This document discusses Newton's first law of motion, which states that an object at rest stays at rest and an object in motion stays in motion with constant velocity unless acted upon by an external force. It provides examples to illustrate this, such as a hovercraft moving at constant velocity, meaning no net external force is acting on it, and blocks sliding across different surfaces. It also discusses the concept of inertia and how it relates to an object's mass, with more massive objects being harder to accelerate. Newton's first law is known as the law of inertia because it describes an object's tendency to maintain its state of motion in the absence of external forces.

1. Key Terms
inertia
net force
equilibrium
Inertia
A hovercraft, such as the one in Figure 2.1, glides along the surface of the
water on a cushion of air. A common misconception is that an object on
which no force is acting will always be at rest. This situation is not always
the case. If the hovercraft shown in Figure 2.1 is moving at a constant
velocity, then there is no net force acting on it. To see why this is the case,
consider how a block will slide on different surfaces.
First, imagine a block on a deep, thick carpet. If
you apply a force by pushing the block, the block
will begin sliding, but soon after you remove the
force, the block will come to rest. Next, imagine
pushing the same block across a smooth, waxed
floor. When you push with the same force, the block
will slide much farther before coming to rest. In
fact, a block sliding on a perfectly smooth surface
would slide forever in the absence of an applied
force.
In the 1630s, Galileo concluded correctly that it
is an object’s nature to maintain its state of motion
or rest. Note that an object on which no force is
acting is not necessarily at rest; the object could
also be moving with a constant velocity. This
concept was further developed by Newton in 1687
and has come to be known as Newton’s first law
of motion.
Newton’s First Law
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.
Inertia is the tendency of an object not to accelerate. Newton’s first law
is often referred to as the law of inertia because it states that in the absence
of a net force, a body will preserve its state of motion. In other words,
Newton’s first law says that when the net external force on an object is zero,
the object’s acceleration (or the change in the object’s velocity) is zero.
Newton’s First Law Main Ideas
Explain the relationship
between the motion of an object
and the net external force
acting on the object.
Determine the net external
force on an object.
Calculate the force required to
bring an object into equilibrium.
©George
Hunter/SuperStock
Hovercraft on Air A hovercraft floats on a cushion of air above
the water. Air provides less resistance to motion than water does.
FIGURE 2.1
inertia the tendency of an object to
resist being moved or, if the object is
moving, to resist a change in speed
or direction
Forces and the Laws of Motion 123
SECTION 2
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Differentiated Instruction
Preview Vocabulary
Latin Word Origins The root of inertia
comes from Latin. The root word can
mean “idleness,” “unskilled,” or “inactive.”
Inertia has the same root word as the
term inert, a word used in chemistry to
describe something that is chemically
inactive. The Group 18 elements, such as
neon and argon, are sometimes called
the inert gases.
Inertia
Purpose Help students develop a
kinesthetic sense of inertia.
Materials physics book, calculator
Procedure Tell students that they will
be able to feel the effects of inertia.
First, tell them to hold the physics book
upright between their hands, palms
facing inward. Have them move the
book from side to side (oscillating a
distance of 30 cm) at regular time
intervals. Tell the students to note the
effort involved in changing the motion
of the book. Repeat the demonstration
with the calculator and have students
note the much smaller effort required.
Plan and Prepare
Teach
Demonstration
Inclusion
Kinesthetic learners may benefit from a simple
activity which demonstrates inertia. Take
students outside or into the gymnasium. Mark
off a 25-meter race course. Set cones at 23
and 25 meters. Tell students that the goal is for
them to run as fast as possible and then to
come to a complete stop between the cones.
Allow students to warm up and then start
from one end.
After students have completed their run,
ask them to describe what they experienced.
They should have noticed that while it was
easy to stop their feet, they may have felt as
if their upper body was still moving forward.
Explain to them that they were experiencing
inertia. Use this experience as a starting point
for a classroom discussion about Newton’s
first law.
Forces and the Laws of Motion 123
SECTION 2

2. PHYSICS
Spec. Number PH 99 PE C04-002-002-A
Boston Graphics, Inc.
617.523.1333
resistance
F
gravity
F
forward
F
ground-on-car
F
The sum of forces acting on an object is the net force.
