If your solution to stated problem does not match the given solution, you should self-critique per instructions at

http://vhcc2.vhcc.edu/dsmith/geninfo/labrynth_created_fall_05/levl1_22/levl2_81/file3_259.htm.

Your solution, attempt at solution. 

If you are unable to attempt a solution, give a phrase-by-phrase interpretation of the problem along with a statement of what you do or do not understand about it.  This response should be given, based on the work you did in completing the assignment, before you look at the given solution.

018. Vectors

Question:  `q001.  Note that this assignment contains 8 questions.

.  The Pythagorean Theorem: the hypotenuse of a right triangle has a length  c such that c^2 = a^2 + b^2, where a and b are the lengths of the legs of the triangle.  Sketch a right triangle on a set of coordinate axes by first locating the  point (7, 13).  Then sketch a straight line from the origin of the coordinate system to this point to form the hypotenuse of  the triangle.  Continue by sketching a line straight down from (7, 13) to the x axis to form one leg of the triangle, then form the other leg by continuing straight along the x axis back to the origin.  How long are these two legs?  How long therefore is the  hypotenuse?

Your solution: 

Confidence rating:
 

Given Solution: 

The leg from (7, 13) to the x axis drops from the point (7, 13) to the point (7,0) and so has length 13.  The  second leg runs from (7,0) to the origin, a distance of 7 units.

The legs therefore have lengths a = 13 and b = 7, so that the hypotenuse  c satisfies c^2 = a^2 + b^2 and we have

c = `sqrt(a^2 + b^2) = `sqrt( 13^2 + 7^2 ) = `sqrt( 216)  = 14.7, approximately.

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Question:  `q002.  Sketch a right triangle on a set of coordinate axes whose leg along the x axis has length 12 and whose hypotenuse has length 15.  What must be the  length of the second leg of the triangle?

Your solution: 

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Given Solution: 

Let c stand for the length of the hypotenuse and a for the length of the known side, with b standing for the  length of the unknown side.  Then since c^2 = a^2 + b^2, b^2 = c^2 - a^2 and

b = `sqrt(c^2 - a^2) = `sqrt( 15^2 - 12^2) = `sqrt(225 -  144) = `sqrt(81) = 9.

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Question:  `q003.  If a line of length L is directed from the origin of an x-y coordinate system at an angle `theta with the positive x axis, then the x and y coordinates  of the point where the line ends will be y = L * sin(`theta) and x = L * cos(`theta).

Sketch a line of length 10, directed from the origin at an angle of 37 degrees with the positive x axis.  Without doing any  calculations estimate the x and y coordinates of this point.  Give your results and explain how you obtained your estimates.

Your solution: 

Confidence rating:
 

Given Solution: 

The line will run closer to the x axis then to the y axis, since the line is directed at an angle below 45 degrees.   It won't run a whole lot closer but it will run significantly closer, which will make the x coordinate greater than the y coordinate.  Since the line itself has length 10, it will run less than 10 units along either the x or the y axis.  It turns out that the x  coordinate is very close to 8 and the y coordinate is very close to 6.  Your estimates should have been reasonably close to these values.

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Question:  `q004.  Using your calculator you can calculate sin(37 deg) and cos(37 deg).  First be sure your calculator is in degree mode (this is the default mode for most  calculators so if you don't know what mode your calculator is in, it is probably in degree mode).  Then using the sin and cos buttons on your calculator you can find sin(37 deg) and cos(37 deg).  What are these  values and what should therefore be the x and y coordinates of the line directed from the origin at 37 degrees from the x axis?

Your solution: 

Confidence rating:
 

Given Solution: 

sin(37 deg) should give you a result very close but not exactly equal to .6.

cos(37 deg)  should give you a result very close but not exactly equal to .8.

Since the x coordinate is L cos(`theta), then for L = 10 and `theta = 37  deg we have x coordinate 10 * cos(37 deg) = 10 * .8 = 8 (your result should be slightly different than this approximate value).

