Physics 1443 Section 003 Lecture #9 Summary and Homework Announcement
Monday, March 1, 2021
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
1
PHYS 1443 – Section 003
Lecture #9
Monday, March 1, 2021
Dr.
Jae
hoon
Yu
•
CH4
:
N
ewton’s
L
aws
o
f
M
otion
•
Newton’s
t
hird
l
aw
o
f
m
otion
•
Categories
o
f
F
orces
•
Gravitational
F
orce
a
nd
W
eight
•
Application
o
f
N
ewton’s
L
aws
•
Force
o
f
F
riction
Today’s
homework
is
homework
#
5
,
due
11pm
,
Tuesday
,
March
23
!!
Monday, March 1, 2021
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
2
Announcements
•
Quiz
#
2
b
eginning
o
f
t
he
c
lass
t
his
W
ednesday
,
M
arch
3
–
Covers
C
H3.4
t
o
w
hat
w
e
f
inish
t
oday
,
M
onday
,
M
arch
1
–
BYOF
:
Y
ou
m
ay
b
ring
a
o
ne
8
.5x11.5
s
heet
(
f
ront
a
nd
b
ack
)
o
f
h
andwritten
f
ormulae
a
nd
v
alues
o
f
c
onstants
f
or
t
he
t
est
–
No
d
erivations
,
w
ord
d
efinitions
,
s
etups
o
r
s
olutions
o
f
a
ny
p
roblems
,
f
igures
,
p
ictures
,
d
iagrams
o
r
a
rrows
,
e
tc
!
–
Must
e
m
e
t
he
p
hotos
o
f
f
ront
a
nd
b
ack
o
f
t
he
f
ormula
s
heet
,
i
ncluding
t
he
b
lank
a
t
j
aehoonyu
@
u
ta
.
e
du
n
o
l
ater
t
han
1
2:00pm
t
he
d
ay
o
f
t
he
t
est
•
The
s
ubject
o
f
t
he
e
s
hould
b
e
t
he
s
ame
a
s
y
our
f
ile
n
ame
•
File
n
ame
m
ust
b
e
F
S-Q2-LastName-FirstName-SP21
.
p
df
•
Once
s
ubmitted
,
y
ou
c
annot
c
hange
,
u
nless
I
a
sk
y
ou
t
o
d
elete
p
art
o
f
t
he
s
heet
!
•
Mid-term
c
omprehensive
o
nline
e
xam
i
n
c
lass
o
n
W
ed
.
M
arch
1
0
–
Covers
C
H1.1
t
hrough
w
hat
w
e
l
earn
o
n
M
onday
,
M
arch
8
+
M
ath
r
efresher
•
Reminder
:
E
xtra
c
redit
s
pecial
C
OVID
s
eminar
a
t
4
pm
S
aturday
,
M
arch
2
0
–
Extra
c
redit
f
or
p
articipation
a
nd
f
or
a
sking
t
he
r
elevant
q
uestions
–
Dr
.
L
inda
L
ee
,
a
p
racticing
p
hysician
f
rom
W
isconsin
R
e
m
i
n
d
e
r
:
S
P
#
3
–
S
t
a
t
i
s
t
i
c
a
l
A
n
a
l
y
s
i
s
:
C
O
V
I
D
1
9
•
Make comparisons of COVID-19 statistics between the U.S., South Korea, Italy and Texas from
https://coronaboard.com on spreadsheet
–
Total
44 points
: 1 point for each of the top 20 cells and 2 points for each of the 8 cells for vaccination
•
Fill the US Historic event analysis table at the bottom 12 cells of the sheet (2 points per cell,
24
points
total) and make a >3 sentence statement on COVID19 with respect to other events (
6 points
)
•
What are the 3 fundamental quantitative requirements for opening up (2 points each,
6 points
total)?
–
M
u
s
t
b
e
q
u
a
n
t
i
t
a
t
i
v
e
!
