1. The vectors should be parallel to achieve a resultant 7 m long (the unprimed case shown below), antiparallel (in opposite directions) to achieve
a resultant√1 m long (primed case shown), and perpendicular to achieve a
resultant 32 + 42 = 5 m long (the double-primed case shown). In each
sketch, the vectors are shown in a “head-to-tail” sketch but the resultant
is not shown. The resultant would be a straight line drawn from beginning
to end; the beginning is indicated by A (with or without primes, as the
case may be) and the end is indicated by B.
A
❜
✲❜
✲
B
✻
✛
B
❜A
✲❜
A
❜
2. A sketch of the displacements is shown. The resultant (not shown) would
be a straight line from
start (Bank) to finish
(Walpole). With a careful
drawing, one should find
that the resultant vector
has length 29.5 km at 35◦
west of south.
B
✲❜
West
Bank
❅
❅
❅
East
❍❍

❇ ❍
❍
❇
❍❍
❅
❇
❍❍
❅
❇
❍❍ ❅
❇
❍❍ ❅
❇
❍❍
❅
❇N
❍
❅
❘
❅
Walpole
South
3. The x component of a is given by ax = 7.3 cos 250◦ = −2.5 and the y
component is given by ay = 7.3 sin 250◦ = −6.9. In considering the variety
of ways to compute these, we note that the vector is 70◦ below the −x axis,
so the components could also have been found from ax = −7.3 cos 70◦ and
ay = −7.3 sin 70◦ . In a similar vein, we note that the vector is 20◦ from
the −y axis, so one could use ax = −7.3 sin 20◦ and ay = −7.3 cos 20◦ to
achieve the same results.
4. The angle described by a full circle is 360◦ = 2π rad, which is the basis of
our conversion factor. Thus,
(20.0◦ )
2π rad
= 0.349 rad
360◦
and (similarly) 50.0◦ = 0.873 rad and 100◦ = 1.75 rad. Also,
(0.330 rad)
360◦
= 18.9◦
2π rad
and (similarly) 2.10 rad = 120◦ and 7.70 rad = 441◦ .
5. The textbook’s approach to this sort of problem is through the use of
Eq. 3-6, and is illustrated in Sample Problem 3-3. However, most modern graphical calculators can produce the results quite efficiently using
rectangular ↔ polar “shortcuts.”
(−25)2 + 402 = 47.2 m
(a)
(b) Recalling that tan (θ) = tan (θ + 180◦ ), we note that the two possibilities for tan−1 (40/ − 25) are −58◦ and 122◦ . Noting that the vector
is in the third quadrant (by the signs of its x and y components) we
see that 122◦ is the correct answer. The graphical calculator “shortcuts” mentioned above are designed to correctly choose the right
possibility.
6. The x component of r is given by 15 cos 30◦ = 13 m and the y component
is given by 15 sin 30◦ = 7.5 m.
7. The point P is displaced vertically by 2R, where R is the radius of the
wheel. It is displaced horizontally by half the circumference of the wheel,
or πR. Since R = 0.450 m, the horizontal component of the displacement
is 1.414 m and the vertical component of the displacement is 0.900 m. If
the x axis is horizontal and the y axis is vertical, the vector displacement
(in meters) is r = (1.414 î + 0.900 ĵ). The displacement has a magnitude
of
|r| = (πR)2 + (2R)2 = R π 2 + 4 = 1.68 m
and an angle of
tan−1
2R
πR
= tan−1
2
= 32.5◦
π
above the floor. In physics there are no “exact” measurements, yet that
angle computation seemed to yield something exact. However, there has
to be some uncertainty in the observation that the wheel rolled half of a
revolution, which introduces some indefiniteness in our result.
8. Although we think of this as a three-dimensional movement, it is rendered
effectively two-dimensional by referring measurements to its well-defined
plane of the fault.
(a) The magnitude of the net displacement is
= |AD|2 + |AC|2 = 172 + 222 = 27.8 m .
|AB|
is |AD| sin 52.0◦ =
(b) The magnitude of the vertical component of AB
13.4 m.
9. The length unit meter is understood throughout the calculation.
(a) We compute the √
distance from one corner to the diametrically opposite corner: d = 3.002 + 3.702 + 4.302 = 6.42.
.....................................................................................................................................................
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.....................................................................................................................................................
d
L
h
w
(b) The displacement vector is along the straight line from the beginning
to the end point of the trip. Since a straight line is the shortest
distance between two points, the length of the path cannot be less
than the magnitude of the displacement.
