Forces and the Laws of Motion -...
Transcript of Forces and the Laws of Motion -...
Chapter 4
Forces and the Laws of Motion
Assumption College
Mr. Steve
Section 1: Changes in Motion◦ Force
◦ Force Diagrams
Section 2: Newton’s First Law◦ Inertia
◦ Equilibrium
Section 3: Newton’s Second and Third Laws◦ Newton’s Second Law
◦ Newton’s Third Law
Section 4: Everyday Forces◦ Weight
◦ The Normal Force
◦ The Force of FrictionChapter 4: Forces | Physics | Mr. Steve
Chapter 4 Preview
CHANGES IN MOTION
Chapter 4.1
Chapter 4: Forces | Physics | Mr. Steve
Assumption College
Mr. Steve
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Changes in Motion
What is a force?
◦ an action exerted on an object which may
change the object’s state of rest or motion
What is a newton (N)?
◦ the SI unit for force
Chapter 4: Forces | Physics | Mr. Steve
What does exert mean?
to put forth or into
use, to push/pull on
2s
mkgN
F
Whenever there is an interaction between
two objects, there is a force upon each of
the objects. When the interaction ceases, the
two objects no longer experience the force.
Forces only exist as a result of an
interaction.
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.1
Changes in Motion
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.1
Changes in Motion
In general, a force is any influence that causes
an object to experience a change in:
1. speed
2. direction
3. shape
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.1
Changes in Motion
Chapter 4: Forces | Physics | Mr. Steve
Forces can cause accelerations
p. 118
Chapter 4.1
Changes in Motion
Forces describe the interactions between an
object and its environment
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.1
Changes in Motion
Chapter 4: Forces | Physics | Mr. Steve
Forces exerted on an object can change
the object’s velocity with respect to time.
p. 119
?
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Changes in Motion
Forces can act through contact or at a
distance
What is a contact force?
◦ a force between two objects that are in
contact with each other
What is a field force?
◦ a non-contact force that acts over a distance,
such as gravity or magnetic
attraction/repulsion
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.1
Changes in Motion
Fundamental Forces
StrongForce that holds nuclei together
WeakForce involved in the transmutation
of quarks. (Quantum physics)
ElectromagneticForce between electric charges,
between magnets
GravityForce of attraction between masses
Chapter 4: Forces | Physics | Mr. Steve
NUCLEAR FORCES
Which one do you think is the weakest?
Chapter 4: Forces | Physics | Mr. Steve
Which is the strongest? Strong InteractionWhich is the weakest? GravityWhich ones have the greatest range?Gravity &
Electromagnetic –
infinite range!
Chapter 4.1
Changes in Motion
Chapter 4: Forces | Physics | Mr. Steve
part of the
electromagnetic force
p. 119
Chapter 4.1
Changes in Motion
Force is a vector quantity
◦ example: pushing open a door
Chapter 4: Forces | Physics | Mr. Steve
When applying a force, both magnitude and direction matter!
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Changes in Motion
What is normal force?
◦ the component of a contact force that acts on
a surface in a direction perpendicular to the
surface
Chapter 4: Forces | Physics | Mr. Steve
nF
The normal force is what prevents us
from sinking through the chair, the floor,
the ground, etc.
Fn is an action-reaction force pair of an object's
weight
Chapter 4: Forces | Physics | Mr. Steve
larperpendicunormal
Chapter 4.1
Changes in Motion
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Changes in Motion
What is equilibrium?
1. a state in which opposing forces are balanced
2. a state in which the net force on an object is
zero
Equilibrium
◦ all forces are balanced
◦ there is no acceleration
◦ velocity is constant
could be at rest, 0
could be moving
Chapter 4: Forces | Physics | Mr. Steve
0 F
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Changes in Motion
A free-body diagram helps us study a
situation
What is a free-body diagram?
◦ is a pictorial representation to analyze all the
forces acting on an object
Chapter 4: Forces | Physics | Mr. Steve
A book sits on a table.1) Draw a simple representation.
◦ e.g., a box or a circle
2) Draw and label all forces.
i. Is gravity acting on the body?
ii. Is there a normal force?
iii. Is anything applying a force?
iv. Any frictional forces?
