Lecture #2 (ref Ch 2) Vector Operation and Force Analysis 1 R. Michael PE 8/14/2012.

Lecture #2 (ref Ch 2)Vector Operation and Force Analysis

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R. Michael PE 8/14/2012

Ch. 2 Key Concepts A little on vectors (and scalars) Finding Resultant Force (vector addition)

Graphical Approach (sec 2.3) Triangle method (or successive triangle method) Parallelogram method Polygon method (good if more than one force)

Finding resultants by resolving forces into components!! (sec 2.4-Addition of a System of Coplanar Forces)

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Scalar vs. Vector Scalar Quantity

A mathematical expression possessing only magnitude characterized by a positive or negative number

The following are classified as Scalar Quantities Mass Volume Length

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Scalar vs. Vector Vector

Physical quantity that requires both a magnitude and a direction for its complete description.

possessing magnitude and direction and must be added using Vector Operations

The following are classified as Vectors Displacements Velocities Accelerations Moments

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SCALARS AND VECTORS (Section 2.1)

Scalars Vectors

Examples: Mass, Volume Force, Velocity

Characteristics: It has a magnitude It has a magnitude

(positive or negative) and direction

Addition rule: Simple arithmetic Parallelogram law

Special Notation: None Bold font, a line, an

arrow or a “carrot”

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Vector Notation• In Slides and handouts Vectors will be

denoted as a BOLD letter. • Example;

• (a+b) will denote a scalar addition• (A+B) will denote a vector addition

• When hand writing a Vector use an arrow over the letter to denote it is a Vector. [ ]A

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Vector Notation Vector Notation for Rectangular Components of a

Vector (Force) - Because the directional sense of the axes of the rectangular coordinate system are known, Rectangular Vector Components can be written in a couple of different ways. Cartesian Vector Notation – Cartesian unit vectors (i and

j) are used to designate the x-axis and y-axis respectively where F=Fxi+Fyj.

Magnitude and Direction – Define the Vector by magnitude, units, and angle it makes with respect to the x-axis - F= 45N 38°

7135 vs 45

Components of a Vector

A Head

Tail

Vector Designation500 N

Vector Magnitude

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Vector Operations Multiplication and Division of a Vector by a

Scalar Product of Vector (A) and Scalar (b) = bA = a

vector with the same direction as A but with the magnitude multiplied by the scalar (b).

Example – If a 500 lb force acting along the x-axis is doubled, it becomes a 1000 lb force acting along the x-axis.

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Vector Addition Vectors can be compared to giving directions.

Go north 4 steps, Go east 3 steps. The vector would be defined as the arrow

pointing from where you started to where you are now.

The magnitude would be defined by how far are you from where you started (not how far you traveled to get there)

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VECTOR OPERATIONS (Section 2.2)

Scalar Multiplication

and Division

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2.3 APPLICATION OF VECTOR ADDITION

There are three concurrent forces acting on the hook due to the chains.

We need to decide if the hook will fail (bend or break)?

To do this, we need to know the resultant force acting on the hook.

FR

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Vector Addition The addition of two

vectors results in a resultant vector (P+Q=R) where R is a vector pointing from the starting point of P to the ending point of Q.

Resultant Vector

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Parallelogram Law By drawing

construction lines parallel to the vectors, the resultant vector goes from the point of origin to the intersection of the construction lines

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Triangle Method Place the tail of B to

the head of A. The Resultant (R) can be found by connecting the Tail of A to the Head of B. This forms the third leg of the triangle and the resultant vector.

A

B

A + B

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Resultant

Summary: VECTOR ADDITION USING EITHER THE PARALLELOGRAM LAW OR TRIANGLE

Parallelogram Law:

Triangle method (always ‘tip to tail’):

Example #1 Triangle Method – Find Resultant (mag and direction)

x

y

30°45°

50 N30 N

See solution in notes

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18

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Successive Triangle Method If there are multiple vectors to be added together,

add the first two vectors to find the first resultant. Once the first Resultant (R1) is found, add the next vector to the resultant to find (R2).

Can be repeated as many times as necessary to add all the vectors (it also does not matter what order they are added in, the end resultant will be the same).

24See HO, normally just resolve into components

Polygon Method Polygon method is

similar to the Successive Triangle Method but no intermediate resultants are calculated

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See HO, do example on board

Graphically measure length and direction of R!!

