Lecture Note
University
Massachusetts Institute of TechnologyCourse
Multivariable CalculusPages
1
Academic year
2022
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General case So it is this vector. Why? Well, if we consider the right triangle formed by this vector, wecan read off its components from this right triangle. We begin by noting that the length of w is equal to the length of v. So the length of w is v2.This side is v2, the length of w. What about this side? The cosine of theta is v2 on thisside, and the sine of theta is v2 on this other side. The vertical component of velocity can be calculated as v2 cosine theta, where v is themagnitude of the velocity vector and theta is the angle between v and the horizontal axis.The horizontal component is found by calculating v2 sine of theta, which has a negativesign because it is in the x direction and not towards or away from you. Good, those were warmups. In general, v is just pointing in some arbitrary direction. So thegeneral case is more complicated than those two, we don't immediately see how to do it. If you just look at this picture, it's hard to do. There is a trick to figuring out what happensto a general v. We will draw a rectangle around v and imagine rotating the whole rectangleinstead of just rotating v. This is an example of what a before picture looks like. So, in this case we will write v. And we are going to think of v, let’s draw a rectangularframe around v. The bottom of the frame is blue, red, and red. OK. Now imagine rotating this picture withthe "v" shapes sitting inside a rectangular frame of color. After rotating the rectangle by theta radians, the figure will appear as shown here. Theta isthe angle by which we rotated the rectangle; w is our new vector of interest. This image makes sense. Now, in the before picture, v is v1, comma v2. The blue vector is v1 comma 0. The redvector is 0 comma v2. Let's look at this for a second. The blue vector is v1 comma 0, and it's in the x direction. The red vector is 0 comma v2,and it's in the y direction. There are two red vectors here (they have the same length), butyou can see that they're both going straight up. Thus, they are both 0 comma v2. We will now explore the after picture. So the first question for you is: Are the two red vectors in the after picture the same vectoror not? Thoughts can be divided by halves. Think over. We began with a rectangular box, rotated it by angle theta to get this figure,and found two vectors in the picture. Are they the same? Try to solve it.
Vector Components: Analyzing the General Case
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