3) Check, should the body be in
equilibrium?
Chapter 4: Forces | Physics | Mr. Steve
gF
nF
Steps to drawing a Free-Body Diagram
Some textbooks may use
mg or W instead.
vectors originate from the
center of mass
Chapter 4.1
Changes in Motion
Chapter 4: Forces | Physics | Mr. Steve
Recall: What’s a system?
p. 120
Force Diagram Free-Body Diagram (of car)
1. A book is at rest on a tabletop.
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Chapter 4.1
Free-Body Diagrams
2. An acrobat is suspended motionless
from the ceiling by a rope.
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Chapter 4.1
Free-Body Diagrams
1) Draw a simple representation.
◦ e.g. a box or a circle
2) Draw and label all forces.
i. Is gravity acting on the body?
ii. Is there a normal force?
iii. Is anything applying a force?
iv. Any frictional forces?
3) Check, should the body be in equilibrium?
3. An egg is free-falling from a nest in a
tree. Neglect air resistance.
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Chapter 4.1
Free-Body Diagrams
1) Draw a simple representation.
◦ e.g. a box or a circle
2) Draw and label all forces.
i. Is gravity acting on the body?
ii. Is there a normal force?
iii. Is anything applying a force?
iv. Any frictional forces?
3) Check, should the body be in equilibrium?
4. An egg is falling (not freely, do not
neglect air resistance) from a nest in a
tree.
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Chapter 4.1
Free-Body Diagrams
force of air resistance
RF
5. A rightward force is applied to a book in
order to move it across a desk with a
rightward acceleration. Consider
frictional forces. Neglect air resistance.
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Chapter 4.1
Free-Body Diagrams
force applied
appFforce of friction
fF
6. A rightward force is applied to a book in
order to move it across a desk at
constant velocity. Consider frictional
forces. Neglect air resistance.
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Chapter 4.1
Free-Body Diagrams
7. A Bangkok taxi is (actually) stopping at a
red light.
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Chapter 4.1
Free-Body Diagrams
8. A skydiver is descending with a constant
velocity. Of course, consider air
resistance.
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Chapter 4.1
Free-Body Diagrams
9. A car is parked on a sloped street.
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Chapter 4.1
Free-Body Diagrams
Chapter 4: Forces | Physics | Mr. Steve
, cosg yn gF FF
gF
,g yF Check:
Is Fn < Fg?
Is Fn = Fg,y?
nF
,g xF
fF, sing xf gF FF
9. A car is parked on a sloped street.
10. A hot air balloon is accelerating upward.
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Chapter 4.1
Free-Body Diagrams
force of buoyancy
buoyancyF
11. A car is coasting to the right and
slowing down.
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Chapter 4.1
Free-Body Diagrams
12. A soccer ball while being kicked at a 45º
angle.
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Chapter 4.1
Free-Body Diagrams
13. A soccer ball that has already been
kicked at a 45º angle.
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Chapter 4.1
Free-Body Diagrams
14. A race car crashing head-on into a brick
wall.
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Chapter 4.1
Free-Body Diagrams
Chapter 4: Forces | Physics | Mr. Steve
Review Sample Problem A on p. 121 then do Practice A on p. 122 (#1-2) at your seats now, independently or in groups of no more than 2
Do in your hw/classwork notebook
SHOW ALL WORK*
(NO WORK = NO CREDIT)
*That means show me that you did the thinking and work. Don’t just show me that you copied!
Chapter 4.1: Changes in Motion
Classwork: Practice A
1. A truck pulls a trailer on a flat stretch of road. The forces acting on the trailer are the force due to gravity (250 000 N downward), the force exerted by the cable connecting the trailer to the truck (20 000 N to the right). The forces acting on the truck are the force due to gravity (80 000 N downward), the force exerted by the road (80 000 upward), the force exerted by the cable (20 000 N to the left), and the force causing the truck to move forward 26 400 N to the right).
a) Draw and label a free-body diagram of the trailer.
b) Draw and label a free-body diagram of the truck.
