4.65∘\begin{align*} \theta & = 40^\circ - 35.3516^\circ \\ & = 4.6484 ^\circ \\ & = 4.65 ^\circ \end{align*}

Therefore, the compass reading is

4.65∘,South of West  (Answer)4.65^\circ, \text{South of West} \ \qquad \ {\color{Orange} \left( \text{Answer} \right)}

This is the same with Problem 5.

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College Physics by Openstax Chapter 3 Problem 5

Suppose you first walk 12.0 m in a direction 20º west of north and then 20.0 m in a direction 40.0º south of west. How far are you from your starting point, and what is the compass direction of a line connecting your starting point to your final position? (If you represent the two legs of the walk as vector displacements A and B, as in Figure 3.54, then this problem finds their sum R=A+B.)

Figure 3.54

Solution:

Consider Figure 3.5A shown below.

Figure 3.5A

Before we can use cosine law to solve for the magnitude of R, we need to solve for the interior angle 𝛽 first. The value of 𝛽 can be calculated by inspecting the figure and use simple knowledge on geometry. It is equal to the sum of 20° and the complement of 40°. That is

β=20∘+(90∘−40∘)=70∘\beta = 20^\circ +\left( 90^\circ -40^\circ \right) = 70^\circ

We can use cosine law to solve for R.

R2=A2+B2−2ABcos⁡βR2=(12.0 m)2+(20.0 m)2−2(12.0 m)(20.0 m)cos⁡70∘R=(12.0 m)2+(20.0 m)2−2(12.0 m)(20.0 m)cos⁡70∘R=19.4892 mR=19.5 m  (Answer)\begin{align*} R^2 & =A^2+B^2 -2AB \cos \beta \\ R^2 & = \left( 12.0\ \text{m} \right) ^2+\left( 20.0\ \text{m} \right)^2-2 \left( 12.0\ \text{m} \right) \left( 20.0\ \text{m} \right) \cos 70^\circ \\ R & = \sqrt{ \left( 12.0\ \text{m} \right) ^2+\left( 20.0\ \text{m} \right)^2-2 \left( 12.0\ \text{m} \right) \left( 20.0\ \text{m} \right) \cos 70^\circ} \\ R & =19.4892 \ \text{m} \\ R & =19.5 \ \text{m} \ \qquad \ {\color{DarkOrange} \left( \text{Answer} \right)} \end{align*}

We can solve for α using sine law.

sin⁡αB=sin⁡βRsin⁡α20.0 m=sin⁡70∘19.4892 msin⁡α=20.0 sin⁡70∘19.4892α=sin⁡−1(20.0 sin⁡70∘19.4892)α=74.6488∘\begin{align*} \frac{\sin \alpha}{B} & = \frac{\sin \beta}{R} \\ \frac{\sin \alpha}{20.0\ \text{m}} & = \frac{\sin 70^\circ }{19.4892 \ \text{m}} \\ \sin \alpha & = \frac{20.0 \ \sin 70^\circ }{19.4892} \\ \alpha & = \sin ^{-1} \left( \frac{20.0 \ \sin 70^\circ }{19.4892} \right) \\ \alpha & = 74.6488 ^\circ \end{align*}

Then we solve for the value of θ by subtracting 70° from α.

θ=74.6488∘−70∘=4.65∘\theta=74.6488 ^\circ -70 ^\circ = 4.65^\circ

Therefore, the compass reading is

4.65∘,South of West  (Answer)4.65^\circ, \text{South of West} \ \qquad \ {\color{Orange} \left( \text{Answer} \right)}
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