ch 4 forces & newton’s laws · 2018. 10. 22. · 90.0 km/h. at that speed the forces...
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Ch 4 Forces & Newton’s Laws
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M1M2
M3
Pulley Clamps
Lab Table
Aluminum Support Rods
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M3
T1 T2
M3 g
Protractor
Wooden Block
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T1 T2
M3 g
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T1 cos T2 cos
T1
T2 cos
cos
Horizontal Force Components
Equation
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T1 sin T2 sin M3 g
T2 cos
cos
sin T2 sin M3 g
Vertical Components Equation
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T1sin T2 sin
M3 g
T1 cos T2 cos
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Static Equilibrium
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Horizontal forces balance.
T1 cos T2 cos
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Vertical forces balance.
T1 sin T2 sin M g
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A mass, (M 23 kg ), is
suspended by two ropes from a
ceil ing. Rope 1 makes an angle
of 60 deg from the ceil ing,
while Rope 2 makes an angle of
40 deg . What are the
tensions T1 and T2?
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T1
T2 cos
cos
T1 sin T2 sin M g
T2 cos
cos
sin T2 sin M g
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1.
TL is the tension of the
horizontal cable on the
left, attached to the wall.
TR is the tension of the
angled cable on the right,
attached to the ceil ing.
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1.
TL is the tension of the
horizontal cable on the
left, attached to the wall.
TR is the tension of the
angled cable on the right,
attached to the ceil ing.
Fy 0 TR sin 37 deg( ) 625 N Fx 0 TR cos 37 deg( ) TL
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Setting up a T1 T2 Problem
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Find the tensions T1 for the rope
attached to the left, and T2 for the
rope attached to the right.
Fx 0 T1 cos 43.0deg( ) T2 cos 55 deg( )
Fy 0 T1 sin 43.0deg( ) T2 sin 55 deg( ) 43.8kg9.8 N
kg
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This horizontal equation substitutes
into the vertical equation, enab ling us to
solve for T2.
T1
T2 cos 55 deg( )
cos 43.0deg( )
T2 cos 55 deg( )
cos 43.0deg( )
sin 43.0deg( ) T2 sin 55 deg( ) 43.8kg9.8 N
kg
0.574
0.731T2 0.682
0.819T2 429.24N
0.535T2 0.819T2 429.24N
1.354T2 429.24N
T2429.24N
1.354
T1
T2 cos 55 deg( )
cos 43.0deg( )
T2 317N
T1317N( ) cos 55 deg( )
cos 43.0deg( )
T1 249N
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4.1 Concepts of Force & Mass
•Force is a push or
pull.
•Mass is a measure of
the quantity of matter.
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O-Stax Ch 4 Page 168 Prob 9.
Suppose two children push horizontally, but in exactly opposite directions, on a
third child in a wagon. The first child exerts a force of 75.0 N, the second a force of
90.0 N, friction is 12.0 N, and the mass of the third child plus wagon is 23.0 kg.
(a) What is the system of interest if the acceleration of the child in the wagon is to
be calculated? (b) Draw a free-body diagram, including all forces acting on the
system. (c) Calculate the acceleration. (d) What would the acceleration be if
friction were 15.0 N?
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O-Stax Ch 4 Page 168 Prob 10.
A powerful motorcycle can produce an acceleration of 3.50 m/s2 while traveling at
90.0 km/h. At that speed the forces resisting motion, including friction and air
resistance, total 400 N. (Air resistance is analogous to air friction. It always
opposes the motion of an object.) What is the magnitude of the force the
motorcycle exerts backward on the ground to produce its acceleration if the mass
of the motorcycle with rider is 245 kg?
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O-Stax Ch 4 Page 168 Prob 11.
The rocket sled shown in Figure 4.33 accelerates at a rate of 49.0 m/s2 . Its
passenger has a mass of 75.0 kg. (a) Calculate the horizontal component of the
force the seat exerts against his body. Compare this with his weight by using a
ratio. (b) Calculate the direction and magnitude of the total force the seat exerts
against his body.
