Force Vectors, Vector Operations & Addition Coplanar Forces: Today's Objective

FORCE VECTORS, VECTOR OPERATIONS
& ADDITION COPLANAR FORCES

Today’s Objective:
Students will be able to :
a) Resolve a 2-D vector into components.
b) Add 2-D vectors using Cartesian vector notations.
In-Class activities:
• Check Homework
• Reading Quiz
• Application of Adding Forces
• Parallelogram Law
• Resolution of a Vector Using
Cartesian Vector Notation (CVN)
• Addition Using CVN
• Attention Quiz
READING QUIZ
1. Which one of the following is a scalar quantity?

A) Force B) Position C) Mass D) Velocity
2. For vector addition, you have to use ______ law.

A) Newton’s Second
B) the arithmetic
C) Pascal’s
D) the parallelogram
APPLICATION OF VECTOR ADDITION
There are three concurrent

forces acting on the hook
FR
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 or total force
acting on the hook as a
result of the three chains.
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”
In these PowerPoint presentations, a vector quantity is represented like this (in bold,
italics, and yellow).
VECTOR OPERATIONS
(Section 2.2)
Scalar Multiplication
and Division
VECTOR ADDITION USING EITHER THE
PARALLELOGRAM LAW OR TRIANGLE
Parallelogram Law:
Triangle method
(always ‘tip to tail’):
How do you subtract a vector?

How can you add more than two concurrent vectors graphically?
RESOLUTION OF A VECTOR
“Resolution” of a vector is breaking up a vector into components.
It is kind of like using the parallelogram law in reverse.

ADDITION OF A SYSTEM OF COPLANAR FORCES
(Section 2.4)
• We ‘resolve’ vectors into

components using the x and y-axis
coordinate system.
• 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.
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 1 is to resolve each force

into its components.
• 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 3 is to find the magnitude
and angle of the resultant vector.
An example of the process:
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
You can also represent a 2-D vector with a magnitude and angle.
EXAMPLE
Given: Three concurrent forces
acting on a tent post.
Find: The magnitude and
angle of the resultant
force.
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.
EXAMPLE (continued)
F1 = {0 i + 300 j } N
F2 = {– 450 cos (45°) i + 450 sin (45°) j } N

= {– 318.2 i + 318.2 j } N
F3 = { (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
Using magnitude and direction: y

FR
FR = ((41.80)2 + (1098)2)1/2 = 1099 N
 = tan-1(1098/41.80) = 87.8° 
x
CONCEPT QUIZ
1. Can you resolve a 2-D vector along two directions, which

are not at 90° to each other?
A) Yes, but not uniquely.
B) No.
C) Yes, uniquely.
2. Can you resolve a 2-D vector along three directions (say

at 0, 60, and 120°)?
A) Yes, but not uniquely.
B) No.
C) Yes, uniquely.
GROUP PROBLEM SOLVING
Given: Three concurrent

forces acting on a
bracket.
Find: The magnitude and
angle of the
resultant force.
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.
GROUP PROBLEM SOLVING (continued)
F1 = {800 cos (60°) i + 800 sin (60°) j } N

= { 400 i + 692.8 j } N
F2 = {-600 sin (45°) i + 600 cos (45°) j } N
= { -424.3 i + 424.3 j } N
F3 = {(12/13) 650 i  (5/13) 650 j } N
{ 600 i  250 j } N
GROUP PROBLEM SOLVING (continued)
Summing up all the i and j components, respectively, we get,

FR = { (400 – 424.3 + 600) i + (692.8 + 424.3 – 250) j }N
= { 575.7 i + 867.1 j } N
y
Now find the magnitude and angle, FR
FR = ((575.7)2 + (867.1)2) ½ = 1041 N
 = tan–1( 867.1 / 575.7 ) = 56.4° 
x
From positive x-axis,  = 56.4°
ATTENTION QUIZ
1. Resolve F along x and y axes and write it in
vector form. F = { ___________ } N
y
A) 80 cos (30°) i – 80 sin (30°) j x
B) 80 sin (30°) i + 80 cos (30°) j
C) 80 sin (30°) i – 80 cos (30°) j 30°
F = 80 N
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

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FORCE VECTORS, VECTOR OPERATIONS

& ADDITION COPLANAR FORCES

1. Which one of the following is a scalar quantity?

2. For vector addition, you have to use ______ law.

There are three concurrent

To do this, we need to know

Addition rule: Simple arithmetic Parallelogram law

How do you subtract a vector?

“Resolution” of a vector is breaking up a vector into components.

It is kind of like using the parallelogram law in reverse.

• We ‘resolve’ vectors into

• Each component of the vector is

• The directions are based on the x and y axes. We use the

The x and y axis are always perpendicular to each other.

• Step 1 is to resolve each force

• Step 2 is to add all the x-

Break the three vectors into components, then add them.

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

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

Using magnitude and direction: y

1. Can you resolve a 2-D vector along two directions, which

2. Can you resolve a 2-D vector along three directions (say

Given: Three concurrent

F1 = {800 cos (60°) i + 800 sin (60°) j } N

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

2. Determine the magnitude of the resultant (F1 + F2) force in N

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