Chapter 3 Two-Dimensional Kinematics
- Understand the rules of vector addition and subtraction using analytical methods.
- Apply analytical methods to determine vertical and horizontal component vectors.
- Apply analytical methods to determine the magnitude and direction of a resultant vector.
Analytical methods of vector addition and subtraction employ geometry and simple trigonometry rather than the ruler and protractor of graphical methods. Part of the graphical technique is retained, because vectors are still represented by arrows for easy visualization. However, analytical methods are more concise, accurate, and precise than graphical methods, which are limited by the accuracy with which a drawing can be made. Analytical methods are limited only by the accuracy and precision with which physical quantities are known.
Resolving a Vector into Perpendicular Components
Analytical techniques and right triangles go hand-in-hand in physics because (among other things) motions along perpendicular directions are independent. We very often need to separate a vector into perpendicular components. For example, given a vector like in Figure 1, we may wish to find which two perpendicular vectors, and , add to produce it.
and are defined to be the components of the x- and y-axes. The three vectors and form a right triangle:
Note that this relationship between vector components and the resultant vector holds only for vector quantities (which include both magnitude and direction). The relationship does not apply for the magnitudes alone. For example, if east, north, and north-east, then it is true that the vectors However, it is not true that the sum of the magnitudes of the vectors is also equal. That is,
If the vector is known, then its magnitude and its angle (its direction) are known. To find and its x- and y-components, we use the following relationships for a right triangle.
Suppose, for example, that is the vector representing the total displacement of the person walking in a city considered in Chapter 3.1 Kinematics in Two Dimensions: An Introduction and Chapter 3.2 Vector Addition and Subtraction: Graphical Methods.
Then and so that
Calculating a Resultant Vector
If the perpendicular components and of a vector are known, then can also be found analytically. To find the magnitude and direction of a vector from its perpendicular components and we use the following relationships:
Note that the equation is just the Pythagorean theorem relating the legs of a right triangle to the length of the hypotenuse. For example, if and are 9 and 5 blocks, respectively, then blocks, again consistent with the example of the person walking in a city. Finally, the direction is as before.
DETERMINING VECTORS AND VECTOR COMPONENTS WITH ANALYTICAL METHODS
Equations and are used to find the perpendicular components of a vector—that is, to go from and to and Equations and are used to find a vector from its perpendicular components—that is, to go from and to and Both processes are crucial to analytical methods of vector addition and subtraction.
Adding Vectors Using Analytical Methods
To see how to add vectors using perpendicular components, consider Figure 5, in which the vectors and are added to produce the resultant
If and represent two legs of a walk (two displacements), then is the total displacement. The person taking the walk ends up at the tip of There are many ways to arrive at the same point. In particular, the person could have walked first in the x-direction and then in the y-direction. Those paths are the x– and y-components of the resultant, and If we know and we can find and using the equations and When you use the analytical method of vector addition, you can determine the components or the magnitude and direction of a vector.
Step 1. Identify the x- and y-axes that will be used in the problem. Then, find the components of each vector to be added along the chosen perpendicular axes. Use the equations and to find the components. In Figure 6, these components are and The angles that vectors and make with the x-axis are and respectively.
Step 2. Find the components of the resultant along each axis by adding the components of the individual vectors along that axis. That is, as shown in Figure 7,
Components along the same axis, say the x-axis, are vectors along the same line and, thus, can be added to one another like ordinary numbers. The same is true for components along the y-axis. (For example, a 9-block eastward walk could be taken in two legs, the first 3 blocks east and the second 6 blocks east, for a total of 9, because they are along the same direction.) So resolving vectors into components along common axes makes it easier to add them. Now that the components of are known, its magnitude and direction can be found.
Step 3. To get the magnitude of the resultant, use the Pythagorean theorem:
Step 4. To get the direction of the resultant:
The following example illustrates this technique for adding vectors using perpendicular components.
Example 1: Adding Vectors Using Analytical Methods
Add the vector to the vector shown in Figure 8, using perpendicular components along the x– and y-axes. The x– and y-axes are along the east–west and north–south directions, respectively. Vector represents the first leg of a walk in which a person walks in a direction north of east. Vector represents the second leg, a displacement of a direction north of east.
The components of and along the x– and y-axes represent walking due east and due north to get to the same ending point. Once found, they are combined to produce the resultant.