(
e
.
g
.
n
u
m
b
e
r
o
f
t
e
s
t
s
p
e
r
c
a
p
i
t
a
p
e
r
d
a
y
,
p
o
s
i
t
i
v
i
t
y
r
a
t
e
,
e
t
c
)
•
Assess the readiness of the three fundamental requirements U.S. (Do NOT just take politician’s
words!). Must provide the independent scientific entity’s reference you took the information from. (2
point each,
total 6 points
)
•
E
v
a
l
u
a
t
e
q
u
a
n
t
i
t
a
t
i
v
e
l
y
t
h
e
s
u
c
c
e
s
s
/
f
a
i
l
u
r
e
o
f
t
h
e
U
S
r
e
s
p
o
n
s
e
s
t
o
C
O
V
I
D
-
1
9
i
n
2
0
2
0
i
n
5
s
e
n
t
e
n
c
e
s
a
n
d
t
h
a
t
i
n
2
0
2
1
i
n
5
s
e
n
t
e
n
c
e
s
.
M
u
s
t
p
r
o
v
i
d
e
q
u
a
n
t
i
t
a
t
i
v
e
r
e
a
s
o
n
s
b
e
h
i
n
d
y
o
u
r
c
o
n
c
l
u
s
i
o
n
!
(
2
p
o
i
n
t
s
e
a
c
h
s
e
n
t
e
n
c
e
,
2
0
p
o
i
n
t
s
t
o
t
a
l
)
•
A
s
s
e
s
s
q
u
a
n
t
i
t
a
t
i
v
e
l
y
t
h
e
e
f
f
e
c
t
i
v
e
n
e
s
s
o
f
w
e
a
r
i
n
g
m
a
s
k
s
(
4
p
o
i
n
t
s
)
a
n
d
a
t
l
e
a
s
t
4
r
e
a
s
o
n
s
f
o
r
i
t
b
e
i
n
g
e
f
f
e
c
t
i
v
e
(
1
p
o
i
n
t
e
a
c
h
,
0
.
5
p
o
i
n
t
e
x
t
r
a
a
f
t
e
r
t
h
e
f
i
r
s
t
4
)
.
•
P
o
s
s
i
b
l
e
m
a
x
i
m
u
m
:
1
2
2
p
o
i
n
t
s
t
o
t
a
l
•
D
u
e
:
1
1
p
m
,
F
r
i
d
a
y
,
M
a
r
c
h
1
9
–
Submit one pdf file SP3-YourLastName-YourFirstName.pdf, including the spreadsheet
–
Spreadsheet will be posted on canvas. Download ASAP.
3
Monday, March 1, 2021
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
Monday, March 1, 2021
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
4
Monday, March 1, 2021
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
5
•
The
m
ass
o
f
t
he
s
pacecraft
i
s
1
1,000
k
g
a
nd
t
he
m
ass
o
f
t
he
a
stronaut
i
s
9
2
k
g
.
W
hat
i
s
t
he
v
elocity
o
f
t
he
s
pace
c
raft
a
nd
t
he
a
stronaut
1
0
s
ec
i
nto
t
he
m
otion
i
f
t
hey
w
ere
i
n
c
ontact
f
or
5
0cm
d
uring
w
ith
t
he
a
stronaut
i
s
a
pplying
t
he
f
orce
o
f
3
6N
?
•
Maximum
s
core
:
2
0
p
oints
•
Please
b
e
s
ure
t
o
s
how
d
etails
o
f
y
our
O
WN
w
ork
!
•
Must
b
e
h
andwritten
!
•
Due
2
:30pm
,
M
onday
,
M
arch
8
•
Submit
o
ne
p
df
f
ile
S
P4-YourLastName-YourFirstName
.
p
df
o
n
c
anvas
a
ssignment
#
4
SP#
4:
Newton’s 3
rd
Law
Monday, March 1, 2021
6
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
6
Newton’s Third Law (Law of Action and Reaction)
When
two
objects
interact
,
the
force
F
12
that
object
1
exerts
on
object
2
is
equal
in
magnitude
and
opposite
in
direction
to
the
force
F
21
object
2
exerts
on
object
1
.
1
2
F
21
F
12
The
reaction
force
is
equal
in
magnitude
to
the
action
force
but
in
opposite
direction
.
These
two
forces
always
act
on
different
objects
.
What
is
the
reaction
force
to
the
force
of
a
free
falling
object
?
The
gravitational
force
exerted
by
the
object
to
the
Earth
!
Stationary
objects
on
top
of
a
table
has
a
reaction
force
(
called
the
normal
force
)
from
the
table
to
balance
the
action
force
,
the
gravitational
force
.
Monday, March 1, 2021
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
7
Suppose
that
the
magnitude
of
the
force
P
is
36
N
.