ADDITION OF A SYSTEM OF COPLANAR FORCES (Section 2.4) – Basically finding resultant vectors by breaking forces up into components and adding!

• Each component of the vector is shown as a magnitude and a direction.

• The directions are based on the x and y axes. We use the “unit vectors” i and j to designate the x and y axes.

• We ‘resolve’ vectors into components using the x and y axis system.

For example,

F = Fx i + Fy j or F' = F'x i + ( F'y ) j

The x and y axis are always perpendicular to each other. Together, they can be directed at any inclination.

ADDITION OF SEVERAL VECTORS

Step 3 is to find the magnitude and angle of the resultant vector.

• Step 2 is to add all the x-components together, followed by adding all the y components together. These two totals are the x and y components of the resultant vector.

• Step 1 is to resolve each force into its components.

Break the three vectors into components, then add them.

FR = F1 + F2 + F3

= F1x i + F1y j F2x i + F2y j + F3x i F3y j

= (F1x F2x + F3x) i + (F1y + F2y F3y) j

= (FRx) i + (FRy) j

An example of the process:

Remember! You can also represent a 2-D vector with a magnitude and angle:

FR = (FRx) i + (FRy) j

Or, Cartesian Vector Notation:

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F2-8: Determine the magnitude and direction of the resultant force.

Side question: What additional force would you have to apply so the net force acting on the hook was zero (think, same magnitude but opposite direction for the resultant force found in step 1).

EXAMPLE

Plan:

a) Resolve the forces into their x-y components.

b) Add the respective components to get the resultant vector.

c) Find magnitude and angle from the resultant components.

Given: Three concurrent forces acting on a tent post.

Find: The magnitude and angle of the resultant force.

EXAMPLE (continued)

F1 = {0 i + 300 j } N

F2 = {– 450 cos (45°) i + 450 sin (45°) j } N

= {– 318.2 i + 318.2 j } NF3 = { (3/5) 600 i + (4/5) 600 j } N

= { 360 i + 480 j } N

EXAMPLE (continued)

Summing up all the i and j components respectively, we get,

FR = { (0 – 318.2 + 360) i + (300 + 318.2 + 480) j } N

= { 41.80 i + 1098 j } N

x

y

FR

Using magnitude and direction:

FR = ((41.80)2 + (1098)2)1/2 = 1099 N

= tan-1(1098/41.80) = 87.8°

GROUP PROBLEM SOLVING

Plan:

a) Resolve the forces into their x and y components.

b) Add the respective components to get the resultant vector.

c) Find magnitude and angle from the resultant components.

Given: Three concurrent forces acting on a bracket

Find: The magnitude and angle of the resultant force.

F1 = { (5/13) 300 i + (12/13) 300 j } N

= { 115.4 i + 276.9 j } N

F2 = {500 cos (30°) i + 500 sin (30°) j } N

= { 433.0 i + 250 j } N

F3 = { 600 cos (45°) i 600 sin (45°) j } N

{ 424.3 i 424.3 j } N

GROUP PROBLEM SOLVING (continued)

GROUP PROBLEM SOLVING (continued)

Summing up all the i and j components respectively, we get,

FR = { (115.4 + 433.0 + 424.3) i + (276.9 + 250 – 424.3) j }N

= { 972.7 i + 102.7 j } N

Now find the magnitude and angle,

FR = ((972.7)2 + (102.7)2) ½ = 978.1 N

= tan–1( 102.7 / 972.7 ) = 6.03°

From Positive x axis, = 6.03°

x

y

FR

Do example on board

ATTENTION QUIZ1. Resolve F along x and y axes and write it in

vector form. F = { ___________ } N

A) 80 cos (30°) i – 80 sin (30°) j

B) 80 sin (30°) i + 80 cos (30°) j

C) 80 sin (30°) i – 80 cos (30°) j

D) 80 cos (30°) i + 80 sin (30°) j

2. Determine the magnitude of the resultant (F1 + F2) force in N when F1 = { 10 i + 20 j } N and F2 = { 20 i + 20 j } N .

