Chapter 4: Forces and the Laws of Motion
MAY THE FORCE BE WITH YOU 
A Review….
 Force
 push or pull exerted on some object
 - Unit of Force is a NEWTON (N)
 1 kg x m/s2
 - Force is a VECTOR
 Has a magnitude and direction
Review
 Acceleration is a
change in velocity per
unit of time interval.
 It has both magnitude
and direction.
 Vectors have both
magnitude and
direction.
 What causes motion?
 Why do some objects
accelerate at higher
rates than others do?
4.1 Changes in Motion
 How does force affect the motion of an
object?
 Forces can cause
 Changes in velocity (fig 4-1)
 Stationary objects to move (throwing a ball)
 Moving objects to stop (catching a ball)
 A change of direction (hitting a ball)
 These all involve a change in velocity an
acceleration
Force the cause of acceleration, or the change in an object’s velocity
4.1 Weight vs. Mass
 The weight of an object is a measure of the
magnitude of the gravitational force exerted
on an object. (mass is a measure of an object’s
inertia)
 It’s the result of the objects mass interacting with




Earth’s gravitational field.
Terms we often use to describe weight are really
units of force
Ex. A ¼ lb. stick of butter has a weight equivalent
to a force of 1N.
1 lb. =4.448N
1N=0.225 lb
Contact and Field Forces
 Forces can act through contact or at a
distance
 Contact force is a force that arises from the
physical contact of two objects (like a push or a
pull)
 Usually easy to identify
 Ex:
 frictional force
 Tension force
 air resistance
 Applied force
Field Forces
 Forces that do not involve physical contact are
called field forces
 The presence of an object affects the space
around it so that a force is exerted on any other
object placed within that space.
 The region of influence is called a field
 Ex: Gravity – earth exerts a force on an object
even when it’s not in immediate contact with the
object (also electrical and magnetic)
 Ex: the attraction or repulsion between electrical
charges
Forces
 Applied Force Fapp a force that is applied to an object by
a person or another object. (a person pushing a desk
across the room)
 Gravity Force F grav (also known as weight) the force with
which the earth or other large objects attract another
object towards itself (not the same as mass)
 Normal Force Fnorm the support force exerted upon an
object that is in contact with another stable object. (like a
book resting upon a surface, the surface is exerting an
upward force on the book in order to support the weight of
the book)
Forces
 Friction Force Ffrict the force exerted by a
surface as an object moves across it or makes
an effort to move across it. (Both sliding and
static friction)
 Friction results from the two surfaces being
pressed together closely, causing intermolecular
attractive forces between molecules
 Air Resistance Force Fair frictional force that
acts upon objects as they travel through the
air (usually negligible except for objects
traveling at high speeds or having high
surface area)
Forces
 Tension Force Ftens Force transmitted through
a string, rope, cable or wire when it is pulled
tight by forces acting from opposite ends.
 Spring Force Fspring the force exerted by a
compressed or stretched spring upon any
object that is attached to it.
 Mass the amount of matter that is contained
by the object. (how much stuff is there)
 Weight the force of gravity acting upon an
object (the pull of the earth on that stuff)
Force Diagrams
 If you push a toy car, it does not move as fast
as it does when you give a harder push.
 The effect of force depends on its magnitude
 The effect of force on an object’s motion also
depends on the direction of the force
 A force diagram is a diagram of the objects
involved in a situation and the forces exerted
on the objects
(a) force diagram
shows all the forces
acting in a situation
(b) free-body diagram shows only the forces acting
on a particular object
Force Diagrams
 Show force vectors as arrows
 Tail of the arrow is attached to the object on
which the force is acting
 Used as tools to analyze collisions
 For now, disregard the size and shape of the
object and assume that all forces act on a
point at the center of an object
FREE BODY DIAGRAMS
 A free-body diagram helps analyze a situation
 Used to analyze only the forces affecting the motion of a
single object
 Work just like vector diagrams – find component and
resultant forces
 Ex. Engineers use to analyze test-car crashes to determine
which forces affect the car and passengers
Tow truck example
 Figure 4-4
 There are many forces acting on this car
 The tow truck exerts a force on the car in the
direction of the cable
 The road exerts forces on the car
 The car is acted on by gravitational force
 To draw a free-body diagram, you must first isolate
and identify all the forces acting on the car
Free Body Diagrams isolate an object
and the forces acting on it
Steps to draw free body diagrams:
1. Draw a diagram to represent the
isolate object (b). Draw in same position
as the actual object.
2. Draw and label vector arrows of all
external forces acting on the object (c). All
forces are assumed to act on a single point at the
center of an object.
Label the arrow with the size of the force
b) The force of the tow truck exerts 5800 N
on the car. The arrow is pointing in the
same direction as the force of the cable
on the car.
a)
Drawing Free Body Diagrams
3. The gravitational force acting
on the car is 14700 N directed
toward the center of the
earth (d).