Consider a car traveling at a constant velocity. Newton’s first law tells us
that the net external force on the car must be equal to zero. However,
Figure 2.2 shows that many forces act on a car in motion. The vector Fforward
represents the forward force of the road on the tires. The vector Fresistance,
which acts in the opposite direction, is due partly to friction between the
road surface and tires and is due partly to air resistance. The vector Fgravity
represents the downward gravitational force on the car, and the vector
Fground-on-car represents the upward force that the road exerts on the car.
To understand how a car under the influence of so many forces can
maintain a constant velocity, you must understand the distinction between
external force and net external force. An external force is a single force that
acts on an object as a result of the interaction between the object and its
environment. All four forces in Figure 2.2 are external forces acting on the
car. The net force is the vector sum of all forces acting on an object.
When many forces act on an object, it may move in a particular
direction with a particular velocity and acceleration. The net force is the
force, which when acting alone, produces exactly the same change in
motion. When all external forces acting on an object are known, the net
force can be found by using the methods for finding resultant vectors.
Although four forces are acting on the car in Figure 2.2, the car will main-
tain a constant velocity if the vector sum of these forces is equal to zero.
Mass is a measure of inertia.
Imagine a basketball and a bowling ball at rest side by side on the ground.
Newton’s first law states that both balls remain at rest as long as no net
external force acts on them. Now, imagine supplying a net force by
pushing each ball. If the two are pushed with equal force, the basketball
will accelerate more than the bowling ball. The bowling ball experiences
a smaller acceleration because it has more inertia than the basketball.
As the example of the bowling ball and the basketball shows, the
inertia of an object is proportional to the object’s mass. The greater the
mass of a body, the less the body accelerates under an applied force.
Similarly, a light object undergoes a larger acceleration than does a heavy
object under the same force. Therefore, mass, which is a measure of the
amount of matter in an object, is also a measure of the inertia of an object.
net force a single force whose
external effects on a rigid body are the
same as the effects of several actual
forces acting on the body
Net Force Although several forces
are acting on this car, the vector sum
of the forces is zero, so the car moves
at a constant velocity.
FIGURE 2.2
INERTIA
Place a small ball on the rear end
of a skateboard or cart. Push the
skateboard across the floor and
into a wall. You may need to either
hold the ball in place while push-
ing the skateboard up to speed or
accelerate the skateboard slowly
so that friction holds the ball
in place. Observe what happens
to the ball when the skateboard
hits the wall. Can you explain
your observation in terms of
inertia? Repeat the procedure
using balls with different masses,
and compare the results.
MATERIALS
skateboard or cart
•
toy balls with various masses
•
SAFETY
Perform this experiment
away from walls and
furniture that can be
damaged.
Chapter 4
124
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Problem Solving
Teacher’s Notes
If students have trouble keeping the ball
in place while accelerating the skate-
board, they can tape a wooden block
onto the skateboard to keep the ball
from rolling off the back. Students
should recognize that when the skate-
board hits the wall, the ball continues
moving forward due to its inertia.
Teach continued
QuickLab
Take it Further
During liftoff, astronauts on a space shuttle
experience tremendous forces. Challenge
students to research the forces that act on
astronauts as they lift off the launch pad and
leave Earth’s atmosphere. Ask students to
choose one segment of the astronauts’ path
and create a drawing illustrating the net forces
working at that point in time. Suggest that
students research education websites hosted
by NASA in order to get started.
124 Chapter 4

3. Determining Net Force
Sample Problem B Derek leaves his physics book on top of a
drafting table that is inclined at a 35° angle. The free-body
diagram at right shows the forces acting on the book. Find the net
force acting on the book.
ANALYZE Define the problem, and identify
the variables.
Given: Fgravity-on-book = Fg = 22 N
Ffriction = Ff = 11 N
Ftable-on-book = Ft = 18 N
Unknown: Fnet = ?
Select a coordinate system, and apply it to the free-body diagram.
Choose the x-axis parallel to and the y-axis perpendicular
to the incline of the table, as shown in (a). This coordinate
system is the most convenient because only one force
needs to be resolved into x and y components.
PLAN Find the x and y components of all vectors.
Draw a sketch, as shown in (b), to help find the components
of the vector Fg. The angle θ is equal to 180°- 90° - 35° = 55°.
cos θ =
Fg, x
_
Fg
sin θ =
Fg, y
_
Fg
Fg,x = Fg cos θ Fg,y = Fg sin θ
Fg,x = (22 N)(cos 55°) = 13 N Fg,y = (22 N)(sin 55°) = 18 N
Add both components to the free-body diagram, as shown in (c).
SOLVE Find the net force in both the x and y directions.