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.1: Changes in Motion
Classwork: Practice A
Chapter 4: Forces | Physics | Mr. Steve
20000 Ntruck on trailerF
250000 NnF
80000 NgF
80000 NnF
250000 NgF
20000 Ntrailer on truckF
26400 NappF
S I D E V I E W
trailer truck
2. A physics book is at rest on a desk.
Gravitation force pulls the book downward.
The desk exerts an upward force on the book
that is equal in magnitude to the gravitational
force. Draw a free-body diagram of the book.
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.1: Changes in Motion
Classwork: Practice A
gF
nF
Chapter 4.1: Changes in Motion
Formative Assessment
Do Formative Assessment 4.1 p. 122 (#1-6),
independently or in groups of no more than 2
Do in your hw/classwork notebook
SHOW ALL WORK*
Draw ALL Diagrams!
*That means show me that you did the thinking
and work. Don’t just show me that you copied!
Chapter 4: Forces | Physics | Mr. Steve
NEWTON’S FIRST LAW
Chapter 4.2
Chapter 4: Forces | Physics | Mr. Steve
Assumption College
Mr. Steve
Chapter 4.2
Newton’s First Law
An object on which no force is acting will
remain at a constant velocity.
Chapter 4: Forces | Physics | Mr. Steve
True or False
An object on which no force is acting will always be at rest
Voca
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Newton’s First Law
What is inertia?
◦ the tendency of an object to resist being
moved or, if the object is moving, to resist a
change in speed or direction
Chapter 4: Forces | Physics | Mr. Steve
ความเฉ่ือย -สมบตัขิองเทหวตัถทุีจ่ะคงสภาพน่ิงอยูอ่ยา่ง
เดมิตลอดไป หรอืคงสภาพเคลือ่นทีอ่ยา่งสม่ํา
เสมอในแนวเสน้ตรงตลอดไป
Chapter 4.2
Newton’s First Law
Chapter 4: Forces | Physics | Mr. Steve
Newton’s 1st law of motion
An object at rest remains at rest, and an object
in motion continues in motion with constant
velocity (that is, constant speed in a straight
line) unless the object experiences a net
external force.The Law of Inertia
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Chapter 4: Forces | Physics | Mr. Steve
If the car were to abruptly stop and the seat belts were not being worn, then the passengers in
motion would continue in motion. Assuming a negligible amount of friction between the
passengers and the seats, the passengers would likely be propelled from the car and be hurled
into the air. Once they leave the car, the passengers become projectiles and continue along a
parabolic trajectory.
Chapter 4.2
Newton’s First Law
notice, the driver’s inertia causes him to fly out of the car
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4: Forces | Physics | Mr. Steve
The sum of forces acting on an
object is the net force?
fF
gF
nF
appF
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Newton’s First Law
What is an external force?
◦ a single force that acts on an object as a result
of the interaction between the object and its
environment
What is net force?
◦ a vector sum of all forces acting on an object
◦ a single force whose external effects on a
rigid body are the same as the effects of
several actual forces acting on the body
Chapter 4: Forces | Physics | Mr. Steve
alternatively…
Chapter 4.2
Newton’s First Law
Chapter 4: Forces | Physics | Mr. Steve
Force Examples
Internal forces External forces
gF
springF appF
RF
nF
fF
TF
magneticF
electricF
(T = tension)
(R = air resistance/drag)
Chapter 4.2
Newton’s First Law
Mass is a measure of inertia
Chapter 4: Forces | Physics | Mr. Steve
inertiamass is proportional to
Chapter 4.2
Newton’s First Law
What is equilibrium?
◦ the state in which the net force on an object
is zero
Equilibrium
◦ all forces are balanced
◦ there is no acceleration
◦ velocity is constant
could be at rest, 0
could be moving
Chapter 4: Forces | Physics | Mr. SteveReca
ll
0 F
Chapter 4: Forces | Physics | Mr. Steve
p. 127
gF
buoyancyF
bobonfishF
bobonmanF
Chapter 4.2: Newton’s First Law
Sample Problem B (p. 125)
Yingluck leaves her physics book on top
of a drafting table that is inclined at a 35º
angle.
Find the net force acting on the book.
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Step 1: Draw the free-body diagram.
