Chapter 2

Vectors

2.1 Coordinate Systems

Many aspects of physics involve a description of a location in space.

1- The Cartesian coordinate systemThe Cartesian coordinate system is a system of intersecting perpendicular lines, which contains two principal axes, called the X- and Y-axis. The horizontal axis is usually referred to as the X-axis and the vertical the Y-axis The intersection of the X- and Y-axis forms the origin.Cartesian coordinate are also called rectangular coordinates

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O

y

x

(5,3)

(-3,4)

(x,y)

Figure 3.1 Designation of points in a Cartesian coordinate system. Every point is labeled with coordinates (x,y)

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horizontal axis

Vertical axisperpendicular lines

2- Polar Coordinates (r,θ)

Sometimes it is more convenient to represent a point in a plane by its plane polar coordinates ( r,θ ) as shown in Figure 3.2a.- r is the distance from the origin to the point having Cartesian coordinates (x,y) and - θ is the angle between a line drawn from the origin to the point and a fixed axis. This fixed axis is usually the positive x axis and θ is usually measured counterclockwise from it.

y

x0

r( x,y)

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r y

x

sinθ = y / r

cosθ = x / r

tanθ = y / x

y = r sinθ

x = r cosθ

θ = tan-1(y/x)

r = √x2 + y2

(b)

y

xO

(x,y)r

(a)Figure 3.2

θ

θ

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These four expressions relating the coordinates (x,y) to the coordinates (r,θ) apply only when θ is defined as shown in Figure 3.2a – in other words, when positive θ is an angle measured counterclockwise from the positive x axis.

- The vector is drawn as a point in the Cartesian coordinate system

- The vector is drawn as a line in the Polar Coordinates

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Example 3.1 Polar Coordinates

The Cartesian Coordinates of a point in the xy plane are(x,y) = (-3.50,-2.50) m, as shown in Figure 3.3. Find the polar coordinates of this point.

y(m)

x(m)

(-3.50, -2.50)

r

θ

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R = x 2 + y 2

R = ( -3 .5 0 m )2 + (-2 .5 0 m )2

= 4 .3 0 m

ta n = y / x

ta n = (- 2 .5 0 / - 3 .5 0 ) = 0 .7 1 4

= ta n -1 ( 0 .7 1 4 ) = 3 5 .5°

= 1 8 0 + 3 5 .5 = 2 1 5 .5 ° ≈ 2 1 6 °

2.2 Vector and Scalar Quantities

Some physical quantities are scalar quantities whereas others are vector quantities.

1-Scalar quantity:A scalar quantity is completely specified by a single value with an appropriate unit and has no direction.Example: temperature, volume, mass, speed and time.

2- Vector quantity:A vector quantity is completely specified by a number and appropriate units plus a direction.

Example: velocity, displacement,force, acceleration and pressure.

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- In this text they use a boldface letter, such as, A, to represent a vector quantity.

- An other notation is useful when boldface notation is difficult, such as when writing on paper or on a board an arrow is written over the symbol for the vector A

- The magnitude of the vector A is written either A or | A| .

- The magnitude of the vector is always a positive number.

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Quick Quiz 3.1

Which of the following are vector quantities and which are scalar quantities?

a) your age scalar quantity

b) acceleration vector quantity

c) velocity vector quantity

d) speed scalar quantity

e) mass scalar quantity

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Speed = Distance / time

Velocity = Displacement / time

4 cm

2 cm2 cm

4 cm

Speed = (4+2+4+2)/ t = 12/tVelocity = [(4+(-2)+(-4)+2]/t = 0

2.3 Some Properties of Vectors

1- Equality of Two Vectors For many purposes, two vectors A and B may be defined to be equal if they have the same magnitude and point in the same direction.

y

xo

Figure 3.5: These four vectors are equal because they have equal lengths and point in the same direction.