Diagram (d) shows another free-body diagram of the
book, now with forces acting only along the x- and y-axes.
For the x direction: For the y direction:
ΣFx = Fg,x - Ff ΣFy = Ft - Fg,y
ΣFx = 13 N - 11 N = 2 N ΣFy = 18 N - 18 N = 0 N
Find the net force.
Add the net forces in the x and y directions together as
vectors to find the total net force. In this case, Fnet = 2 N
in the +x direction, as shown in (e). Thus, the book
accelerates down the incline.
CHECK YOUR
WORK
The box should accelerate down the incline, so the answer
is reasonable.
Continued
35°
11 N
18 N
18 N
13 N
22 N
11 N
13 N
18 N
18 N
= 2 N
Fnet
= 18 N
table-on-book
F
= 22 N
gravity-on-book
F
= 11 N
friction
F
11 N 18 N
22 N
TSI Graphics
HRW • Holt Physics
PH99PE-C04-002-007-A
Tips and Tricks
To simplify the problem,
always choose the
coordinate system in which
as many forces as possible
lie on the x- and y-axes.
(a)
(b)
(c)
(d)
(e)
Forces and the Laws of Motion 125
orrectionKey=C
H_CNLESE586694_C04S2.indd 125 3/26/2013 9:55:51 PM
Classroom Practice
Determining Net Force
An agriculture student is designing a
support to keep a tree upright. Two
wires have been attached to the tree
and placed at right angles to each other.
One wire exerts a force of 30.0 N on the
tree; the other wire exerts a 40.0 N
force. Determine where to place a third
wire and how much force it should exert
so that the net force acting on the tree
is equal to zero.
Answer: 50.0 N at 143° from the 40.0 N
force and at 127° from the 30.0 N force
A flying, stationary kite is acted on by
a force of 9.8 N downward. The wind
exerts a force of 45 N at an angle of
50.0° above the horizontal. Find the
force that the string exerts on the kite.
Answer: 38 N, 40° below the horizontal
Alternative Approaches
For free-body diagrams, it sometimes helps to
try a different arrangement of the vectors.
Show students that, by arranging the force
vectors on the coordinate system in such a way
that more vectors lie on one of the axes, there
will be fewer vectors to resolve into compo-
nents. As a result, calculating the solution will
take fewer steps.
Forces and the Laws of Motion 125

4. ©NASA/Reuters/Corbis
Determining Net Force (continued)
1. A man is pulling on his dog with a force of 70.0 N directed at an angle of +30.0° to
the horizontal. Find the x and y components of this force.
2. A gust of wind blows an apple from a tree. As the apple falls, the gravitational force
on the apple is 2.25 N downward, and the force of the wind on the apple is 1.05 N
to the right. Find the magnitude and direction of the net force on the apple.
3. The wind exerts a force of 452 N north on a sailboat, while the water exerts a force
of 325 N west on the sailboat. Find the magnitude and direction of the net force on
the sailboat.
Astronaut
Workouts
G
ravity helps to keep bones strong. Loss of bone
density is a serious outcome of time spent in
space. Astronauts routinely exercise on treadmills
to counteract the effects of microgravity on their skeletal
systems. But is it possible to increase the value of their
workouts by increasing their mass? And does it matter if
they run or walk?
A team of scientists recruited runners to help find out.
The runners used treadmills that measured the net force
on their legs, or ground reaction force, while they ran and
walked. The runners’ inertia was changed by adding
masses to a weighted vest. A spring system supported
them as they exercised. Although the spring system did
not simulate weightless conditions, it kept their weight the
same even as their inertia was changed by the added
mass. This mimicked the situation in Earth orbit, where a
change in mass does not result in a change in weight.
The scientists were surprised to discover that ground
reaction force did not increase with mass while the
subjects were running. Ground reaction force did increase
with mass while the subjects were walking. But overall,
ground reaction force for running was still greater. So
astronauts still need to run, not walk—and they can’t
shorten their workouts by carrying more mass.
Tips and Tricks
If there is a net force in both the x and y directions, use vector
addition to find the total net force.
Chapter 4
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Problem Solving
TAKE IT FURTHER
After solving Practice B(2), ask students to
identify the x and y components of the net
force. The x component of the net force is
+1.05 N and the y component is –2.25 N.
Such extension of the problem can be
considered as the evaluation method.