Chapter 4: Forces | Physics | Mr. Steve
35˚
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Chapter 4.2: Newton’s First Law
Sample Problem B (p. 125)
gF
nFfF
Given:
◦ Fg= 22N
◦ Ff= 11N
◦ Fn= 18N
Wanted:
◦ Fnet= ?
Chapter 4: Forces | Physics | Mr. Steve
35˚
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Chapter 4.2: Newton’s First Law
Sample Problem B (p. 125)
fF
nF
gF
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Step 2: Select a coordinate system. Apply.
Chapter 4: Forces | Physics | Mr. Steve
35˚
To simplify the problem, always choose the
coordinate system in which as many forces as
possible lie on the x- and y- axes
fF
nF
gF
Step 3: Find the x- and y-components on
all vectors.
Chapter 4: Forces | Physics | Mr. Steve
35˚
35˚
55˚
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Chapter 4: Forces | Physics | Mr. Steve
55˚
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ygF ,
xgF ,
gF
Chapter 4.2: Newton’s First Law
Sample Problem B (p. 125)
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Chapter 4: Forces | Physics | Mr. Steve
55˚
ygF ,
xgF ,
gF
55cos55cos ,
,
gxg
g
xgFF
F
F
N1355cos)N22(, xgF
55sin55sin ,
,
gyg
g
ygFF
F
F
N1855sin)N22(, ygF
Chapter 4.2: Newton’s First Law
Sample Problem B (p. 125)
Chapter 4: Forces | Physics | Mr. Steve
18 N
18 N
11 N
13 N
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Chapter 4.2: Newton’s First Law
Sample Problem B (p. 125)
ygF ,
xgF ,
nF
fF
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Step 3: Find the net force in both thex- and y-directions.
Chapter 4: Forces | Physics | Mr. Steve
for the x-direction for the y-direction
fxgx FFF , nygy FFF ,
)N11(N13xF N18N)18(yF
N2xF N0yF
Chapter 4: Forces | Physics | Mr. Steve
Yingluck’s book accelerates down the incline
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direction in 2 xNFnet
Chapter 4.2: Newton’s First Law
Sample Problem B (p. 125)
Chapter 4: Forces | Physics | Mr. Steve
Review Sample Problem B on p. 125 then do Practice B on p. 126 (#1-3) at your seats now, independently or in groups of no more than 2
Do in your hw/classwork notebook
SHOW ALL WORK*
(NO WORK = NO CREDIT)
*That means show me that you did the thinking and work. Don’t just show me that you copied!
Chapter 4.2: Newton’s First Law
Classwork: Practice B
Chapter 4.2: Newton’s First Law
Formative Assessment Do Formative Assessment 4.2 p. 127 (#1-5),
independently or in groups of no more than 2
Do in your hw/classwork notebook
SHOW ALL WORK*
Draw ALL Diagrams!
M4/1: Due _____ (10 points)
M4/2: Due _____ (10 points)
M4/3: Due _____ (10 points)
*That means show me that you did the thinking and work. Don’t just show me that you copied!
Chapter 4: Forces | Physics | Mr. Steve
NEWTON’S SECOND AND
THIRD LAWS
Chapter 4.3
Chapter 4: Forces | Physics | Mr. Steve
Assumption College
Mr. Steve
Chapter 4.3
Newton’s 2nd and 3rd Laws
Chapter 4: Forces | Physics | Mr. Steve
Newton’s 2nd law of motion
∑F=m∙a
Chapter 4.3
Newton’s 2nd and 3rd Laws
Chapter 4: Forces | Physics | Mr. Steve
massforce
onacceleratiforce
if a is constant…
if m is constant…
Chapter 4.3
Newton’s 2nd and 3rd Laws
Chapter 4: Forces | Physics | Mr. Steve
onacceleratimass
1 1 onacceleratimass
What is the relationship between mass
and acceleration?
or simply…
is inversely proportional to
Chapter 4.3
Newton’s 2nd and 3rd Laws
Chapter 4: Forces | Physics | Mr. Steve
Newton’s 3rd law of motion
If 2 objects interact, the magnitude of the force
exerted on object 1 by object 2 is equal to the
magnitude of the force simultaneously exerted
on object 2 by object 1, and are opposite in
direction. For every action there is an
equal and opposite reaction.