4. The road exerts an upward
force of 13690 N (e).
5. Because of the interaction
between the road and the
car’s tires, the road also
exerts a backward force of
friction equal to 775 N (f).
Drawing Free Body Diagrams
6. Make sure that only the
forces acting on the car are
included in the free body
diagram. (leave off the forces
the car is exerting on other
objects)
7. F is the completed free-body
diagram.
A free-body diagram can be used
to find the net external force
acting on an object, using the
rules for vector analysis.
Section Review 1-6
NEWTON’S LAWS OF MOTION
 3 Laws of Motion
Newton’s 1st Law
 Predicts the behavior of stationary objects and
moving objects. (Insert chart from
physicsclassroom.com)
 An object at rest remains at rest and an object in
motion continues in motion with a constant velocity
unless acted on by an outside force
Law of Inertia
 Also known as the LAW OF INERTIA
 Inertia the tendency of an object not to
accelerate
 When the net external force on an object is zero,
its acceleration (or change in velocity) is zero
 If the forces acting upon an object are balanced,
then the acceleration of that object will be 0 m/s2
 Objects tend to “keep on doing what they’re
doing”.
Newton’s First Law
 Acceleration is determined by net force
 Net external force is the total force resulting
from a combination of external forces on an
object; sometimes called the resultant force
 It’s the vector sum of all the forces acting on an
object
 When all external forces are known, the net
external force can be found by finding the
resultant vector
 An objects acceleration is determined by all the
forces acting on it.
Newton’s first law concepts
 Mass is a measurement of inertia
 A basketball and a golf ball that are side by side on
the ground remain at rest as long as no net external
force acts on them.
 If you strike each ball with a golf club, which one
will accelerate more?
 The basketball experiences a smaller acceleration
because it has more inertia than the golf ball.
 The inertia of an object is proportional to its
mass.
 The greater the mass of the body, the less the
body accelerates under an applied force.
Newton’s First law concepts
 Objects in motion tend to stay in motion.
 The Zebra’s stripes stay in motion!
Equilibrium
 Objects that are either at rest or moving with
constant velocity are said to be in
equilibrium.
 All forces are balanced
 Newton’s first law describes objects in
equilibrium, whether they are at rest or
moving with constant velocity.
 For equilibrium to occur, the net external
force must be zero.
Equilibrium
 The bob on the fishing line is
at rest.
 We know Fnet is equal to zero
 If a fish bites the bait, the fish
exerts a force on the line, the
bob accelerates downward.
 To return the bobber to
equilibrium , the person must
apply force to the fishing line.
Equilibrium
 The force that brings an accelerating object
into equilibrium, must be equal and opposite
to the force causing the object to accelerate.
 An object is in equilibrium when the vector
sum of the forces acting on it is equal to zero.
 The easiest way to do this is to resolve forces
into their x and y components.
 When the sum of all forces in the x direction is
zero (Fy=0). Then the vector sum is equal to
zero and the body is in equilibrium.
Newton’s Second Law
 Newton’s Second Law
 Predicts the behavior of objects for which all
existing forces are NOT balanced.
 States: The acceleration of an object is
dependent on two variables: Fnet and mass.
Newton’s Second Law
 Acceleration is directly proportional to the
Fnet acting on the object and inversely
proportional to the object's mass.
 As force is increased, acceleration is increased
 As mass is increased, acceleration is decreased
Acceleration = Net Force divided by mass
a=  Fnet/m
Net Force = mass * acceleration
 Fnet=m*a
Newton’s Second Law
 Remember that
1 Newton (N) = kg * m/s2
a= F/m
Example: Roberto and Laura are studying across
from each other at a wide table. Laura slides
a 2.2 kg book toward Roberto. If the net
external force acting on the book is 2.6 N to
the right, what is the book’s acceleration?
EXAMPLE 4B
 Given: m=2.2 kg
 Fnet= F=2.6N to the right
 a=?
 Use Newton’s second law and solve for a.
a= F/m
Practice 4B
Misconception Alert
 Be aware of the common misconception that
sustaining motion requires a constant force.
 Read the scenarios and we will discuss the
answers!
Newton’s 3rd Law
 Forces result from an object’s interaction
 Such as contact interactions (normal, friction,
tensional and applied)
 Action-at-a-distance interactions (gravitational,
electrical, magnetic)
 When you sit in a chair, you exert a downward
force on the chair and the chair exerts an upward
force on your body
 These two forces are called action and reaction
pairs
 “a pair of simultaneous equal but opposite forces resulting
from the interaction of two objects”
Newton’s Third Law
 Newton’s Third law deals with action and
reaction pairs
 For every action there is an equal and opposite
reaction.