Following the method outlined above, we first find the components of and along the x– and y-axes. Note that and We find the x-components by using which gives
Similarly, the y-components are found using
The x– and y-components of the resultant are thus
Now we can find the magnitude of the resultant by using the Pythagorean theorem:
Finally, we find the direction of the resultant:
This example illustrates the addition of vectors using perpendicular components. Vector subtraction using perpendicular components is very similar—it is just the addition of a negative vector.
Subtraction of vectors is accomplished by the addition of a negative vector. That is, Thus, the method for the subtraction of vectors using perpendicular components is identical to that for addition. The components of are the negatives of the components of The x– and y-components of the resultant are thus
and the rest of the method outlined above is identical to that for addition. (See Figure 10.)
Analyzing vectors using perpendicular components is very useful in many areas of physics, because perpendicular quantities are often independent of one another. The next module, Chapter 3.4 Projectile Motion, is one of many in which using perpendicular components helps make the picture clear and simplifies the physics.
PHET EXPLORATIONS: VECTOR ADDITION
Learn how to add vectors. Drag vectors onto a graph, change their length and angle, and sum them together. The magnitude, angle, and components of each vector can be displayed in several formats.
- The analytical method of vector addition and subtraction involves using the Pythagorean theorem and trigonometric identities to determine the magnitude and direction of a resultant vector.
- The steps to add vectors and using the analytical method are as follows:
Step 1: Determine the coordinate system for the vectors. Then, determine the horizontal and vertical components of each vector using the equations
Step 2: Add the horizontal and vertical components of each vector to determine the components and of the resultant vector,
Step 3: Use the Pythagorean theorem to determine the magnitude, of the resultant vector
Step 4: Use a trigonometric identity to determine the direction, of
1: Suppose you add two vectors and What relative direction between them produces the resultant with the greatest magnitude? What is the maximum magnitude? What relative direction between them produces the resultant with the smallest magnitude? What is the minimum magnitude?
2: Give an example of a nonzero vector that has a component of zero.
3: Explain why a vector cannot have a component greater than its own magnitude.
4: If the vectors and are perpendicular, what is the component of along the direction of What is the component of along the direction of
Problems & Exercises
1: Find the following for path C in Figure 12: (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.
2: Find the following for path D in Figure 12: (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.
3: Find the north and east components of the displacement from San Francisco to Sacramento shown in Figure 13.
4: 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 and as in Figure 14, then this problem asks you to find their sum
Note that you can also solve this graphically. Discuss why the analytical technique for solving this problem is potentially more accurate than the graphical technique.
5: Repeat Exercise 4 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, Discuss how taking another path to reach the same point might help to overcome an obstacle blocking you other path.
6: You drive in a straight line in a direction 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.
7: Do Exercise 4 again using analytical techniques and change the second leg of the walk to straight south. (This is equivalent to subtracting from —that is, finding (b) Repeat again, but now you first walk north and then east. (This is equivalent to subtract from —that is, to find Is that consistent with your result?)
8: 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 from in Figure 15. She then correctly calculates the length and orientation of the third side What is her result?
9: You fly in a straight line in still air in the direction south of west. (a) Find the distances you would have to fly straight south and then straight west to arrive at the same point. (This determination is equivalent to finding the components of the displacement along the south and west directions.) (b) Find the distances you would have to fly first in a direction south of west and then in a direction west of north. These are the components of the displacement along a different set of axes—one rotated
10: A farmer wants to fence off his four-sided plot of flat land. He measures the first three sides, shown as and in Figure 16, and then correctly calculates the length and orientation of the fourth side
What is his result?
11: In an attempt to escape his island, Gilligan builds a raft and sets to sea. The wind shifts a great deal during the day, and he is blown along the following straight lines: north of west; then south of east; then south of west; then straight east; then east of north; then south of west; and finally north of east. What is his final position relative to the island?
12: Suppose a pilot flies in a direction north of east and then flies in a direction north of east as shown in Figure 17. Find her total distance from the starting point and the direction of the straight-line path to the final position. Discuss qualitatively how this flight would be altered by a wind from the north and how the effect of the wind would depend on both wind speed and the speed of the plane relative to the air mass.
- analytical method
- the method of determining the magnitude and direction of a resultant vector using the Pythagorean theorem and trigonometric identities
Problems & Exercises
(a) 1.56 km
(b) 120 m east
North-component 87.0 km, east-component 87.0 km
30.8 m, 35.8 west of north
(a) south of west
(b) north of east
(a) 18.4 km south, then 26.2 km west
(b) 31.5 km at south of west, then 5.56 km at west of north
south of east