Chapter 4.3
Newton’s 2nd and 3rd Laws
Check Your Understanding
False—the object with less mass will
experience a greater acceleration.
Chapter 4: Forces | Physics | Mr. Steve
True or False
Equal forces are applied to 2 objects of different masses,
both objects will experience the same acceleration.
Chapter 4.3
Newton’s 2nd and 3rd Laws
Forces always exist in pairs.
◦ a.k.a. an action-reaction pair
These forces coexist, simultaneously
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.3
Newton’s 2nd and 3rd Laws
Chapter 4: Forces | Physics | Mr. Steve
What about field forces? Do you think they
exist in action-reaction pairs?
Field forces also exist in pairs.
Chapter 4: Forces | Physics | Mr. Steve
𝐹 =↓ 𝑚 ↑ 𝑎
𝐹 =↑ 𝑚 ↓ 𝑎Force of
MAN on EARTH
Force of
EARTH on MAN
relative to Earth,
man has very small mass
magnitude of
forces are equal
relative to man,
Earth has very large mass
therefore, man’s acceleration is very large
therefore, Earth’s acceleration is very small
Remember:
1. 3rd law states forces are equal and opposite
2. when force is constant, mass is inversely
proportional to acceleration
Chapter 4.3
Newton’s 2nd and 3rd Laws
Chapter 4: Forces | Physics | Mr. Steve
Conceptual Challenge1. Gravity and Rocks Calculate the force of gravity
on a 2 kg rock. Calculate the force of gravity on a 1 kg rock. The force due to gravity is twice as great on a 2 kg rock as it is on a 1kg rock. Why doesn’t the 2 kg rock have a greater free-fall acceleration?
A greater force acts on the heavier rock, but the heavier rock also has greater mass (greater inertia), so the acceleration is the same.Free-fall acceleration is independent of mass.
p. 130
Remember:
The force of gravity is not constant, but the
acceleration of gravity is constant!
Chapter 4.3
Newton’s 2nd and 3rd Laws
Chapter 4: Forces | Physics | Mr. Steve
Conceptual Challenge2. Leaking Truck A truck loaded with sand
accelerates at 0.5 m/s2 on the highway. If the driving force on the truck remains constant, what happens to the truck’s acceleration if sand leaks at a constant rate from a hole in the truck bed?
The acceleration will increase as the mass decreases.
p. 130
Chapter 4: Forces | Physics | Mr. Steve
Review Sample Problem C on p. 129 then do Practice C on p. 130 (#1-5) at your seats now, independently or in groups of no more than 2
Do in your hw/classwork notebook
SHOW ALL WORK*
(NO WORK = NO CREDIT)
*That means show me that you did the thinking and work. Don’t just show me that you copied!
Chapter 4.3: Newton’s 2nd and 3rd Laws
Classwork: Practice C
Do Formative Assessment 4.3 p. 132 (#1-5), independently or in groups of no more than 2
Do in your hw/classwork notebook
SHOW ALL WORK*
Draw ALL Diagrams!
M4/1: Due _____ (10 pts)
M4/2: Due _____ (10 pts)
*That means show me that you did the thinking and work. Don’t just show me that you copied!
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.3: Newton’s 2nd and 3rd Laws
Formative Assessment
EVERYDAY FORCES
Chapter 4.4
Chapter 4: Forces | Physics | Mr. Steve
Assumption College
Mr. Steve
Chapter 4.4
Everyday Forces
This section includes:
1. WEIGHT
2. THE NORMAL FORCE
3. THE FORCE OF FRICTION
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.4
Everyday Forces
This section includes:
1. WEIGHT
2. THE NORMAL FORCE
3. THE FORCE OF FRICTION
Chapter 4: Forces | Physics | Mr. Steve
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Everyday Forces
WEIGHT
What is weight?
◦ a measure of the gravitational force exerted
on an object
An object’s weight can change with the
location of the object in the universe
Weight is not an inherent property of an
object like mass.