Figure 4.33
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O-Stax Ch 4 Page 168 Prob 12.
Repeat the previous problem for the situation in which the rocket sled decelerates
at a rate of 201 m/s2 . In this problem, the forces are exerted by the seat and
restraining belts.
Figure 4.33
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O-Stax Ch 4 Page 168 Prob 13.
The weight of an astronaut plus his space suit on the Moon is only 250 N. How
much do they weigh on Earth? What is the mass on the Moon? On Earth?
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4.2 Newton’s 1st Law of Motion
•Objects at rest tend to
stay at rest.
•Objects in motion tend
to stay in motion.
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4.2 Newton’s 1st Law of Motion
• The object is only compelled
to change its state (whether
stationary or moving at
constant speed) if it is acted
upon by an outside net
force.
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Inertia & Mass
• Inertia is the tendency of an object to remain at rest or in motion.
• Inertia depends on mass.
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Non-Inertial Frame of Reference
• Objects show evidence of acceleration due to an unknown force.
• Newton’s first Law of Inertia does not appear to hold true.
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Inertial Frame of Reference
• Newton’s Law of Inertia holds true.
• Zero acceleration is assumed, whether the object is at rest or moving at constant velocity.
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Inertial Frame of Reference
• Zero acceleration
• Van is parked.
• Van is moving at
constant velocity.
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4.3 Newton’s 2nd Law
• Acceleration of a mass is the result of an unbalanced force.
• It takes a force to cause an acceleration.
• A net force of zero results in the object staying at rest or staying at constant speed.
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Units & the 2nd Law of Motion
N kgm
s2
F ma
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Free-Body Diagrams (FBD)
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Free-Body Diagrams (FBD)
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Page 128 Problem 11
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Page 128 Problem 11
• A net force causes acceleration of the block.
• Since the block is moving along the horizontal, it is accelerating along the horizontal.
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Page 128 Problem 11
•Therefore, only the horizontal components of the force vectors are taken into consideration.
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F m a
a F
m
45.0N cos 65°( ) 25.0N
5.00kg
a 1.20m
s2
A negative
acceleration is
a deceleration.
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4.4 Vector Nature of 2nd Law
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4.5 Newton’s 3rd Law of Motion
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4.5 Newton’s 3rd Law of Motion
• The force of Object A on Object B has the same magnitude as the force of Object B on Object A.
• The directions of the forces are opposite.
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Newton’s 3rd Law
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4.6 Universal Gravitation
FG M1 M2
r2
G 6.671011
N m
2
kg2
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4.6 Universal Gravitation
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4.6 Universal Gravitation
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4.6 Universal Gravitation
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Mass of the Earth
F m gG ME m( )
RE2
gG ME
RE2 ME
g RE2
G
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Mass of the Earth
ME
g RE2
G
ME
9.80m
s2
6.38106
m 2
6.671011
N m
2
kg2
5.981024
kg
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4.7 Weight
• Gravitational force that Earth exerts on an object.
• For a given mass, the object’s weight varies with the mass of the planet or moon.
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Relation Between
Mass & Weight
• An object’s mass remains
constant in different
gravitational fields.
• It’s weight, being a force,
DOES change in the presence
of different gravitational fields.
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4.8 Normal Force
• A normal force is the perpendicular force to the surface in question.
• In coefficient of friction problems, this is the pressing force of the surface upon which an object rests or slides.
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4.8 Normal Force
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Static Friction
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PHYSICS LAB: Electronic Worksheet
Determination of the
Coefficients of Static
& Kinetic Friction
Copyright © 2010, by R. Faucher
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4.9 Static and Kinetic Frictional Forces
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Orientation Effects on m
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FN Fs Fk
data0 1 2
0
1
2
3
4
2.5
3.5
4.3
7.1
12
FN data0
N Normal Force against block.
Fs data1
N1 Static Force against block.
Fk data2
N2 Kinetic Force against block.