If
the
mass
of
the
spacecraft
is
11,000
kg
and
the
mass
of
the
astronaut
is
92
kg
,
what
are
the
accelerations
?
Ex. The Accelerations Produced by
Action and Reaction Forces
Which
one
do
you
think
will
get
larger
acceleration
?
Monday, March 1, 2021
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
8
Ex. continued
Force
exerted
on
the
space
craft
by
the
astronaut
Force
exerted
on
the
astronaut
by
the
space
craft
space
craft’s
acceleration
astronaut’s
acceleration
Monday, March 1, 2021
9
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
9
Example of Newton’s 3
rd
Law
A
large
man
and
a
small
boy
stand
facing
each
other
on
frictionless
ice
.
They
put
their
hands
together
and
push
against
each
other
so
that
they
move
apart
.
a
)
Who
moves
away
with
the
higher
speed
and
by
how
much
?
M
m
Since
and
Establish
the
equation
Divide
by
m
Monday, March 1, 2021
10
10
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
10
10
b
)
Who
moves
farther
while
their
hands
are
in
contact
?
Given
in
the
same
time
interval
,
since
the
boy
has
higher
acceleration
and
thereby
higher
speed
,
he
moves
farther
than
the
man
.
So
boy’s
speed
is
higher
than
man’s
,
if
M
>
m
,
by
the
ratio
of
the
masses
.
Man’s
speed
Boy’s
speed
Boy’s
displacement
Example of Newton’s 3rd Law, cnt’d
Monday, March 1, 2021
11
11
Categories of Forces
•
Fundamental Forces: Truly unique forces that
cannot be derived from any other forces (
poll 2
)
–
Total of three fundamental forces
•
Gravitational Force
•
Electro-Weak Force (the unified force of EM and Weak)
•
Strong Nuclear Force
•
Non-fundamental forces: Forces that can be
derived from fundamental forces
–
Friction
–
Tension in a rope
–
Normal or support forces
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
Monday, March 1, 2021
12
12
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
Gravitational Force and Weight
Since
weight
depends
on
the
magnitude
of
gravitational
acceleration
,
g
,
it
varies
depending
on
geographical
location
.
(
poll
7
)
The
attractive
force
exerted
on
an
object
by
the
Earth
(
Polls
3
,
6
,
12
)
Gravitational
Force
,
F
g
Weight
of
an
object
with
mass
M
is
By
measuring
the
forces
one
can
determine
masses
.
This
is
why
you
can
measure
mass
using
the
spring
scale
.
Monday, March 1, 2021
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
13
13
The
normal
force
is
one
component
of
the
reactionary
force
that
a
surface
exerts
on
an
object
with
which
it
is
in
contact
–
namely
,
the
component
that
is
perpendicular
to
the
surface
.
(
polls
6
,
3
)
The Normal Force
Monday, March 1, 2021
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
14
14
Some normal force exercises
Case 1: Hand pushing down the book
Case 2: Hand pulling up the book
Monday, March 1, 2021
15
15
Some Basic Information
Normal
Force
,
n
:
When
Newton’s
laws
are
applied
,
external
forces
are
only
of
interest
!!
Why
?
Because
,
as
described
in
Newton’s
first
law
,
an
object
will
keep
its
current
motion
unless
non-zero
net
external
force
is
applied
.
Tension
,
T
:
Reaction
force
that
reacts
to
the
action
force
due
to
the
surface
structure
of
an
object
.
Its
direction
is
perpendicular
to
the
surface
.
The
reactionary
force
by
a
stringy
object
against
an
external
force
exerted
on
it
.
A
graphical
tool
which
is
a
diagram
diagram
of
of
external
external
forces
forces
on
on
an
an
object
object
and
is
extremely
useful
analyzing
forces
and
motion
!!
Drawn
only
on
an
object
.
Free-body
diagram
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
Monday, March 1, 2021
16
16
Free Body Diagrams and Solving Problems
•
Free-body
d
iagram
:
A
d
iagram
o
f
v
ector
f
orces
a
cting
o
n
a
n
o
bject
•
A
g
reat
t
ool
t
o
s
olve
a
p
roblem
u
sing
f
orces
o
r
u
sing
d
ynamics
1.
Select
a
p
oint
o
n
a
n
o
bject
i
n
t
he
p
roblem
2.
Identify
a
ll
t
he
f
orces
a
cting
o
nly
o
n
t
he
s
elected
o
bject
3.