(c) It can be greater, however. The fly might, for example, crawl along
the edges of the room. Its displacement would be the same but the
path length would be + w + h.
(d) The path length is the same as the magnitude of the displacement if
the fly flies along the displacement vector.
(e) We take the x axis to be out of the page, the y axis to be to the
right, and the z axis to be upward. Then the x component of
the displacement is w = 3.70, the y component of the displacement is 4.30, and the z component is 3.00. Thus d = 3.70 î +
4.30 ĵ + 3.00 k̂. An equally correct answer is gotten by interchanging
.............................................................................................................
the length, width, and height.
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w
h
(f) Suppose the path of the fly is as shown by the dotted lines on the
upper diagram. Pretend there is a hinge where the front wall of the
room joins the floor and lay the wall down as shown on the lower
diagram. The shortest walking distance between the lower left back
of the room and the upper right front corner is the dotted straight
line shown on the diagram. Its length is
Lmin = (w + h)2 + 2 = (3.70 + 3.00)2 + 4.302 = 7.96 m .
B
and C
(and denote the result
10. We label the displacement vectors A,
of their vector sum as r). We
choose east as the î direction (+x
direction) and north as the ĵ direction (+y direction). All distances are understood to be in
kilometers. We note that the an and the x axis is
gle between C
◦
60 . Thus,
west
north
✛ ✻
✲
❄
south
A
❡
A = 50 î
= 30 ĵ
B
= 25 cos (60◦ ) î + 25 sin (60◦ ) ĵ
C
+B
+C
= 62.50 î + 51.65 ĵ
r = A
✁✁✕
✁
east
✁
✁
❡
✻
B
✁
✁C
✲ ❡
which means that its magnitude is
|r| = 62.502 + 51.652 ≈ 81 km .
and its angle (counterclockwise from +x axis) is tan−1 (51.65/62.50) ≈
40◦ , which is to say that it points 40◦ north of east. Although the resultant
to
r is shown in our sketch, it would be a direct line from the “tail” of A
the “head” of C.
11. The diagram shows the displacement vectors for the two segments of her
and B,
and the total (“final”) displacement vector, labeled
walk, labeled A
r. We take east to be the +x direction and north to be the +y direction.
and the x axis is 60◦ . Where the
We observe that the angle between A
units are not explicitly shown, the distances are understood
B
to be in meters. Thus, the components
.......
..
North
.........................................................................................................
.
.
.
.
◦
.
.
..
are Ax = 250 cos 60 = 125 and
........ ..
of A
...... ..
A
.....
.
.
.
.
.
.....
...
..... r
Ay = 250 sin 30◦ = 216.5. The compo......
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.
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..
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.
nents of B are Bx = 175 and By = 0.
.
......
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The components of the total displace.....
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.
.
..
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ment are rx = Ax + Bx = 125 + 175 =
......
...
......
... ..........
.
... ......
300 and ry = Ay + By = 216.5 + 0 =
... ......
..........
.......
216.5.
East
(a) The magnitude of the resultant displacement is
|r| = rx2 + ry2 = 3002 + 216.52 = 370 m .
(b) The angle the resultant displacement makes with the +x axis is
ry
216.5
−1
−1
tan
= tan
= 36◦ .
rx
300
(c) The total distance walked is d = 250 + 175 = 425 m.
(d) The total distance walked is greater than the magnitude of the re and B
are not
sultant displacement. The diagram shows why: A
collinear.
B
and C
(and denote the result
12. We label the displacement vectors A,
of their vector sum as r). We
choose east as the î direction (+x
direction) and north as the ĵ direction (+y direction). All distances are understood to be in
kilometers. Therefore,
A
B
C
r = A + B + C
= 3.1 ĵ
= −2.4 î
❡✛
B
❡
✻
A
❡
C
❄
= −5.2 ĵ
= −2.1 î − 2.4 ĵ
which means that its magnitude is
|r| = (−2.1)2 + (−2.4)2 ≈ 3.2 km .
west
north
✛ ✻
✲
❄
south
east
and the two possibilities for its angle are
−2.4
−1
tan
= 41◦ , or 221◦ .
−2.1
We choose the latter possibility since r is in the third quadrant. It should
be noted that many graphical calculators have polar ↔ rectangular “shortcuts” that automatically produce the correct answer for angle (measured
counterclockwise from the +x axis). We may phrase the angle, then, as
221◦ counterclockwise from East (a phrasing that sounds peculiar, at best)
or as 41◦ south from west or 49◦ west from south. The resultant r is not
shown in our sketch; it would be an arrow directed from the “tail” of A
to the “head” of C.