Friction as a Function of Normal Force
Normal Force (Weight of Wood Block)
Sta
tic
& K
inet
ic F
rict
ion
5 .5
0
Fs
N
Fk
newton
best_fits
best_fitk
120 FN
newton
Fs slopes slopes
slopek slopek
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Friction as a Function of Normal Force
Normal Force (Weight of Wood Block)
Sta
tic
& K
inet
ic F
rict
ion
5 .5
0
Fs
N
Fk
newton
best_fits
best_fitk
120 FN
newton
Fs slopes slopes
slopek slopek
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0 5 10 150
1
2
3
4
5
6Friction as a Function of Normal Force
Normal Force (Weight of Wood Block)
Sta
tic
& K
inet
ic F
rict
ion
Fs
N
Fk
newton
best_fits
best_fitk
FN
newton
slopes 0.465
slopek 0.331
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What information do the slopes
of Fs
FN
and Fk
FN
tel l us from
the straight-l ine graph?
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4.9 Static and Kinetic Frictional Forces
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Coefficient of Friction #41 Fw 45.0 N Fa 36.0 N
ms 0.650 mk 0.420
If calculated Fa Fs , then
block wil l move.
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Coefficient of Friction #41
ms
Fs
FN
Fa
Fw
Holds true if block is static or
moves at constant speed, just
l ike in the friction lab we did.
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Coefficient of Friction #41 If calculated Fa Fs , then
block wil l move.
Fs ms FN ms Fw 29.3 N
36.0 N 29.3N Block moves.
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Coefficient of Friction #41
mk
Fk
Fw
F m a
a F
m
Fa Fk
m
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Coefficient of Friction #41
aFa Fw mk
Fw
9.8N
kg
a 3.72m
s2
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Static & Kinetic Friction
ms
Fs
FN
mk
Fk
FN
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4.8 The Normal Force
Definition of the Normal Force
The normal force is one component of the force that a surface
exerts on an object with which it is in contact – namely, the
component that is perpendicular
to the surface.
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4.8 The Normal Force
N 26
0N 15N 11
N
N
F
F
N 4
0N 15N 11
N
N
F
F
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Apparent Weight
• This is the weight that you feel in an elevator, regardless of your true weight on Earth.
• It is what a bathroom scale would read if you were on one in an elevator. (The elevator floor provides an upward force to the scale, and from the scale to your feet.)
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4.8 The Normal Force
Apparent Weight
The apparent weight of an object is the reading of the scale.
It is equal to the normal force the scale exerts on the man.
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4.8 The Normal Force
mamgFF Ny
mamgFN
apparent
weight
true
weight
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Elevator Problem #37
• Since an elevator accelerates, use F=ma.
• Draw a FBD of all the forces involved.
• The algebraic sum of the colinear forces gives the net force.
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Elevator Problem #37
F m a
vo 0 vf 45m
s mA 57 kg FN
FN mA g mA a
FN mA a mA g
FN a g( )mA
g 9.80m
s2
t 15sec
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Elevator Problem #37
FN a g( )mA
FN
45m
s
15 s
9.8m
s2
57 kg
FN 730N
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Elevator Problem # 36a
Mp 95.0kg g 9.8m
s2
FN
a 1.80m
s2
g 9.8m
s2
a 1.80m
s2
F m a
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Elevator Problem # 36a
FN Mp g( ) Mp a
FN Mp a Mp g
FN Mp a g( )
FN 1.1 103
N
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Elevator Problem # 36b
• Constant speed means NO
acceleration.
• Since acceleration is the result
of an unbalanced force, and
there is no acceleration, the
forces are balanced.
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Elevator Problem # 36b
• The magnitude of the normal
force, FN, is therefore the same
as the magnitude of the
person’s weight, mpg.