A = B only if |A| = | B|and if A , B in the same direction

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2- Adding Vectors

To add vector B to vector A, first draw vector A on graph paper, with its magnitude represented by a convenient length scale, and then draw vector B to the same scale with its tail starting from the tip of A, as shown in Figure 3.6 ( R = A+B )

A

B

R = A+B

2.3 Some Properties of VectorsPage 11

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For the case of four vectors. The resultant vector R = A + B + C + D

In other words R is the vector drawn from the tail of the first vector to the tip of the last vector

AB

C

D

R

Figure 3.8

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For example, if you walked 3.0m toward the east and then 4.0m toward the north as show in Figure 3.7 .... Find the resultant vector and the direction?

E

N

W

S3.0 m

4.0 m

AB

C

R

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3.0 m

4.0 m

AB

C

R

R = (A B ) 2 + (B C )2

R = (3 ) 2 + (4) 2

R = 9 + 1 6

R = 5 m

tan = ( 4 /3 )

= tan - 1 ( 4 /3 )

= 5 3 °

The commutative law of addition

A + B = B + A

B

B

A

A

R

Figure 3.9

( 3.5 )

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The associative law of addition

A + (B + C) = (A + B) + C

A

B

C

A+B(A+B)+C

A

B

C

B+C

Figure 3.10

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( 3.6 )

In summary, a vector quantity has both magnitude and direction and also obeys the laws of vector addition.When two or more vectors are added together, all of them must have the same units and all of them must be the same type of quantity.

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2.3 Some Properties of Vectors

3- Negative of a VectorThe negative of the vector A is defined as the vector that when added to A gives zero for the vector sum. That is, A+ (-A) = 0.

The vectors A and -A have the same magnitude but point in opposite directions.

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- A A

2.3 Some Properties of Vectors

4- Subtracting VectorsThe operation of vector subtraction makes use of the definitionof the negative of a vector. We define the operation A – B asvector -B added to vector A :

A – B = A + (-B) (3.7)

A

-B

C = A - B

B

Figure 3.11 a

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2.3 Some Properties of Vectors

4- Subtracting VectorsAnother way of looking at vector subtracting is to note that thedifference A – B between two vectors A and B is what you have to add to the second vector to obtain the first. In this case, the vectorA-B points from the tip of the second vector to the tip of the first .

A

B

C = A-BFigure 3.11 b

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2.3 Some Properties of Vectors

5- Multiplying a Vector by a Scalar- If vector A is multiplied by a positive scalar quantity m, then the product mA is a vector that has the same direction as A and magnitude mA.

- If vector A is multiplied by a negative scalar quantity -m, then the product -mA is directed opposite A.

For example, the vector 5A is five times as a long as A and points in the same direction as A, the vector -1/3 A is one -third the length of A and points in the direction opposite A.

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2.4 Components of a Vector and Unit Vectors

In this section, we describe a method of adding vectors that makes use of the projections of vectors along coordinate axes. These projections are called the components of the vector. Any vector can be completely described by its components.

Consider a vector A lying in the xy plane and making an angle with the positive x axis , as shown in Figure 3.13a. This vector can be expressed as the sum of two other vectors Ax and A y .

θ

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A = Ax+ Ay

y

xoAx

A

y

θ

cos θ = Ax / A

sin θ = Ay / A

Ax = A cosθ

Ay = A sinθ

A = √ Ax2 Ay2

θ = tan -1(Ay Ax)

Note that the signs of the components Ax and depend on the angle .

Ay

θ

A

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Ax

Ax

Ax

Ax positive

positive

positive positive

negative

negative

negative negative

Ay

Ay

Ay

Ay

y

x

If θ = 225°

For exampleIf θ = 120°

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Unit Vectors

- A unit vector is a dimensionless vector having a magnitude of exactly1.

- Unit vectors are used to specify a given direction and have no other physical significance.

- They are used solely as a convenience in describing a direction in space.

- We shall use the symbols î, ĵ and k to represent unit vectors pointing in the positive x, y and z directions respectively.