13. We write r = a + b. When not explicitly displayed, the units here are
assumed to be meters. Then rx = ax + bx = 4.0 − 13 = −9.0 and ry =
ay + by = 3.0 + 7.0 = 10. Thus r = (−9.0 m) î + (10 m) ĵ . The magnitude
of the resultant is
r = rx2 + ry2 = (−9.0)2 + (10)2 = 13 m .
The angle between the resultant and the +x axis is given by tan−1 (ry /rx ) =
tan−1 10/(−9.0) which is either −48◦ or 132◦ . Since the x component of
the resultant is negative and the y component is positive, characteristic of
the second quadrant, we find the angle is 132◦ (measured counterclockwise
from +x axis).
14. The x, y and z components (with meters understood) of r are
rx = cx + dx
ry = cy + dy
= 7.4 + 4.4 = 12
= −3.8 − 2.0 = −5.8
rz = cz + dz
= −6.1 + 3.3 = −2.8 .
15. The vectors are shown on the diagram. The x axis runs from west
to east and the y axis run
from south to north. Then
ax = 5.0 m, ay = 0, bx =
−(4.0 m) sin 35◦ = −2.29 m,
and by = (4.0 m) cos 35◦ =
3.28 m.
N
.............
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.. . .. .
.... . . .....
.
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◦
...
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......................................................................................................................................................................
.
...
a + b
b
35
50.5
a
E
(a) Let c = a + b. Then cx = ax + bx = 5.0 m − 2.29 m = 2.71 m and
cy = ay + by = 0 + 3.28 m = 3.28 m. The magnitude of c is
c = c2x + c2y = (2.71 m)2 + (3.28 m)2 = 4.3 m .
(b) The angle θ that c = a + b makes with the +x axis is
θ = tan−1
cy
3.28 m
= 50.4◦ .
= tan−1
cx
2.71 m
The second possibility (θ = 50.4◦ + 180◦ = 126◦ ) is rejected because
it would point in a direction opposite to c.
(c) The vector b − a is found by adding −a to b. The result is shown
on the diagram to the
right.
Let c = b −
a. Then cx = bx −
ax = −2.29 m − 5.0 m =
−7.29 m and cy = by −
ay = 3.28 m. The
magnitude of c is c =
= 8.0 m .
N
....................
........................
........... ...........
........
. ....
........
...
........
...
........
...
........
........
...
........
...
........
...
........
...
........
...
........
...
........
...
........
...
........
........
...
........
...
........
...
........
...
........
...
........
...
........
...
........
...
........
........
...
........
...
....
... ..........
....................................................................................................................................................................
.....
b
c2x + c2y
−a + b
W
−a
(d) The tangent of the angle θ that c makes with the +x axis (east) is
tan θ =
3.28 m
cy
= −4.50, .
=
cx
−7.29 m
There are two solutions: −24.2◦ and 155.8◦ . As the diagram shows,
the second solution is correct. The vector c = −a + b is 24◦ north of
west.
16. All distances in this solution are understood to be in meters.
(a) a + b = (3.0 î + 4.0 ĵ) + (5.0 î − 2.0 ĵ) = 8.0 î + 2.0 ĵ .
(b) The magnitude of a + b is
|a + b| =
8.02 + 2.02 = 8.2 m .
(c) The angle between this vector and the +x axis is tan−1 (2.0/8.0) =
14◦ .
(d) b − a = (5.0 î − 2.0 ĵ) − (3.0 î + 4.0 ĵ) = 2.0 î − 6.0 ĵ .
(e) The magnitude of the difference vector b − a is
|b − a| = 2.02 + (−6.0)2 = 6.3 m .
(f) The angle between this vector and the +x axis is tan−1 (−6.0/2.0) =
−72◦ . The vector is 72◦ clockwise from the axis defined by î.
17. All distances in this solution are understood to be in meters.
(a) a +b = (4.0 + (−1.0)) î + ((−3.0) + 1.0) ĵ + (1.0 + 4.0) k̂ = 3.0 î − 2.0 ĵ +
5.0 k̂ .
(b) a −b = (4.0 − (−1.0)) î + ((−3.0) − 1.0) ĵ + (1.0 − 4.0) k̂ = 5.0 î − 4.0 ĵ −
3.0 k̂ .
(c) The requirement a −b +c = 0 leads to c = b −a, which we note is the
opposite of what we found in part (b). Thus, c = −5.0 î+4.0 ĵ+3.0 k̂ .