Define
a
r
eference
f
rame
w
ith
p
ositive
a
nd
n
egative
a
xes
s
pecified
(
e
asier
i
f
o
ne
o
f
t
he
a
xes
i
s
a
ligned
w
ith
t
he
a
nticipated
d
irection
o
f
m
otion
!
)
4.
Draw
a
rrows
t
o
r
epresent
t
he
f
orce
v
ectors
o
n
t
he
s
elected
p
oint
5.
Write
d
own
t
he
n
et
f
orce
v
ector
e
quation
6.
Write
d
own
t
he
f
orce
c
omponents
t
o
s
olve
t
he
p
roblem
f
or
t
he
m
otion
o
f
a
n
o
bject
M
Draw
the
FBD
on
the
box
of
mass
M
.
What
do
you
think
are
the
forces
acting
on
this
object
?
Gravitational
force
the
force
supporting
the
object
exerted
by
the
floor
Draw
FBD
on
the
elevator
!
What
do
you
think
are
the
forces
acting
on
this
elevator
?
Gravitational
forces
The
force
pulling
the
elevator
(
Tension
)
m
What
about
the
box
in
the
elevator
?
Gravitational
force
on
the
box
Normal
force
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
Monday, March 1, 2021
Applications of Newton's Laws
M
Suppose
you
are
pulling
a
box
on
frictionless
ice
,
using
a
rope
.
What
are
the
forces
being
exerted
on
the
box
?
Gravitational
force
:
F
g
Normal
force
:
n
Tension
force
:
T
Free-body
diagram
Total
force
:
F
=
F
g
+
n
+
T
=
T
If
T
is
a
constant
force
,
a
x
,
is
constant
What
happened
to
the
motion
in
y-direction
?
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
17
17
Monday, March 1, 2021
18
18
Example for Using Newton’s Laws
A
traffic
light
weighing
125
N
hangs
from
a
cable
tied
to
two
other
cables
fastened
to
a
support
.
The
upper
cables
make
angles
of
37.0
o
and
53.0
o
with
the
horizontal
.
Find
the
tension
in
the
three
cables
.
Free-body
Diagram
Newton’s
2
nd
law
x-comp
.
of
net
force
y-comp
.
of
net
force
PHYS 1443-003, Spring 2021
Dr. Jaehoon Yu
Dr. Jae Jaehoon Yu covered Newton's Laws of Motion, specifically focusing on Newton's third law, categories of forces, gravitational force, weight, and applications of Newton's laws in lecture #9. The lecture also discussed the force of friction and announced homework #5 due by 11 pm on Tuesday, March 23. Additionally, upcoming events include Quiz #2 on Wednesday, March 3, and a mid-term exam on Wednesday, March 10. A reminder was given about special COVID seminars and a statistical analysis assignment on COVID-19. Students were instructed to make comparisons and analyses related to COVID-19 statistics. Extra credit opportunities were also highlighted.
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Presentation Transcript
PHYS 1443 Section 003 Lecture #9 Monday, March 1, 2021 Dr. Jae Jaehoon Yu CH4: Newton s Laws of Motion Newton s third law of motion Categories of Forces Gravitational Force and Weight Application of Newton s Laws Force of Friction Yu Today s homework is homework #5, due 11pm, Tuesday, March 23!! Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 1
Announcements Quiz #2 beginning of the class this Wednesday, March 3 Covers CH3.4 to what we finish today, Monday, March 1 BYOF: You may bring a one 8.5x11.5 sheet (front and back) of handwritten formulae and values of constants for the test No derivations, word definitions, setups or solutions of any problems, figures, pictures, diagrams or arrows, etc! Must email me the photos of front and back of the formula sheet, including the blank at jaehoonyu@uta.edu no later than 12:00pm the day of the test The subject of the email should be the same as your file name File name must be FS-Q2-LastName-FirstName-SP21.pdf Once submitted, you cannot change, unless I ask you to delete part of the sheet! Mid-term comprehensive online exam in class on Wed. March 10 Covers CH1.1 through what we learn on Monday, March 8 + Math refresher Reminder: Extra credit special COVID seminar at 4pm Saturday, March 20 Extra credit for participation and for asking the relevant questions Dr. Linda Lee, a practicing physician from Wisconsin Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 2