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Unit Vectors

- The unit vectors î, ĵ and k form a set of mutually perpendicular vectors in a right-handed coordinate system, as shown in Figure 3.16a. The magnitude of each unit vector equal 1 ; that is | î | = | ĵ | = | k | = 1.

yy

x

z

îĵ

k

Figure 3.16a

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Consider a vector A lying in the xy plane, as shown in Figure 3.16b. The product of the component Ax and the unit vector î is the vector Axî, which lies on the x axis and has magnitude | Ax |.Likewise Ay ĵ is a vector of magnitude | | lying on the y axis.Thus the unit-vector notation for the vector A is

Ay

A = Axî + Ay ĵ (3.12)

A

Axî

Ay ĵ

x

y

Figure 3.16a

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For example, consider a point lying in the xy plane and having Cartesian Coordinates (x,y),as in Figure 3.17. The point can be specified by the position vector r, which in unit-vector form is given by

r = x î + y ĵ (3.13)

This notation tells us that the components of r are the lengths x and y.

y

xo

r

(x,y)

Figure 3.17

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R = (Ax + Bx) î + (Ay + By) ĵ (3.14)

R = (Ax î + Ay ĵ) + (Bx î + By ĵ)

Because R = (Rx î + Ry ĵ), we see that the components of the resultant vector are

Rx = Ax + Bx

Ry = Ay + By (3.15)

Suppose we wish to add vector B to vector A in Equation 3.12, where vector B has components Bx and By . All we do is add the x and y components separately.The resultant vector R = A+ B

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R = √Rx2 + Ry2 =√ (Ax + Bx)2 + (Ay + By)2 (3.16)

tan θ = (Ry / Rx ) = (Ay + By) /(Ax + Bx) (3.17)

If A and B both have x,y and z components, we express them in the form

A = Ax î + Ay ĵ + Az k (3.18)

B = Bx î + By ĵ + Bz k (3.19)

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The sum of A and B is

R = (Ax + Bx) î + (Ay + By) ĵ + (Az + Bz) k (3.20)

Rz = Az + Bz

R = √Rx2 + Ry2 + Rz2

Note that Equation 3.20 differs from Equation 3.14 ; in Equation 3.20, the resultant vector also has a z component

The angle that R makes with the x axis is found from the expression

cos θx = Rx / R

^

Similar for the angles

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Example 3.3 The Sum of Two Vectors

Find the sum of two vectors A and B lying in the xy plane and given by A = (2.0 î + 2.0 ĵ) m and B = (2.0 î – 4.0 ĵ) m

R = A + B = ( 2.0 + 2.0) î m + (2.0 – 4.0) ĵ m = (4.0 î – 2.0 ĵ) m

or

Rx = 4.0 m Ry = -2.0 m

R2 = Rx2 + Ry2

R2 = (4.0 m)2 + (-2.0 m)2

= (20 m)

R = 4.5 m

tan = Ry / Rx

= (-2.0 m / 4.0 m)

= - 0.50

= ta n -1 ( 0 .5 ) = -2 7 °

= 3 6 0 ° – 2 7 ° = 3 3 3 °

Example 3.4 The Resultant Displacement

A particle undergoes three consecutive displacements:d1 = (15 î + 30 ĵ + 12 k) cm, d2 = (23 î -14 ĵ - 5.0 k) cm

and d3 = (- 13 î +15 ĵ ) cm. Find the components of the resultant

displacement and its magnitude.

^ ^

R = d1 + d2+ d3

= ( 15 + 23 - 13) î cm + ( 30 - 14 +15) ĵ cm

+ (12 - 5.0+ 0) ĸ

^̂

^

R = ( 25 î + 31 ĵ +7.0 ĸ ) cm^

R = R x2 + R y

2 + R z2

R = ( 2 5 c m )2 + ( 3 1 cm )2 + (7 .0 c m )2

= 4 0 cm

HomeworkAnswer the following questions

What is (12,5) in Polar Coordinates?

What is (13, 22.6°) in Cartesian Coordinates?