18. Many of the operations are done efficiently on most modern graphical
calculators using their built-in vector manipulation and rectangular ↔
polar “shortcuts.” In this solution, we employ the “traditional” methods
(such as Eq. 3-6).
(a) The magnitude of a is 42 + (−3)2 = 5.0 m.
(b) The angle between a and the +x axis is tan−1 (−3/4) = −37◦ . The
vector is 37◦ clockwise from the axis defined by î .
√
(c) The magnitude of b is 62 + 82 = 10 m.
(d) The angle between b and the +x axis is tan−1 (8/6) = 53◦ .
(e) a + b = (4 + 6) î + ((−3) + 8) ĵ = 10 î + 5 ĵ √
, with the unit meter
understood. The magnitude of this vector is 102 + 52 = 11 m; we
rounding to two significant figures in our results.
(f) The angle between the vector described in part (e) and the +x axis
is tan−1 (5/10) = 27◦ .
(g) b − a = (6 − 4) î + (8 − (−3)) ĵ = 2 î + 11 ĵ , with
√ the unit meter
understood. The magnitude of this vector is 22 + 112 = 11 m,
which is, interestingly, the same result as in part (e) (exactly, not just
to 2 significant figures) (this curious coincidence is made possible by
the fact that a ⊥ b).
(h) The angle between the vector described in part (g) and the +x axis
is tan−1 (11/2) = 80◦ .
(i) a − b = (4 − 6) î + ((−3) − 8) ĵ = −2 î − 11 ĵ , with the unit meter
understood. The magnitude of this vector is (−2)2 + (−11)2 =
11 m.
(j) The two possibilities presented by a simple calculation for the angle
between the vector described in part (i) and the +x direction are
tan−1 (11/2) = 80◦ , and 180◦ + 80◦ = 260◦ . The latter possibility is
the correct answer (see part (k) for a further observation related to
this result).
(k) Since a − b = (−1)(b − a), they point in opposite (antiparallel) directions; the angle between them is 180◦ .
19. Many of the operations are done efficiently on most modern graphical
calculators using their built-in vector manipulation and rectangular ↔
polar “shortcuts.” In this solution, we employ the “traditional” methods
(such as Eq. 3-6). Where the length unit is not displayed, the unit meter
should be understood.
(a) Using unit-vector notation,
a = 50 cos (30◦ ) î + 50 sin (30◦ ) ĵ
b = 50 cos (195◦ ) î + 50 sin (195◦ ) ĵ
c = 50 cos (315◦ ) î + 50 sin (315◦ ) ĵ
a + b + c =
30.4 î − 23.3 ĵ .
The magnitude of this result is 30.42 + (−23.3)2 = 38 m.
(b) The two possibilities presented by a simple calculation for the angle
between the vector described in part (a) and the +x direction are
tan−1 (−23.2/30.4) = −37.5◦ , and 180◦ + (−37.5◦ ) = 142.5◦ . The
former possibility is the correct answer since the vector is in the
fourth quadrant (indicated by the signs of its components). Thus,
the angle is −37.5◦ , which is to say that it is roughly 38◦ clockwise
from the +x axis. This is equivalent to 322.5◦ counterclockwise from
+x.
(c) We find a−b+c = (43.3−(−48.3)+35.4) î−(25−(−12.9)+(−35.4)) ĵ =
127 î + 2.6 ĵ in unit-vector notation. The magnitude of this result is
√
1272 + 2.62 ≈ 1.3 × 102 m.
(d) The angle between the vector described in part (c) and the +x axis
is tan−1 (2.6/127) ≈ 1◦ .
(e) Using unit-vector notation, d is given by
d = a + b − c
= −40.4 î + 47.4 ĵ
which has a magnitude of (−40.4)2 + 47.42 = 62 m.
(f) The two possibilities presented by a simple calculation for the angle between the vector described in part (e) and the +x axis are
tan−1 (47.4/(−40.4)) = −50◦ , and 180◦ + (−50◦ ) = 130◦ . We choose
the latter possibility as the correct one since it indicates that d is in
the second quadrant (indicated by the signs of its components).
20. Angles are given in ‘standard’ fashion, so Eq. 3-5 applies directly. We
use this to write the vectors in unit-vector notation before adding them.
However, a very different-looking approach using the special capabilities
of most graphical calculators can be imagined. Where the length unit is
not displayed in the solution below, the unit meter should be understood.