Reminder: SP #3 Statistical Analysis : COVID19 Make comparisons of COVID-19 statistics between the U.S., South Korea, Italy and Texas from https://coronaboard.com on spreadsheet Total 44 points: 1 point for each of the top 20 cells and 2 points for each of the 8 cells for vaccination Fill the US Historic event analysis table at the bottom 12 cells of the sheet (2 points per cell, 24 points total) and make a >3 sentence statement on COVID19 with respect to other events (6 points) What are the 3 fundamental quantitative requirements for opening up (2 points each, 6 points total)? Must be quantitative! (e.g. number of tests per capita per day, positivity rate, etc) Assess the readiness of the three fundamental requirements U.S. (Do NOT just take politician s words!). Must provide the independent scientific entity s reference you took the information from. (2 point each, total 6 points) Evaluate quantitatively the success/failure of the US responses to COVID-19 in 2020 in 5 sentences and that in 2021 in 5 sentences. Must provide quantitative reasons behind your conclusion! (2 points each sentence, 20 points total) Assess quantitatively the effectiveness of wearing masks (4 points) and at least 4 reasons for it being effective (1 point each, 0.5 point extra after the first 4). Possible maximum: 122 points total Due: 11pm, Friday, March 19 Submit one pdf file SP3-YourLastName-YourFirstName.pdf, including the spreadsheet Spreadsheet will be posted on canvas. Download ASAP. Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 3
PHYS1443-003, Spring 21, Special Project #3, Statistical Analysis - COVID19 Date? &? time? of? your? COVID-19? Data:? South?Korea Name: Items U.S.A Italy Texas Total? Population Total COVID-19? Confirmed? cases Cases per 1M people Total COVID-19? Deaths Death per 1M people Total COVID-19? Testing Testing per 1M people Poditivity Rate Total COVID-19 Vaccination to date Per 1M people US?Vietnam?War US?World?War?II COVID-19 US H1N1 Time? period? (mm/dd/yy? -? MM/DD/YY) Duration? in? Months US Historic Event Analysis Total? deaths Death? per? month Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 4
SP#4: Newtons 3rd Law The mass of the spacecraft is 11,000 kg and the mass of the astronaut is 92 kg. What is the velocity of the space craft and the astronaut 10 sec into the motion if they were in contact for 50cm during with the astronaut is applying the force of 36N? Maximum score: 20 points Please be sure to show details of your OWN work! Must be handwritten! Due 2:30pm, Monday, March 8 Submit one pdf file SP4-YourLastName-YourFirstName.pdf on canvas assignment #4 Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 5
Newtons Third Law (Law of Action and Reaction) When two objects interact, the force F12 that object 1 exerts on object 2 is equal in magnitude and opposite in direction to the force F21 object 2 exerts on object 1. F12 2 F21 1 The reaction force is equal in magnitude to the action force but in opposite direction. These two forces always act on different objects on different objects. What is the reaction force to the force of a free falling object? The gravitational force exerted by the object to the Earth! Stationary objects on top of a table has a reaction force (called the normal force) from the table to balance the action force, the gravitational force. Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 6 6
Ex. The Accelerations Produced by Action and Reaction Forces Which one do you think will get larger acceleration? Suppose that the magnitude of the force P is 36 N. If the mass of the spacecraft is 11,000 kg and the mass of the astronaut is 92 kg, what are the accelerations? Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 7
Ex. continued Force exerted on the space craft by the astronaut Force exerted on the astronaut by the space craft space craft s acceleration astronaut s acceleration Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 8
Example of Newtons 3rd Law A large man and a small boy stand facing each other on frictionless ice. They put their hands together and push against each other so that they move apart. a) Who moves away with the higher speed and by how much? F F F12 12 ma ma =0 Ma Ma = 0 = = = = F bx 12x F by 12 y F F21 21= = - - F F12 12 F F m 21x Mx M 21y My and Since Ma m F m = = ma = a Establish the equation F = Ma Divide by m Mx bx bx Mx Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 9 9
Example of Newtons 3rd Law, cntd = + = v Mxi v a t a t Man s speed Mxf Mx Mx M M = + = a t = v v a t = a t v Boy s speed bxf Mx bxi bx bx Mxf m m So boy s speed is higher than man s, if M>m, by the ratio of the masses. b) Who moves farther while their hands are in contact? 1 2 Ma t m bx = + = 2 2 v t a t Boy s displacement bxi bx Mx 2 Man s displacement 1 2 M m Mx m = = 2 x a t b Mx M Given in the same time interval, since the boy has higher acceleration and thereby higher speed, he moves farther than the man. Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 10 10