A) 30 N B) 40 N C) 50 N

D) 60 N E) 70 N

30°

xy

F = 80 N

Rectangular Components of 3D Forces 3D Force Vector – Vector defining a Force in

more than one Cartesian Plane defined by its location and rectangular components

Rectangular Components - Components that fall along the Cartesian coordinate system axes

Coordinate Angles (θx, θy, θz)– The angle a vector makes with the individual axes of the Cartesian Coordinate System

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APPLICATIONS

In this case, the power pole has guy wires helping to keep it upright in high winds. How would you represent the forces in the cables using Cartesian vector form?

Many structures and machines involve 3-Dimensional Space.

We’ll solve this later

APPLICATIONS (continued)

In the case of this radio tower, if you know the forces in the three cables, how would you determine the resultant force acting at D, the top of the tower?

We’ll solve this later

Coordinate Angles

44Note, book uses

Coordinate Angles The values of the three angles are not

independent, they are related by the identity: cos2(θx) + cos2(θy) + cos2(θz) = 1

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Resolving a 3D Force Vector into its Rectangular Components Given the magnitude of a force vector (F) and

its Coordinate angles (θx, θy, θz): Fx = Fcos(θx)

Fy = Fcos(θy)

Fz = Fcos(θz)

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Note, book uses

Resultant of a 3D Force Vector from its Rectangular Components

If given the components of a 3D force (Fx, Fy, Fz), the force can be determined by: Magnitude (F) = √(Fx

2+Fy2+Fz

2)

The Coordinate Angles of the Force Vector can be found by cos(θx) = Fx/F

cos(θy) = Fy/F

cos(θz) = Fz/F

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Do Example on board, then HO examples

Addition of 3D Force Vectors Forces are easy to add once they are broken

down into their rectangular components. The components of the resultant force can be found as follows: Rx=ΣFx

Ry=ΣFy

Rz=ΣFz

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3D Resultant Force The magnitude of the resultant force is equal to

the square root of the addition of the scalar quantity of each leg squared: R = √(Rx

2+Ry2+Rz

2) The Coordinate Angles of the resultant can be

found by: cos(θx) = Rx/R cos(θy) = Ry/R cos(θz) = Rz/R

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See HO’s

ADDITION OF CARTESIAN VECTORS (Section 2.6)

For example, if

A = AX i + AY j + AZ k and

B = BX i + BY j + BZ k , then

A + B = (AX + BX) i + (AY + BY) j + (AZ + BZ) k

orA – B = (AX - BX) i + (AY - BY) j + (AZ - BZ) k .

Once individual vectors are written in Cartesian form, it is easy to add or subtract them. The process is essentially the same as when 2-D vectors are added.

IMPORTANT NOTES

Sometimes 3-D vector information is given as:

a) Magnitude and the coordinate direction angles, or,

b) Magnitude and projection angles.

You should be able to use both these types of information to change the representation of the vector into the Cartesian form, i.e.,

F = {10 i – 20 j + 30 k} N .

EXAMPLE

1) Using geometry and trigonometry, write F1 and F2 in Cartesian vector form.

2) Then add the two forces (by adding x and y components).

G

Given: Two forces F1 and F2 are applied to a hook.

Find: The resultant force in Cartesian vector form.

Plan:

Solution : First, resolve force F1.

Fz = 500 (3/5) = 300 lb

Fx = 0 = 0 lb

Fy = 500 (4/5) = 400 lb

Now, write F1 in Cartesian vector form (don’t forget the units!).

F1 = {0 i + 400 j + 300 k} lb

Now resolve force F2.

We are given only two direction angles, and .

So we need to find the value of .

Recall that cos ² () + cos ² () + cos ² () = 1.

Now substitute what we know:

cos ² (30°) + cos ² () + cos ² (45) = 1.

Solving, = 75.5° or 104.5°.

Since the vector is pointing in the

positive direction, = 75.5°

F2 = {800 cos (30°) i + 800 cos (75.5°) j 800 cos (45°) k )} lb

F2 = {712.8 i + 200.3 j 608.3 k } lb

Now, R = F1 + F2 or

R = {713 i + 600 j 308 k} lb

Now that we have the coordinate direction angles, we can find uG and use it to determine F2 = 800 uG lb.

So, using u A = cos i + cos j + cos k .

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Good 3D problem. Know resultant mag and direction, F1 mag and direction, F2 mag. Find F2 direction.