(a) Allowing for the different angle units used in the problem statement,
we arrive at
E
F
=
=
3.73 î + 4.70 ĵ
1.29 î − 4.83 ĵ
G
H
E+F +G+H
= 1.45 î + 3.73 ĵ
= −5.20 î + 3.00 ĵ
=
1.28 î + 6.60 ĵ .
√
(b) The magnitude of the vector sum found in part (a) is 1.282 + 6.602 =
6.72 m. Its angle measured counterclockwise from the +x axis is
tan−1 (6.6/1.28) = 79◦ = 1.38 rad.
21. It should be mentioned that an efficient way to work this vector addition
problem is with the cosine law for general triangles (and since a, b and r
form an isosceles triangle, the angles are easy to figure). However, in the
interest of reinforcing the usual systematic approach to vector addition,
we note that the angle b makes with the +x axis is 135◦ and apply Eq. 3-5
and Eq. 3-6 where appropriate.
(a) The x component of r is 10 cos 30◦ + 10 cos 135◦ = 1.59 m.
(b) The y component of r is 10 sin 30◦ + 10 sin 135◦ = 12.1 m.
√
(c) The magnitude of r is 1.592 + 12.12 = 12.2 m.
(d) The angle between r and the +x direction is tan−1 (12.1/1.59) =
82.5◦ .
22. If we wish to use Eq. 3-5 in an unmodified fashion, we should note that
and the +x axis is 180◦ + 20◦ = 200◦ .
the angle between C
is given by Cx −Ax = 15 cos 200◦ −12 cos 40◦ =
(a) The x component of B
is given by Cy −Ay = 15 sin 200◦ −
−23.3 m, and the y component of B
12 sin 40◦ = −12.8 m. Consequently, its magnitude is (−23.3)2 + (−12.8)2 =
26.6 m.
(b) The two possibilities presented by a simple calculation for the angle
and the +x axis are tan−1 ((−12.8)/(−23.3)) = 28.9◦ , and
between B
◦
180 + 28.9◦ = 209◦ . We choose the latter possibility as the correct
is in the third quadrant (indicated by
one since it indicates that B
the signs of its components). We note, too, that the answer can be
equivalently stated as −151◦ .
with (x, y) components given by (A cos α, A sin α). Simi23. We consider A
larly, B → (B cos β, B sin β). The angle (measured from the +x direction)
for their vector sum must have a slope given by
tan θ =
A sin α + B sin β
.
A cos α + B cos β
The problem requires that we now consider the orthogonal direction, where
tan θ + 90◦ = − cot θ. If this (the negative reciprocal of the above expression) is to equal the slope for their vector difference, then we must have
−
A sin α − B sin β
A cos α + B cos β
=
.
A sin α + B sin β
A cos α − B cos β
Multiplying both sides by A sin α + B sin β and doing the same with
A cos α − B cos β yields
A2 cos2 α − B 2 cos2 β = A2 sin2 α − B 2 sin2 β .
Rearranging, using the cos2 φ + sin2 φ = 1 identity, we obtain
A2 = B 2 =⇒ A = B .
In a later section, the scalar (dot) product of vectors is presented and this
result can be revisited with a more compact derivation.
24. If we wish to use Eq. 3-5 directly, we should note that the angles for
R
and S
are 100◦ , 250◦ and 310◦ , respectively, if they are measured
Q,
counterclockwise from the +x axis.
(a) Using unit-vector notation, with the unit meter understood, we have
P =
=
Q
=
R
S
+R
+S
P + Q
=
10 cos (25◦ ) î + 10 sin (25◦ ) ĵ
12 cos (100◦ ) î + 12 sin (100◦ ) ĵ
8 cos (250◦ ) î + 8 sin (250◦ ) ĵ
9 cos (310◦ ) î + 9 sin (310◦ ) ĵ
=
10.0 î + 1.6 ĵ
√
(b) The magnitude of the vector sum is 102 + 1.62 = 10.2 m and its
angle is tan−1 (1.6/10) ≈ 9.2◦ measured counterclockwise from the
+x axis. The appearance of this solution would be quite different
using the vector manipulation capabilities of most modern graphical
calculators, although the principle would be basically the same.
25. Without loss of generality, we assume a points along the +x axis, and that
b is at θ measured counterclockwise from a. We wish to verify that
r2 = a2 + b2 + 2ab cos θ
where a = |a| = ax (we’ll call it a for simplicity) and b = |b| = b2x + b2y .