FN Mp g
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Elevator Problem # 36b
FN Mp g
Mp g 95.0kg( ) 9.8m
s2
Mp g 931 N
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Elevator Problem # 36c
Mp 95.0kg g 9.8m
s2
FN
a 1.30m
s2
F m a
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Elevator Problem # 36c
FN Mp g( ) Mp a
FN Mp a Mp g
FN Mp a g( )
FN 808N
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4.11 Equilibrium & the 3rd Law
•Static Equilibrium
•Dynamic Equilibrium
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Static Equilibrium
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Equilibrium Definition
•Static (not moving) or
•No acceleration
(moving at constant
speed).
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4.11 Equilibrium Application of Newton’s Laws of Motion
Definition of Equilibrium
An object is in equilibrium when it has zero acceleration.
0xF
0yF
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4.11 Equilibrium Application of Newton’s Laws of Motion
Reasoning Strategy
• Select an object to which the equations of equilibrium are
to be applied.
• Draw a free-body diagram for each object chosen above.
Include only forces acting on the object, not forces the object
exerts on its environment.
• Choose a set of x, y axes for each object and resolve all forces
in the free-body diagram into components that point along these
axes.
• Apply the equations and solve for the unknown quantities.
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4.11 Equilibrium Application of Newton’s Laws of Motion
035sin35sin 21 TT
035cos35cos 21 FTT
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
xx maF
yy maF
When an object is accelerating, it is not in equilibrium.
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
The acceleration is along the x axis so 0ya
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
Force x component y component
1T
2T
D
R
0.30cos1T
0.30cos2T
0
0
D
R
0.30sin1T
0.30sin2T
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
00.30sin0.30sin 21 TTFy
21 TT
x
x
ma
RDTTF
0.30cos0.30cos 21
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
TTT 21
N 1053.10.30cos2
5
DRmaT x
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Two-Body Tension
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Calculate the tension in each of the
three support wires. M=5.0kg
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from University Physics by H. Benson
Fx mA mB ax
ax
Fx
mA mB
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Fx Arepresents the magnitude
of the Normal force FN .
ms
Fs
FN
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2-Body
Double Incline Problem
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FBD Method for finding ax
Fx m1
m1 ax Fx m2
m2 ax
T m1 g sin 1 m1 ax m2 g sin 2 T m2 ax
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UAM Method for finding au
Fu mT
mT au
m2 g sin 2 m1 g sin 1 m1 m2 au
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Atwood’s Machine
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Lab: Find tan theta for block:
1. Just about to slide down.
2. Moving down at constant speed
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Chapter 4
Forces and Newton’s
Laws of Motion
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4.1 The Concepts of Force and Mass
A force is a push or a pull.
Contact forces arise from physical
contact .
Action-at-a-distance forces do not
require contact and include gravity
and electrical forces.
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4.1 The Concepts of Force and Mass
Arrows are used to represent forces. The length of the arrow
is proportional to the magnitude of the force.
15 N
5 N
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4.1 The Concepts of Force and Mass
Mass is a measure of the amount
of “stuff” contained in an object.
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4.2 Newton’s First Law of Motion
An object continues in a state of rest
or in a state of motion at a constant
speed along a straight line, unless
compelled to change that state by a
net force.
The net force is the vector sum of all
of the forces acting on an object.
Newton’s First Law
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4.2 Newton’s First Law of Motion
The net force on an object is the vector sum of
all forces acting on that object.
The SI unit of force is the Newton (N).
Individual Forces Net Force
10 N 4 N 6 N
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4.2 Newton’s First Law of Motion
Individual Forces Net Force
3 N
4 N
5 N
64
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4.2 Newton’s First Law of Motion
Inertia is the natural tendency of an
object to remain at rest in motion at
a constant speed along a straight line.
The mass of an object is a quantitative
measure of inertia.
SI Unit of Mass: kilogram (kg)
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4.2 Newton’s First Law of Motion
An inertial reference frame is one in
which Newton’s law of inertia is valid.
All accelerating reference frames are
noninertial.
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4.3 Newton’s Second Law of Motion
F
Mathematically, the net force is
written as
where the Greek letter sigma
denotes the vector sum.
