Write vectors using their components and find how long they are (their magnitude)
Express a vector in terms of unit vectors
Applications of Vectors
Because vectors have both direction and magnitude, they are valuable tools for solving problems involving such applications as motion and force. Recall the boat example and the quarterback example we described earlier. Here we look at two other examples in detail.
Jane’s car is stuck in the mud. Lisa and Jed come along in a truck to help pull her out. They attach one end of a tow strap to the front of the car and the other end to the truck’s trailer hitch, and the truck starts to pull. Meanwhile, Jane and Jed get behind the car and push. The truck generates a horizontal force of [latex]300[/latex] lb on the car. Jane and Jed are pushing at a slight upward angle and generate a force of [latex]150[/latex] lb on the car. These forces can be represented by vectors, as shown in Figure 5. The angle between these vectors is [latex]15°[/latex]. Find the resultant force (the vector sum) and give its magnitude to the nearest tenth of a pound and its direction angle from the positive [latex]x[/latex]-axis.
Figure 5. Two forces acting on a car in different directions.
To find the effect of combining the two forces, add their representative vectors. First, express each vector in component form or in terms of the standard unit vectors. For this purpose, it is easiest if we align one of the vectors with the positive [latex]x[/latex]-axis. The horizontal vector, then, has initial point [latex](0,0)[/latex] and terminal point [latex](300,0)[/latex]. It can be expressed as [latex]\langle 300, 0\rangle[/latex] or [latex]300{\bf{i}}[/latex].The second vector has magnitude [latex]150[/latex] and makes an angle of [latex]15°[/latex] with the first, so we can express it as [latex]\langle 150\cos({15°}), 150\sin({15°}) \rangle[/latex], or [latex]150\cos({15°}){\bf{i}} + 150\sin({15°}){\bf{j}}[/latex]. Then, the sum of the vectors, or resultant vector, is [latex]{\bf{r}} = \langle 300,0 \rangle + \langle 150\cos({15°}), 150\sin({15°}) \rangle[/latex] , and we have
The angle [latex]θ[/latex] made by [latex]{\bf{r}}[/latex] and the positive [latex]x[/latex]-axis has [latex]\tan{θ} = \frac{150\sin({15°})}{(300 + 150\cos({15°}))} \approx 0.09[/latex], so [latex]θ \approx \tan^{-1}{0.09} \approx 5°[/latex], which means the resultant force [latex]{\bf{r}}[/latex] has an angle of [latex]5°[/latex] above the horizontal axis.
An airplane flies due west at an airspeed of [latex]425[/latex] mph. The wind is blowing from the northeast at [latex]40[/latex] mph. What is the ground speed of the airplane? What is the bearing of the airplane?
Let’s start by sketching the situation described (Figure 6).
Figure 6. Initially, the plane travels due west. The wind is from the northeast, so it is blowing to the southwest. The angle between the plane’s course and the wind is [latex]45^{\circ}[/latex] (Figure not drawn to scale.)
Set up a sketch so that the initial points of the vectors lie at the origin. Then, the plane’s velocity vector is [latex]{\bf{p}} = -425{\bf{i}}[/latex]. The vector describing the wind makes an angle of [latex]225°[/latex] with the positive [latex]x[/latex]-axis:
As a result of the wind, the plane is traveling at approximately [latex]454[/latex] mph relative to the ground.To determine the bearing of the airplane, we want to find the direction of the vector [latex]{\bf{p}} + {\bf{w}}[/latex]:
The overall direction of the plane is [latex]3.57°[/latex] south of west.
An airplane flies due north at an airspeed of [latex]550[/latex] mph. The wind is blowing from the northwest at [latex]50[/latex] mph. What is the ground speed of the airplane?
Approximately [latex]516[/latex] mph
Watch the following video to see the worked solution to the above example.