Since r = a+b then r = |r| = (a + bx )2 + b2y . Thus, the above expression
becomes
2
2
(a + bx )2 + b2y
= a2 +
b2x + b2y
+ 2ab cos θ
a2 + 2abx + b2x + b2y
= a2 + b2x + b2y + 2ab cos θ
which makes a valid equality since (the last term) 2ab cos θ is indeed the
same as 2abx (on the left-hand side). In a later section, the scalar (dot)
product of vectors is presented and this result can be revisited with a
somewhat different-looking derivation.
=A
+B
+C
+ D.
Expressing B
and D
in unit26. The vector equation is R
vector notation, we have 1.69 î + 3.63 ĵ and −2.87 î + 4.10 ĵ, respectively.
Where the length unit is not displayed in the solution below, the unit
meter should be understood.
= −3.18 î + 4.72 ĵ.
(a) Adding corresponding components, we obtain R
(b) and (c) Converting this result to polar coordinates (using Eq. 3-6 or
functions on a vector-capable calculator), we obtain
(−3.18, 4.72) −→ (5.69
124◦ )
which tells us the magnitude is 5.69 m and the angle (measured
counterclockwise from +x axis) is 124◦ .
27. (a) There are 4 such lines, one from each of the corners on the lower
face to the diametrically opposite corner on the upper face. One is
shown on the diagram. Using an xyz coordinate system as shown
(with the origin at the back lower left corner) The position vector
of the “starting point” of the diagonal shown is a î and the position
vector of the ending point is a ĵ + a k̂, so the vector along the line is
the difference a ĵ + a k̂ − a î.
z ...
..
..
The point diametrically opposite
.
.
.........................................................................................................................
.
.
.
.
.
.
the origin has position vector
... .
......... ...
.......
........
...
.......
....... .....
....
..
.......
a.........................
...
a î + a ĵ + a k̂ and this is the vec....... .......
.
.
.
.
.
.
.
....
.... .....
..
...
.
.
.
.
.
.
.
.
.
.
..
.
.
..
....
....
.
.
.
.
.
.
.
.
.
.
...
.
.
.........................................................................................................
tor along the diagonal. Another
...
.
.
.
.
...
..
..
.... ...
...
...
... ...
.
.
.
...
corner of the bottom face is at
...
...
...
.
.
.
.
...
.
.
....
....
.
...
.
...
...
... ...
.
...
a î+a ĵ and the diametrically op...
...
..
.
.
.
.
...
.
.
....
....
.
...
.
... ......
...
...
...
.
posite corner is at a k̂, so another
...
...
.
.
.
...
.
... ...
....
a ......
...
...
... ..
.
.
.
.
.
cube diagonal is a k̂ − a î − a ĵ.
...
.......... ....... ....... ....... ....... ....
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
...
....
....
.
.... .. ....
.
.
.
.
.
...
...
...
.
......
The fourth diagonal runs from a ĵ
...
..... ...... ..
...
.......
.
.......
..
...
....
.......
............
...
...
.......
.
.
.
.
.
.
.
.
.
.
.
.
.
to a î + a k̂, so the vector along
... ... ...
.. .......
...............................................................................................................................
the diagonal is a î + a k̂ − a ĵ.
a
.......
....... x
(b) Consider the vector from the back lower left corner to the front upper
right corner. It is a î+a ĵ+a k̂. We may think of it as the sum of the
vector a î parallel to the x axis and the vector a ĵ + a k̂ perpendicular
to the x axis. The tangent of the angle between the vector and
the x axis is the perpendicular component divided by the parallel
component.
Since
√
√ the magnitude of the perpendicular component is
a2 + a2 =√a 2 and √
the magnitude of the parallel component is a,
tan θ = a 2 /a = 2. Thus θ = 54.7◦ . The angle between the
vector and each of the other two adjacent sides (the y and z axes) is
the same as is the angle between any of the other diagonal vectors
and any of the cube sides adjacent to them.
√
√
(c) The length of any of the diagonals is given by a2 + a2 + a2 = a 3.
....... .....
y
28. Reference to Figure 3-18 (and the accompanying material in that sec to the magnitude-angle notation (as A
tion) is helpful. If we convert B
◦
already is) we have B = (14.4 33.7 ) (appropriate notation especially
if we are using a vector capable calculator in polar mode). Where the
length unit is not displayed in the solution, the unit meter should be understood. In the magnitude-angle notation, rotating the axis by +20◦
amounts to subtracting that angle from the angles previously specified.
= (14.4 13.7◦ ) , where the ‘prime’
= (12.0 40.0◦ ) and B
Thus, A
notation indicates that the description is in terms of the new coordinates.