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4.3 Newton’s Second Law of Motion
Newton’s Second Law
When a net external force acts on an object
of mass m, the acceleration that results is
directly proportional to the net force and has
a magnitude that is inversely proportional to
the mass. The direction of the acceleration is
the same as the direction of the net force.
m
Fa
aF
m
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4.3 Newton’s Second Law of Motion
SI Unit for Force
22 s
mkg
s
mkg
This combination of units is called a newton (N).
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4.3 Newton’s Second Law of Motion
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4.3 Newton’s Second Law of Motion
A free-body-diagram is a diagram that
represents the object and the forces that
act on it.
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4.3 Newton’s Second Law of Motion
The net force in this case is:
275 N + 395 N – 560 N = +110 N
and is directed along the + x axis of the coordinate system.
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4.3 Newton’s Second Law of Motion
If the mass of the car is 1850 kg then, by
Newton’s second law, the acceleration is
2sm059.0kg 1850
N110
m
Fa
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4.4 The Vector Nature of Newton’s Second Law
xx maFyy maF
The direction of force and acceleration vectors
can be taken into account by using x and y
components.
aF
m
is equivalent to
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4.4 The Vector Nature of Newton’s Second Law
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4.4 The Vector Nature of Newton’s Second Law
Force x component y component
+17 N
+(15 N) cos67
0 N
+(15 N) sin67
+23 N +14 N
The net force on the raft can be calculated
in the following way:
P
A
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4.4 The Vector Nature of Newton’s Second Law
2sm 018.0kg 1300
N 23
m
Fa
x
x
2sm 011.0kg 1300
N 14
m
Fa
y
y
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4.5 Newton’s Third Law of Motion
Newton’s Third Law of Motion
Whenever one body exerts a force on a
second body, the second body exerts an
oppositely directed force of equal
magnitude on the first body.
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4.5 Newton’s Third Law of Motion
Suppose that the magnitude of the force 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?
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4.5 Newton’s Third Law of Motion
.astronaut On the
. spacecraft On the
PF
PF
2sm0033.0kg 11,000
N 36
s
sm
Pa
2sm39.0kg 92
N 36
A
Am
Pa
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4.6 Types of Forces: An Overview
In nature there are two general types of forces,
fundamental and nonfundamental.
Fundamental Forces
1. Gravitational force
2. Strong Nuclear force
3. Electroweak force
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4.6 Types of Forces: An Overview
Examples of nonfundamental forces:
friction
tension in a rope
normal or support forces
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4.7 The Gravitational Force
Newton’s Law of Universal Gravitation
Every particle in the universe exerts an attractive force on every
other particle.
A particle is a piece of matter, small enough in size to be
regarded as a mathematical point.
The force that each exerts on the other is directed along the line
joining the particles.
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4.7 The Gravitational Force
For two particles that have masses m1 and m2 and are
separated by a distance r, the force has a magnitude
given by
2
21
r
mmGF
2211 kgmN10673.6 G
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4.7 The Gravitational Force
N 104.1
m 1.2
kg 25kg 12kgmN1067.6
8
2
2211
2
21
r
mmGF
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4.7 The Gravitational Force
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4.7 The Gravitational Force
Definition of Weight
The weight of an object on or above the earth is the
gravitational force that the earth exerts on the object.
The weight always acts downwards, toward the center
of the earth.
On or above another astronomical body, the weight is the
gravitational force exerted on the object by that body.
SI Unit of Weight: newton (N)
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4.7 The Gravitational Force
Relation Between Mass and Weight
2r
mMGW
E
mgW
2r
MGg E
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4.7 The Gravitational Force
2
26
242211
2
sm 80.9
m 106.38
kg 1098.5kgmN1067.6
E
E
R
MGg
On the earth’s surface:
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4.8 The Normal Force
Definition of the Normal Force
The normal force is one component of the force that a surface
exerts on an object with which it is in contact – namely, the
component that is perpendicular
to the surface.
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4.8 The Normal Force
N 26
0N 15N 11
N
N
F
F
N 4
0N 15N 11
N
N
F
F
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4.8 The Normal Force
Apparent Weight
The apparent weight of an object is the reading of the scale.