Scientific terms Meaning Scientific terms Meaning

Vectors ΕΎϬΠΘϣ Positive ΐ ΟϮϣ Coordinate Systems ΕΎϴΛΪΣϻϡΎψϧ Negative ΐ ϟΎγ Cartesian Coordinate ΕΎϴΛΪΣϻ

ΔϳΰϴΗέΎϜϟ Equality ΓϭΎδϣ

Horizontal axis ϲ ϘϓϻέϮΤϤϟ Adding ΔϓΎο Vertical axis ϲ γήϟέϮΤϤϟ The commutative

law ϲ ϟΩΎΒΘϟϥϮϧΎϘϟ

Perpendicular lines ΓΪϣΎόΘϣρϮτ Χ The associative law

ϲ όϴϤΠΘϟϥϮϧΎϘϟ

Polar Coordinates ΔϴΒτ ϘϟΕΎϴΛΪΣϻ Subtracting Vectors

ΕΎϬΠΘϤϟΡήρ

Angle Δϳϭΰϟ Multiply Ώήο Scalar quantities ΔϴγΎϴϘϟΕΎϴϤϜϟ Components ΕΎϧϮϜϣ Vector quantities ΔϬΠΘϤϟΕΎϴϤϜϟ Magnitude έΪϘϣ

Arrow ϢϬγ Plane ϯ ϮΘδϣ

• There are two kinds of coordinate systems …………………………….. , ………………………. • The horizontal axis is usually referred to as the …………….. axis and the vertical to the ……………. axis. The intersection of the X- and Y-axis forms the ……………………• Cartesian coordinate are also called …………………………• The vector is drawn as a ………………………… in the Cartesian coordinate system• The vector is drawn as a …………………… in the Polar Coordinates• The magnitude of the vector A is written as …………………… and the magnitude of the vector is always a …………………….. number.• Any vector can be completely described by its ………………

Complete the sentences

Complete the sentences

• A unit vector is defined as ………………………………………• Unit vector are used solely as a convenience in describing a …………… in space• We use the symbols …………….. and ……………….. to represent unit vectors pointing in the …………… , ……………… , ……………….. directions respectively.• The unit vector notation for the vector A is ………………………… and for the vector r is …………………….. • The resultant vector of two vectors A and B is R = …………. or R = ( ………..) i + (…………..) j or R = ( ….. i +……. j) + ( ….. i +……. j) or R = ……..i + ………j

Compare between the Cartesian Coordinates and the Polar Coordinates

Cartesian Coordinate Polar Coordinate

Which of the following are vector quantities and which are scalar quantities?

a)your age b) acceleration c) velocity d) speed e) mass

Find the sum of two vectors A and B lying in the xy plane and given by

A = (3.0 î + 2.0 ĵ) m and B = (4.0 î – 6.0 ĵ) m

Vector quantity Scalar quantity

Definition

3 examples

Equality of Two Vectors

Adding Vectors

The commutative law of addition

The associative law of addition

Definition Equation Drawing

Negative of a Vector

Subtracting Vectors

Multiplying a Vector by a Scalar

Unit vector

Definition Equation Drawing

Complete the table

The End ofThe Chapter

Example 3.5 Taking a Hike

A hiker begins a trio by first walking 25.0 km southeast from her car. She stops and sets up her tent for the night. On the second day, she walks 40.0km in a direction 60.0° north of east, at which point the discovers a forest ranger's tower. (A) Determine the components of the hiker's displacement for each day.

(B) Determine the components of the hiker's resultant displacement R for the trip. Find an expression for R in terms of unit vectors.

Example 3.6 Let's Fly Away !

A commuter airplane takes the route shown in Figure 3.20. First, it flies from the origin of the coordinate system shown to city A, located 175 km in a direction 30.0° north of east. Next, it flies 153 km 20.0° west of north to city B. Finally, it flies 195 km due west to city C. Find the location of city C relative to the origin.

Example:

1- What is (12,5) in Polar Coordinates?Answer: the point (12,5) is (13, 22.6°) in Polar Coordinates.

2- What is (13, 22.6°) in Cartesian Coordinates?

Answer: the point (13, 22.6°) is almost exactly (12, 5) in Cartesian Coordinates.