Converting these results to (x, y) representations, we obtain
=
A
=
B
9.19 î + 7.71 ĵ
14.0 î + 3.41 ĵ
with the unit meter understood, as already mentioned.
29. We apply Eq. 3-20 and Eq. 3-27.
(a) The scalar (dot) product of the two vectors is a · b = ab cos φ =
(10)(6.0) cos 60◦ = 30.
(b) The magnitude of the vector (cross) product of the two vectors is
|a × b| = ab sin φ = (10)(6.0) sin 60◦ = 52.
30. We consider all possible products and then simplify using relations such
as î · k̂ = 0 and î · î = 1. Thus,
a · b =
ax î + ay ĵ + az k̂ · bx î + by ĵ + bz k̂
= ax bx î · î + ax by î · ĵ + ax bz î · k̂ + ay bx ĵ · î + ay bj ĵ · ĵ + · · ·
= ax bx (1) + ax by (0) + ax bz (0) + ay bx (0) + ay bj (1) + · · ·
which is seen to reduce to the desired result (one might wish to show this
in two dimensions before tackling the additional tedium of working with
these three-component vectors).
31. Since ab cos φ = ax bx + ay by + az bz ,
ax bx + ay by + az bz
.
ab
The magnitudes of the vectors given in the problem are
a = |a| =
(3.0)2 + (3.0)2 + (3.0)2 = 5.2
b = |b| =
(2.0)2 + (1.0)2 + (3.0)2 = 3.7 .
cos φ =
The angle between them is found from
cos φ =
(3.0)(2.0) + (3.0)(1.0) + (3.0)(3.0)
= 0.926 .
(5.2)(3.7)
The angle is φ = 22◦ .
32. We consider all possible products and then simplify using relations such
as î × î = 0 and the important fundamental products
Thus,
a × b
=
î × ĵ = − ĵ × î
ĵ × k̂ = − k̂ × ĵ
=
=
k̂
î
k̂ × î = − î × k̂
=
ĵ .
ax î + ay ĵ + az k̂ × bx î + by ĵ + bz k̂
= ax bx î × î + ax by î × ĵ + ax bz î × k̂ + ay bx ĵ × î + ay bj ĵ × ĵ + · · ·
= ax bx (0) + ax by ( k̂) + ax bz (− ĵ) + ay bx (− k̂) + ay bj (0) + · · ·
which is seen to simplify to the desired result.
33. The area of a triangle is half the product of its base and altitude. The
base is the side formed by vector a. Then the altitude is b sin φ and the
area is A = 12 ab sin φ = 12 |a × b|.
(where q is a scalar) becomes
34. Applying Eq. 3-23, F = qv × B
Fx î+Fy ĵ+Fz k̂ = q (vy Bz − vz By ) î+q (vz Bx − vx Bz ) ĵ+q (vx By − vy Bx ) k̂
which – plugging in values – leads to three equalities:
=
2 (4.0Bz − 6.0By )
−20 =
12 =
2 (6.0Bx − 2.0Bz )
2 (2.0By − 4.0Bx )
4.0
Since we are told that Bx = By , the third equation leads to By = −3.0.
Inserting this value into the first equation, we find Bz = −4.0. Thus, our
answer is
= −3.0 î − 3.0 ĵ − 4.0 k̂ .
B
35. Both proofs shown below utilize the fact that the vector (cross) product of
a and b is perpendicular to both a and b. This is mentioned in the book,
and is fundamental to its discussion of the right-hand rule.
(a) (b ×a) is a vector that is perpendicular to a, so the scalar product of
a with this vector is zero. This can also be verified by using Eq. 3-30,
and then (with suitable notation changes) Eq. 3-23.
(b) Let c = b × a. Then the magnitude of c is c = ab sin φ. Since c is
perpendicular to a the magnitude of a × c is ac. The magnitude of
a × (b ×a) is consequently |a × (b ×a)| = ac = a2 b sin φ. This too can
be verified by repeated application of Eq. 3-30, although it must be
admitted that this is much less intimidating if one is using a math
software package such as MAPLE or Mathematica.
36. If a vector capable calculator is used, this makes a good exercise for getting
familiar with those features. Here we briefly sketch the method. Eq. 3-30
leads to
×B
= 2 2 î + 3 ĵ − 4 k̂ × −3 î + 4 ĵ + 2 k̂ = 44 î + 16 ĵ + 34 k̂ .