It is equal to the normal force the scale exerts on the man.
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4.8 The Normal Force
mamgFF Ny
mamgFN
apparent
weight
true
weight
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4.9 Static and Kinetic Frictional Forces
When an object is in contact with a surface there is a force
acting on that object. The component of this force that is
parallel to the surface is called the
frictional force.
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4.9 Static and Kinetic Frictional Forces
When the two surfaces are
not sliding across one another
the friction is called
static friction.
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4.9 Static and Kinetic Frictional Forces
The magnitude of the static frictional force can have any value
from zero up to a maximum value.
MAX
ss ff
Ns
MAX
s Ff m
10 sm is called the coefficient of static friction.
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4.9 Static and Kinetic Frictional Forces
Note that the magnitude of the frictional force does
not depend on the contact area of the surfaces.
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4.9 Static and Kinetic Frictional Forces
Static friction opposes the impending relative motion between
two objects.
Kinetic friction opposes the relative sliding motion motions that
actually does occur.
Nkk Ff m
10 sm is called the coefficient of kinetic friction.
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4.9 Static and Kinetic Frictional Forces
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4.9 Static and Kinetic Frictional Forces
The sled comes to a halt because the kinetic frictional force
opposes its motion and causes the sled to slow down.
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4.9 Static and Kinetic Frictional Forces
Suppose the coefficient of kinetic friction is 0.05 and the total
mass is 40kg. What is the kinetic frictional force?
N20sm80.9kg4005.0 2
mgFf kNkk mm
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4.10 The Tension Force
Cables and ropes transmit
forces through tension.
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4.10 The Tension Force
A massless rope will transmit
tension undiminished from one
end to the other.
If the rope passes around a
massless, frictionless pulley, the
tension will be transmitted to
the other end of the rope
undiminished.
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4.11 Equilibrium Application of Newton’s Laws of Motion
Definition of Equilibrium
An object is in equilibrium when it has zero acceleration.
0xF
0yF
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4.11 Equilibrium Application of Newton’s Laws of Motion
Reasoning Strategy
• Select an object(s) to which the equations of equilibrium are
to be applied.
• Draw a free-body diagram for each object chosen above.
Include only forces acting on the object, not forces the object
exerts on its environment.
• Choose a set of x, y axes for each object and resolve all forces
in the free-body diagram into components that point along these
axes.
• Apply the equations and solve for the unknown quantities.
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4.11 Equilibrium Application of Newton’s Laws of Motion
035sin35sin 21 TT
035cos35cos 21 FTT
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4.11 Equilibrium Application of Newton’s Laws of Motion
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4.11 Equilibrium Application of Newton’s Laws of Motion
N 3150W
Force x component y component
1T
2T
W
0.10sin1T
0.80sin2T
0
0.10cos1T
0.80cos2T
W
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4.11 Equilibrium Application of Newton’s Laws of Motion
00.80sin0.10sin 21 TTFx
00.80cos0.10cos 21 WTTFy
The first equation gives 21
0.10sin
0.80sinTT
Substitution into the second gives
00.80cos0.10cos0.10sin
0.80sin22
WTT
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4.11 Equilibrium Application of Newton’s Laws of Motion
0.80cos0.10cos0.10sin
0.80sin2
WT
N 5822 T N 1030.3 3
1 T
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
xx maF
yy maF
When an object is accelerating, it is not in equilibrium.
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
The acceleration is along the x axis so 0ya
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
Force x component y component
1T
2T
D
R
0.30cos1T
0.30cos2T
0
0
D
R
0.30sin1T
0.30sin2T
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
00.30sin0.30sin 21 TTFy
21 TT
x
x
ma
RDTTF
0.30cos0.30cos 21
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4.12 Nonequilibrium Application of Newton’s Laws of Motion
TTT 21
N 1053.10.30cos2
5
DRmaT x
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Problem 59 (Honors)
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