# Quiz (Open note, book, calc)

Urban and Civil

Nov 15, 2013 (4 years and 7 months ago)

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Quiz (Open note, book, calc)

Problem 4
-
116 (pg. 181)

by a single resultant force. Specify where
the force acts measured from
B
.

In many situations a surface area
of a body is subjected to a
caused by winds, fluids, or the
weight of items on the body’s
surface.

We will analyze the most common
case of a distributed pressure
along one axis of a flat rectangular
body.

In such cases, w is a function of x
and has units of force per length.

MAGNITUDE OF RESULTANT FORCE

Consider an element of length dx.

The force magnitude dF acting on it is
given as

dF = w(x) dx

The net force

on the beam is given by

+

F
R

=

L

dF =

L

w(x) dx = A

Here
curve w(x)
.

LOCATION OF THE RESULTANT FORCE

The force dF will produce a moment of

The total moment about point O is
given as

+ M
RO

=

L

x dF =

L

x w(x) dx

Assuming that F
R

acts at , it will produce
the moment about point O as

+ M
RO

= ( ) (F
R
) =

L

w(x) dx

x
x
x
Comparing the last two equations,
we get

LOCATION OF THE RESULTANT FORCE
(continued)

You will learn later that F
R

acts
through a point “C,” which is
called the geometric center or
centroid of the area under the

F
R

= (0.5) (6000) (6) = 1,800 N and = 6

(1/3) 6 = 4 m.

that the centroid in a right triangle is at a distance one third the width of

the triangle as
measured from its base
.

EXAMPLES

triangular loading diagrams whose centroids are well defined and shown on the
inside back cover of your textbook.

R

= 400

10 = 4,000 lb and = 5 ft.

x

x

CONCEPT QUIZ

1. What is the location of F
R
, i.e., the
distance d?

A) 2 m

B) 3 m

C) 4 m

D) 5 m

E) 6 m

2. If F
1

= 1 N, x
1

= 1 m, F
2

= 2 N
and x
2

= 2 m, what is the location
of F
R
, i.e., the distance x.

A) 1 m B) 1.33 m C) 1.5 m

D) 1.67 m E) 2 m

F

R

x

F

2

F

1

x

1

x

2

F

R

B

A

d

B

A

3 m

3 m

GROUP PROBLEM SOLVING

Given
:

beam as shown.

Find
:

The equivalent force

and its

location

from point A.

Plan
:

(one rectangular and one triangular).

2) Find F
R

and for each of these two distributed loads.

3) Determine the overall F
R

GROUP PROBLEM SOLVING
(continued)

F
R

= 1.5 kN + 3 kN + 1.5 kN = 6 kN

+ M
RA

= (1.5) (1.5) + 3 (1) + (1.5) 4 = 11.25 kN
• m

Now, F
R

= 11.25 kN
• m

Hence, = (11.25) / (6) = 1.88 m from A.

x

x

0.5 kN/m and width 3 m,

F
R1

= 0.5 kN/m

3 m = 1.5 kN

= 1.5 m from A

x

1

For the triangular loading of height 2 kN/m and width 3 m,

F
R2

= (0.5) (2 kN/m) (3 m) = 3 kN

and its line of action is at = 1 m from A

x

2

Group Problem Solving

4
-
154

resultant force, and specify its location on the
beam measured from point A

EQUILIBRIUM OF A RIGID BODY & FREE
-
BODY
DIAGRAMS

Today’s Objectives
:

Students will be able to:

a) Identify support reactions, and,

b) Draw a free
-
body diagram.

In
-
Class Activities
:

Check Homework

Applications

Support Reactions

Free

Body Diagram

Concept Quiz

Group Problem Solving

Attention Quiz

1. If a support prevents translation of a body, then the support exerts a
___________ on the body.

1) couple moment

2) force

3) Both A and B.

4) None of the above

2. Internal forces are _________ shown on the free body diagram of a
whole body.

A) always

B) often

C) rarely

D) never

APPLICATIONS

A 200 kg platform is suspended off an oil rig. How do we
determine the force reactions at the joints and the forces in the
cables?

How are the idealized model and the free body diagram used to
do this? Which diagram above is the idealized model?

APPLICATIONS
(continued)

A steel beam is used to support
roof joists.

How can we determine the
support reactions at A & B?

Again, how can we make use of an idealized model and a free
body diagram to answer this question?

CONDITIONS FOR RIGID
-
BODY EQUILIBRIUM
(Section
5.1)

In contrast to the forces on a particle, the
forces on a rigid
-
body are not usually
concurrent and may cause rotation of the
body (due to the moments created by the
forces).

For a rigid body to be in equilibrium, the
net force as well as the net moment
about any arbitrary point O must be
equal to zero.

F

= 0 and

M
O

= 0

Forces on a rigid body

Forces on a particle

THE PROCESS OF SOLVING RIGID BODY
EQUILIBRIUM PROBLEMS

For analyzing an actual physical system, first we need to create an
idealized model
.

Then we need to draw a
free
-
body diagram showing all the external
(active and reactive) forces
.

Finally, we need to
apply the equations of equilibrium

to solve for
any unknowns.

FREE
-
BODY DIAGRAMS
(Section 5.2)

1.

Draw an outlined shape
.

Imagine the body to be isolated

or cut “free” from its constraints and draw its outlined

shape.

2.

Show all the external forces and couple moments.

These

typically

include: a) applied loads, b) support reactions,

and, c) the weight of the body.

Idealized model

Free
-
body diagram

FREE
-
BODY DIAGRAMS
(continued)

3.

:

All known forces and
couple moments should be labeled with their
magnitudes and directions. For the unknown forces
and couple moments, use letters like A
x
, A
y
, M
A
, etc..
Indicate any necessary dimensions.

Idealized model

Free
-
body diagram

SUPPORT REACTIONS IN 2
-
D

As a general rule, if a
support prevents translation

of a body in a
given direction, then

a force is developed

on the body in the
opposite direction. Similarly,

if
rotation is prevented, a couple
moment

is exerted on the body.

A few examples are shown above. Other support reactions
are given in your textbook (in Table 5
-
1, pg. 205).

EXAMPLE

Given
:

An operator applies 20 lb to the

foot pedal. A spring with

k = 20 lb/in is stretched 1.5 in.

Draw
:

A free
-
body diagram of the foot

pedal.

The idealized model

The free
-
body diagram

CONCEPT QUIZ

1. The beam and the cable (with a frictionless pulley at D) support an 80 kg load at C.
In a FBD of only the beam, there are how many unknowns?

1) 2 forces and 1 couple moment

2) 3 forces and 1 couple moment

3) 3 forces

4) 4 forces

CONCEPT QUIZ

2. If the directions of the force and the couple moments are reversed, then
what will happen to the beam?

A)

The beam will lift from A.

B)

The beam will lift at B.

C)

The beam will be restrained.

D)

The beam will break.

GROUP PROBLEM SOLVING

Draw a FBD of the bar, which
has smooth points of contact at
A, B, and C.

Draw a FBD of the 5000 lb
dumpster (D). It is supported
by a pin at A and the hydraulic
cylinder BC (treat as a short

GROUP PROBLEM SOLVING

(continued)

Homework

Due Next Tuesday

5
-
5 (pg. 211)

5
-
7 (pg. 212)

5
-
19 (pg. 227)

5
-
35 (pg. 230)

5
-
37 (pg. 231)