2A
· 2A
×B
:
We now apply Eq. 3-23 to evaluate 3C
3 7 î − 8 ĵ · 44 î + 16 ĵ + 34 k̂ = 3 ((7)(44) + (−8)(16) + (0)(34)) = 540 .
37. From the figure, we note that c ⊥ b, which implies that the angle between
c and the +x axis is 120◦ .
(a) Direct application of Eq. 3-5 yields the answers for this and the next
few parts. ax = a cos 0◦ = a = 3.00 m.
(b) ay = a sin 0◦ = 0.
(c) bx = b cos 30◦ = (4.00 m) cos 30◦ = 3.46 m.
(d) by = b sin 30◦ = (4.00 m) sin 30◦ = 2.00 m.
(e) cx = c cos 120◦ = (10.0 m) cos 120◦ = −5.00 m.
(f) cy = c sin 30◦ = (10.0 m) sin 120◦ = 8.66 m.
(g) In terms of components (first x and then y), we must have
−5.00 m
=
p(3.00 m) + q(3.46 m)
8.66 m
=
p(0) + q(2.00 m) .
Solving these equations, we find p = −6.67
(h) and q = 4.33 (note that it’s easiest to solve for q first). The numbers
p and q have no units.
38. We apply Eq. 3-20 with Eq. 3-23. Where the length unit is not displayed,
the unit meter is understood.
√
(a) √
We first note that a = |a| = 3.22 + 1.62 = 3.58 m and b = |b| =
0.52 + 4.52 = 4.53 m. Now,
a · b = ax bx + ay by
(3.2)(0.5) + (1.6)(4.5)
= ab cos φ
= (3.58)(4.53) cos φ
which leads to φ = 57◦ (the inverse cosine is double-valued as is the
inverse tangent, but we know this is the right solution since both
vectors are in the same quadrant).
(b) Since the angle (measured from +x) for a is tan−1 (1.6/3.2) = 26.6◦ ,
we know the angle for c is 26.6◦ −90◦ = −63.4◦ (the other possibility,
26.6◦ + 90◦ would lead to a cx < 0). Therefore, cx = c cos −63.4◦ =
(5.0)(0.45) = 2.2 m.
(c) Also, cy = c sin −63.4◦ = (5.0)(−0.89) = −4.5 m.
(d) And we know the angle for d to be 26.6◦ + 90◦ = 116.6◦ , which leads
to dx = d cos 116.6◦ = (5.0)(−0.45) = −2.2 m.
(e) Finally, dy = d sin 116.6◦ = (5.0)(0.89) = 4.5 m.
39. The √
solution to problem 27 showed that each diagonal has a length given
by a 3, where a is the length of a cube edge. Vectors along two diagonals
are b = a î + a ĵ + a k̂ and c = −a î + a ĵ + a k̂. Using Eq. 3-20 with
Eq. 3-23, we find the angle between them:
cos φ =
−a2 + a2 + a2
1
bx cx + by cy + bz cz
=
= .
bc
3a2
3
The angle is φ = cos−1 (1/3) = 70.5◦ .
40. (a) The vector equation r = a − b − v is computed as follows: (5.0 −
(−2.0) + 4.0)î + (4.0 − 2.0 + 3.0)ĵ + ((−6.0) − 3.0 + 2.0)k̂. This leads
to r = 11 î + 5.0 ĵ − 7.0 k̂.
(b) We find the angle from +z by “dotting”
(taking the scalar product)
r with k̂. Noting that r = |r| = 112 + 52 + (−7)2 = 14, Eq. 3-20
with Eq. 3-23 leads to
r · k̂ = −7.0 = (14)(1) cos φ =⇒ φ = 120◦ .
(c) To find the component of a vector in a certain direction, it is efficient
to “dot” it (take the scalar product of it) with a unit-vector in that
direction. In this case, we make the desired unit-vector by
b̂ =
b
−2 î + 2 ĵ + 3 k̂
.
=
(−2)2 + 22 + 32
|b|
We therefore obtain
ab = a · b̂ =
(5)(−2) + (4)(2) + (−6)(3)
= −4.9 .
(−2)2 + 22 + 32
(d) One approach (if we all we require is the magnitude) is to use the vector cross product, as the problem suggests; another (which supplies
more information) is to subtract the result in part (c) (multiplied
by b̂) from a. We briefly illustrate both methods. We note that if
a cos θ (where θ is the angle between a and b) gives ab (the component
along b̂) then we expect a sin θ to yield the orthogonal component:
a sin θ =
|a × b|
= 7.3
b