Answers
Practice B
1. Fx
= 60.6 N; Fy
= 35.0 N
2. 2.48 N at 25.0° counterclockwise
from straight down
3. 557 N at 35.7° west of north
PROBLEM GUIDE B
Use this guide to assign problems.
SE = Student Edition Textbook
PW = Sample Problem Set I (online)
PB = Sample Problem Set II (online)
Solving for:
Fx
, Fy
SE Sample, 1;
Ch. Rvw. 11–12
PW 3, 4*, 5*
PB 7–10
Fnet
SE Sample, 2–3;
Ch. Rvw. 10, 22a*
PW Sample, 1–2
PB 1–6
*Challenging Problem
Why It Matters
Scientists have determined that the
body adjusts its stride to changes in
inertia differently than it adjusts to
changes in weight. Point out that this
experiment suggests that changing
inertia (mass) does not increase the net
force on runners in microgravity. Ask
students to think about resistance
training and list ways that astronauts
could replace the resistance of gravity
while in space. Then explain that
astronauts use bungee cords to replace
the resistance of gravity during exercise.
However, bungees replace only about
60% of the astronauts’ weight on Earth.
Teach continued
126 Chapter 4

5. ©Tony
Freeman/PhotoEdit
Reviewing Main Ideas
1. If a car is traveling westward with a constant velocity of 20 m/s, what is
the net force acting on the car?
2. If a car is accelerating downhill under a net force of 3674 N, what addi-
tional force would cause the car to have a constant velocity?
3. The sensor in the torso of a crash-test dummy records the magnitude and
direction of the net force acting on the dummy. If the dummy is thrown
forward with a force of 130.0 N while simultaneously being hit from the
side with a force of 4500.0 N, what force will the sensor report?
4. What force will the seat belt have to exert on the dummy in item 3 to hold
the dummy in the seat?
Critical Thinking
5. Can an object be in equilibrium if only one force acts on the object?
Equilibrium
Objects that are either at rest or moving with constant velocity are said to
be in equilibrium. Newton’s first law describes objects in equilibrium,
whether they are at rest or moving with a constant velocity. Newton’s first
law states one condition that must be true for equilibrium: the net force
acting on a body in equilibrium must be equal to zero.
The net force on the fishing bob in
Figure 2.3(a) is equal to zero because the bob is
at rest. Imagine that a fish bites the bait, as
shown in Figure 2.3(b). Because a net force is
acting on the line, the bob accelerates toward
the hooked fish.
Now, consider a different scenario. Suppose
that at the instant the fish begins pulling on the
line, the person reacts by applying a force to
the bob that is equal and opposite to the force
exerted by the fish. In this case, the net force
on the bob remains zero, as shown in
Figure 2.3(c), and the bob remains at rest. In this
example, the bob is at rest while in equilib-
rium, but an object can also be in equilibrium
while moving at a constant velocity.
An object is in equilibrium when the vector
sum of the forces acting on the object is equal
to zero. To determine whether a body is in
equilibrium, find the net force, as shown in
Sample Problem B. If the net force is zero, the
body is in equilibrium. If there is a net force, a
second force equal and opposite to this net
force will put the body in equilibrium.
equilibrium the state in which the net
force on an object is zero
Forces on a Fishing Line (a) The bob on this fishing line is at
rest. (b) When the bob is acted on by a net force, it accelerates. (c) If
an equal and opposite force is applied, the net force remains zero.
FIGURE 2.3
(a)
(b) (c)
Forces and the Laws of Motion 127
SECTION 2 FORMATIVE ASSESSMENT
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Answers to Section Assessment
FIGURE 2.3 Point out that in order for
the bob to be in equilibrium, all the
forces must cancel. You may want to
diagram this situation on the board and
include the force of the water on the
bob (buoyant force).
Ask Other than the forces applied by
the person and the fish, do any other
forces act on the bob?
Answer: yes, the upward (buoyant) force
of the water on the bob and the
downward gravitational force
Assess Use the Formative Assessment
on this page to evaluate student
mastery of the section.
Reteach For students who need
additional instruction, download the
Section Study Guide.
Response to Intervention To reassess
students’ mastery, use the Section Quiz,
available to print or to take directly
online at HMDScience.com.
TEACH FROM VISUALS
Assess and Reteach
1. zero
2. -3674 N
3. 4502 N at 1.655° forward of the side
4. the same magnitude as the net force in
item 3 but in the opposite direction
5. No, either no force or two or more forces
are required for equilibrium.
Forces and the Laws of Motion 127