Categories of Forces Fundamental Forces: Truly unique forces that cannot be derived from any other forces (poll 2) Total of three fundamental forces Gravitational Force Electro-Weak Force (the unified force of EM and Weak) Strong Nuclear Force Non-fundamental forces: Forces that can be derived from fundamental forces Friction Tension in a rope Normal or support forces Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 11
Gravitational Force and Weight The attractive force exerted on an object by the Earth (Polls 3, 6, 12) Gravitational Force, F Fg g Mg Weight of an object with mass M is Since weight depends on the magnitude of gravitational acceleration, g g, it varies depending on geographical location. (poll 7) By measuring the forces one can determine masses. This is why you can measure mass using the spring scale. Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 12
The Normal Force The normal force is one component of the reactionary force that a surface exerts on an object with which it is in contact namely, the component that is perpendicular to the surface. (polls 6, 3) Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 13
Some normal force exercises Case 1: Hand pushing down the book F 11 N 15 N= F = 0 N 26 N N Case 2: Hand pulling up the book F + 11 N 15 N 4 N F = = 0 N N Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 14
Some Basic Information When Newton s laws are applied, external forces are only of interest!! Because, as described in Newton s first law, an object will keep its current motion unless non-zero net external force is applied. Why? Reaction force that reacts to the action force due to the surface structure of an object. Its direction is perpendicular to the surface. Normal Force, n n: The reactionary force by a stringy object against an external force exerted on it. Tension, T T: A graphical tool which is a diagram of external forces on an object forces on an object and is extremely useful analyzing forces and motion!! Drawn only on an object. diagram of external Free-body diagram Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 15
Free Body Diagrams and Solving Problems Free-body diagram: A diagram of vector forces acting on an object A great tool to solve a problem using forces or using dynamics Select a point on an object in the problem Identify all the forces acting only on the selected object Define a reference frame with positive and negative axes specified (easier if one of the axes is aligned with the anticipated direction of motion!) Draw arrows to represent the force vectors on the selected point Write down the net force vector equation Write down the force components to solve the problem for the motion of an object 1. 2. 3. 4. 5. 6. F N F Draw the FBD on the box of mass M. What do you think are the forces acting on this object? Gravitational force the force supporting the object exerted by the floor N FG = M M g FG = M g Draw FBD on the elevator! What do you think are the forces acting on this elevator? F T F T FG = M g Gravitational forces The force pulling the elevator (Tension) F N Me m What about the box in the elevator? Normal force F Gravitational force on the box N 16 FBG = Monday, March 1, 2021 g FGB = PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu m g FG = M m g
Applications of Newton's Laws Suppose you are pulling a box on frictionless ice, using a rope. What are the forces being exerted on the box? M T Gravitational force: F Fg g n= n= - -F Fg Normal force: n n Free-body diagram T Tension force: T T T ax= n= n= - -F Fg Total force: F=F F=Fg g+n+T=T +n+T=T = F = Ma T M x x F Fg=-Mg g + = = = Fg n Ma y F = 0 0 a y y T t T M T M = + = v v a t vxi+ If T T is a constant force, ax, is constant xf xi x F Fg=-Mg g t2 vxit +1 = x = x x f i 2 Monday, March 1, 2021 PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu 17 What happened to the motion in y-direction?
Example for Using Newtons Laws A traffic light weighing 125 N hangs from a cable tied to two other cables fastened to a support. The upper cables make angles of 37.0o and 53.0o with the horizontal. Find the tension in the three cables. y 37o 53o T T1 1 T T2 2 Free-body Diagram 37o 53o x T T3 3 Newton s 2nd law 0 ( )+T2cos 53 ( )= 0 x-comp. of net force = 3 i = i = -T1cos 37 1T = 0.754T 0 F T 2 x ix = 1 ( ) ( ) + = o )+0.754 sin 37 T1= 0.754T2= 75.4N PHYS 1443-003, Spring 2021 Dr. Jaehoon Yu o sin 37 ( sin 53 T T mg ) 0 y-comp. of net force = 3 i 1 2 = ( = 0 F T =1.25T2=125N T2sin 53 y iy = 1 i Monday, March 1, 2021 18