Chapter 4: Forces | Physics | Mr. Steve
inherent-
a quality or attribute that is
fundamental, innate to
something or someone
Why?
gF
Chapter 4.4
Everyday Forces
WEIGHT
Mathematically, weight is defined as…
Chapter 4: Forces | Physics | Mr. Steve
or simply…
or sometimes…
preferred notation!
gg amF
gmFg
gmW
Please don’t use this one because we’ll use W
for other things, like…
Chapter 4.4
Everyday Forces
This section includes:
1. WEIGHT
2. THE NORMAL FORCE
3. THE FORCE OF FRICTION
Chapter 4: Forces | Physics | Mr. Steve
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THE NORMAL FORCE
What is normal force?
◦ the component of a contact force that acts on
a surface in a direction perpendicular to the
surface
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.4
Everyday Forces
Chapter 4.4
Everyday Forces
THE NORMAL FORCE
normal force is always perpendicular to
the contact surface, but NOT always
opposite in direction to gravity or the
applied force.
Chapter 4: Forces | Physics | Mr. Steve
larperpendicunormal
Chapter 4.4
Everyday Forces
Chapter 4: Forces | Physics | Mr. Steve
Notice, the
normal force
is not always
equal and
opposite the
force of
gravity.
Chapter 4: Forces | Physics | Mr. Steve
, cosg ynF mgF
gF
,g yF
How do we calculate the magnitude of Fn?
Check:
Is Fn < Fg?
Is Fn = Fg,y?
nF
Chapter 4.4
Everyday Forces
What if there is no angle? Is the
equation for Fn still correct?
Calculate the magnitude of Fn .
Chapter 4: Forces | Physics | Mr. Steve
cosnF mg
cos(0 )nF mg
nF mg
Chapter 4.4
Everyday Forces
This section includes:
1. WEIGHT
2. THE NORMAL FORCE
3. THE FORCE OF FRICTION
Chapter 4: Forces | Physics | Mr. Steve
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Everyday Forces
THE FORCE OF FRICTION
What is static friction?
◦ the force that resists the initiation (beginning)
of sliding motion between two surfaces that
are in contact and at rest
What is kinetic friction?
◦ the force that opposes the movement of two
surfaces that are in contact and are sliding
over each other
Chapter 4: Forces | Physics | Mr. Steve
sF
kF
Chapter 4.4
Everyday Forces
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.4
Everyday Forces
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.4
Everyday Forces
is a variable force; it depends
on the
Kinetic friction is less than (or equal to)
the maximum static friction
When objects accelerate
Chapter 4: Forces | Physics | Mr. Steve
,maxapp sF F
,maxk sF F
sF
appF
What condition?
?
?
Chapter 4.4
Everyday Forces
Friction is not the same for everything.
Friction can be calculated approximately,
using the coefficient of friction.
The coefficient of friction is specific to
each material.
Chapter 4: Forces | Physics | Mr. Steve
Voca
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Everyday Forces
What is the coefficient of friction (µ)?
◦ the ratio of the magnitude of the force of
friction between two objects in contact to the
magnitude of the normal force with which the
objects press against each other
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.4
Everyday Forces
Chapter 4: Forces | Physics | Mr. Steve
kk
n
F
F ,maxs
s
n
F
F
COEFFICIENT OF STATIC FRICTIONCOEFFICIENT OF KINETIC FRICTION
huh??...Why “max”?
Chapter 4.4
Everyday Forces
Chapter 4: Forces | Physics | Mr. Steve
f nF F
GENERAL EXPRESSION FOR THE FORCE OF FRICTION (using µ)
The force of friction is proportional to the normal force.
What’s the relationship between the force of friction
and the normal force?
Does this make sense?
Chapter 4.4
Everyday Forces
Chapter 4: Forces | Physics | Mr. Steve
frictional coefficients are determined empirically
empirical-
obtained from observation,
experimentation
Why do you think it’s important to have a low coefficient
of friction for the joints of humans?
Chapter 4.4
Everyday Forces
Chapter 4: Forces | Physics | Mr. Steve
Friction: imagine two “teethed”
surfaces rubbing together
Chapter 4.4
Everyday Forces
In free-body diagrams, the force of friction is
always parallel to the surface of contact.