Since the y coordinate is L sin(`theta), then for L = 10 and `theta = 37 deg we have y coordinate 10 * sin(37 deg) = 10 * .6 = 6 (your result should be slightly different than this approximate value).

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Question:  `q005.  Show that the x and y coordinates you obtained in the preceding problem in fact give the legs of a triangle whose hypotenuse is 10.

Your solution: 

Confidence rating:
 

Given Solution: 

If c stands for the hypotenuse of the triangle, then if a = 8 and b = 6 are its legs we have c = `sqrt(a^2 + b^2)  = `sqrt(8^2 + 6^2) = `sqrt(100) = 10.  The same will hold for the more precise values you obtained in the preceding problem.

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Question:  `q006.  A vector drawn on a coordinate system is generally depicted as a line segment of a specified length directed from the origin at a specified angle with  the positive x axis.  The vector is traditionally drawn with an arrow on the end away from the origin.  In the preceding series of problems the line segment has length 10 and was directed at 37 degrees from the  positive x axis.  That line segment could have been drawn with an arrow on its end, pointing away from the origin.

The components of  a vector consist of a vector called the x component drawn from the origin along the x axis from the origin to x  coordinate L cos(`theta), and a vector called the y component drawn from the origin along the y axis to y coordinate L sin(`theta).

What are the x and y components of a vector directed at an angle of 120 degrees, as measured counterclockwise from the  positive x axis, and having length 30 units?

Your solution: 

Confidence rating:
 

Given Solution: 

The x component of this vector is vector along the x axis, from the origin to x =  30 cos(120 degrees) =  -15.

The y component is a vector along the y axis, from the origin to y = 30 sin(120 degrees) = 26, approx..

Note that the x component is to the left and the y component upward.  This is appropriate since the 120 degree angle, has measured counterclockwise  from the positive x axis, takes the vector into the second quadrant, which directs it to the left and upward.

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Question:  `q007.  The angle of a vector as measured counterclockwise from the positive x axis is easily determined if the components of the vector are known.  The  angle is simply arctan( y component / x component ) provided the x component is positive.  If the x component is negative the angle is arctan( y component / x component ) + 180 deg.  If the x component is positive  and the y component negative, arctan( y component / x component ) will be a negative angle, and in this case we generally add 360 degrees in order to obtain an angle between 0 and 360 degrees.

The arctan, or inverse tangent tan^-1, is  usually on a calculator button marked tan^-1.

Find the angle and length of each of the following vectors as measured counterclockwise  from the positive x axis:

A vector with x component 8.7 and y component 5.

A vector with x  component -2.5 and y component 4.3.

A vector with x component 10 and y component -17.3.

Your solution: 

Confidence rating:
 

Given Solution: 

A vector with x component 8.7 and y component 5 makes an angle of arctan (5/8.7) = 30 degrees with the x  axis.  Since the x component is positive, this angle need not be modified.  The length of this vector is found by the Pythagorean Theorem to be length = `sqrt(8.7^2 + 5^2) = 10.

A vector with x component -2.5 and y component 4.3 makes  an angle of arctan (4.3 / -2.5) + 180 deg = -60 deg + 180 deg = 120 degrees with the x axis.  Since the x component is negative, we had to add the 180 degrees.   The length of this vector is found by the Pythagorean Theorem to be length = `sqrt((-2.5)^2 + 4.3^2) = 5.

A vector with x component 10 and y component -17.3 makes an angle of arctan (-17.3 / 10) = -60 degrees with the x axis.  Since the angle is  negative, we add 360 deg to get 300 deg.  The length of this vector is found by the Pythagorean Theorem to be length =  `sqrt(10^2 + (-17.3)^2) = 20.

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If you understand the assignment and were able to solve the previously given problems from your worksheets, you should be able to complete most of the following problems quickly and easily.  If you experience difficulty with some of these problems, you will be given notes and we will work to resolve difficulties.

Question:  `q008.  Find the angle and length of a vector whose x component is 8 and whose y component is -4.

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