The force of kinetic friction is always
opposite the direction of motion.
To determine the direction of the force of
static friction, use the principle of
equilibrium. For an object in equilibrium, the
frictional force must point in the direction
that results in a net force of zero.
Chapter 4: Forces | Physics | Mr. Steve
TIP
S
(se
e p
age 1
35)
Chapter 4: Forces | Physics | Mr. Steve
Review Sample Problem D on p. 137 then do Practice D on p. 137 (#1-3) at your seats now, independently or in groups of no more than 2
Do in your hw/classwork notebook
SHOW ALL WORK*
(NO WORK = NO CREDIT)
*That means show me that you did the thinking and work. Don’t just show me that you copied!
Chapter 4.4: Everyday Forces
Classwork: Practice D
Chapter 4.4
Everyday Forces
Air resistance, or drag, is a form of
friction.
Typical notation:
Chapter 4: Forces | Physics | Mr. Steve
RF
airF
dragF
or…or…
Chapter 4: Forces | Physics | Mr. Steve
Which shape seems to have the least drag?
known as eddies
Voca
bula
ryChapter 4.4
Everyday Forces
What is terminal velocity (vT)?
◦ constant velocity of a falling object when the
upward drag on the object balances the
downward force of gravity.
◦ velocity that occurs at the state of equilibrium
of a falling object with drag
Chapter 4: Forces | Physics | Mr. Steve
Tv
gF
RF
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4: Forces | Physics | Mr. Steve
A dropped ball almost immediately reaches terminal velocity.
Pongwiwat and Sasinat are sliding a 225 kg sofa at a constant speed across a wood floor. Pongwiwat pulls with a force of 225 N at an angle of 13º above the horizontal. Sasinat pushes with a force of 250 N at an angle of 23º below the horizontal. What is the coefficient of kinetic friction between the sofa and the floor?
Chapter 4: Forces | Physics | Mr. SteveEx
am
ple
Chapter 4.4
Classroom Practice: Overcoming Friction
coordinate system
F?
F?F?
F?
Chapter 4: Forces | Physics | Mr. SteveChapter 4: Forces | Physics | Mr. Steve
Known
Fpush
Fpull
needed
needed
wanted
Very Important Note!
1. Calculate the x- and y-components of
both of the applied forces
2. Apply the equilibrium principle to find
needed quantities (Ff and Fn)
a) Add the x-components of all forces
b) Add the y-components of all forces
3. Find µk
Chapter 4: Forces | Physics | Mr. SteveEx
am
ple
Chapter 4.4
Classroom Practice: Overcoming Friction
Ex
am
ple
Chapter 4: Forces | Physics | Mr. Steve
diagram for Fpull
diagram for Fpush
Fpull,xFpush,x
Chapter 4: Forces | Physics | Mr. SteveEx
am
ple Fpull,y Fpush,y
Chapter 4.4
Classroom Practice: Overcoming Friction
Chapter 4: Forces | Physics | Mr. SteveEx
am
ple
Chapter 4.4
Classroom Practice: Overcoming Friction
Apply equilibrium principle
Add up all the forces in the x-direction
Ff
Add up all the forces in the y-direction
FnMake the
connection between
Ff and Fk
eqn to use plug in Ff plug in Fn
Final Answer
(units?)
Chapter 4: Forces | Physics | Mr. Steve
Review Sample Problem E on p. 138-9 then do Practice E on p. 139 (#1-4) at your seats now, independently or in groups of no more than 2
Do in your hw/classwork notebook
SHOW ALL WORK*
(NO WORK = NO CREDIT)
*That means show me that you did the thinking and work. Don’t just show me that you copied!
Chapter 4.4: Everyday Forces
Classwork: Practice E
Do Formative Assessment 4.4 p. 141 (#1-5), independently or in groups of no more than 2
Do in your hw/classwork notebook
SHOW ALL WORK*
Draw diagrams where necessary!
M4/1: Due _____ (10 pts)
M4/2: Due _____ (10 pts)
*That means show me that you did the thinking and work. Don’t just show me that you copied!
Chapter 4: Forces | Physics | Mr. Steve
Chapter 4.4: Everyday Forces
Formative Assessment