 “If 2 objects interact, the magnitude of the
force exerted on object 1 by object 2 is equal in
magnitude of the force simultaneously exerted
on object 2 by object one, and these forces are
opposite in direction.”
 A single isolate force cannot exist!
Action-reaction pair
 In every interaction, there is a pair of forces
acting on the 2 interacting objects
 The size of the forces on the first object equal the
size of the force on the second object
 The direction of the force on the first object is
opposite to the direction of the force on the
second object.
 Forces always come in pairs! Equal and opposite
action-reaction force pairs.
Action- Reaction Pair
 Either force can be called the action or
reaction
 Misconception alert: don’t confuse ‘reaction’
with the everyday use of the word. Reaction
here does not mean something that happens
in response to something, but happens at the
SAME TIME as the action force!
 Most important to remember: each force acts
on a different object.
Action-Reaction forces
 Misconception alert: equal and opposite
forces do not balance each other a inhibit a
change in motion.
 Consider a hammer striking a nail?
 Why doesn’t it remain at rest?
Example
 Think of the flying motion of birds.
 The wings of a bird push air downwards.
 Since forces result from mutual interactions, the
air must also be pushing the bird upwards.
 The size of the force on the air equals the size of
the force on the bird
 The direction of the force on the air (downwards)
is opposite the direction of the force on the bird
(upwards).
A truck driving down the road runs into a bug flying the other
way.
a) Which object has the greater force exerted on it?
b) Which object will change its motion the most?
FTB
FBT
Both objects have the same small force exerted on them during
the collision according to Newton’s third law. The bug squishes
easily!
The bug obviously changes its motion significantly more than
the truck because of its much lower inertia (mass).
Truck:
aT
=F/
M
:
Bug
- aT = - F /
M
The moon moves around the earth.
a) Which object has the greater force exerted on it?
b) Which object will change its motion the most?
FME
FEM
Both objects have the same large force exerted on them during
the collision according to Newton’s third law.
The Moon obviously changes its motion significantly more than
the Earth because of its much lower inertia (mass).
Earth:
aE
=F/
M
Moon
:
- aM = - F /
M
Check for Understanding
 Read the questions on “Check for
Understanding”, answer them and we will
discuss.
 Do Section Review 1-5 pg 140
Laws of Motion Review
 1st Law  what happens when there is no net
force (inertia)
 On object at rest stays at rest and an object in
motion stays in motion
 2nd law  what happens when a net force
does act
 Acceleration depends on net force and mass
 3rd law  forces occur in pairs, no isolated
forces (action-reaction law)
 For every action there is an equal and opposite
4-4 Everyday Forces
 Which weighs more, a bowling ball or a tennis
ball?
 How do you know?
 Imagine one ball in each hand and imagine the
downward forces acting on your hands.
 Because the bowling ball has more mass the force
of gravity pulls more strongly on the bowling ball
and it pushes on your hand with a stronger force.
Weight vs. Mass
 Mass is an inherent property of an object.
The mass of a bowling ball is the same on
earth as it is the moon.
 Weight is NOT an inherent property. It is
dependent on the force of gravity- which
changes with location.
 An astronaut weights 800N (180 lb) on Earth,
but only 130 N (30lb) on the moon.
 Because gravity is smaller on the moon
Weight
 Is weight constant on earth? Does your
weight vary depending on where you are?
 Actually, YES ! G decreases as distance from the
center of Earth increases, so objects weigh less at
higher altitudes than at sea level.
 Also, because earth is not symmetrical, g also
decreases a little as latitude decreases.
 Weight is the magnitude of the force of gravity
acting on an object
The Normal Force
 Normal force (Fn )a force exerted by one
object on another in a direction perpendicular
to the surface of contact.
 One meaning of the word “normal” is
‘perpendicular’.
 It is a support force exerted upon an object that is in
contact with another stable object
 The Fn is ALWAYS perpendicular to the
surface but NOT always opposite of Fg
Perpendicular means at a right angle to
Refrigerator on a ramp
 This is a fridge on a loading
ramp
 Notice the Fn is perpendicular
to the ramp but NOT Fg
 The magnitude of normal force
can be calculated with:
Fn = mg(cos )
The force of Friction
 Friction is the force exerted by a surface as an
object moves or makes an effort to move
across the surface
 Two types of friction
 Static friction (Fs) results when the surfaces of two
objects are at REST relative to one another and a
force exists on one of the objects to set it into
motion
 Kinetic Friction (aka Sliding friction) (Fk) results
when an object slides across a surface
Friction
(a) Because the
jug of is in
equilibrium, any
horizontal force will
cause it to
accelerate
(b) when a small
force is applied,
the jug remains in
equilibrium
because the force
of friction is
equal/opposite to
the applied force.
(c) When a larger
force is applied,
the jug begins to
move as soon as
the applied force
exceeds the
opposing static
friction force
Friction
 Friction opposes the applied force
 Fapplied increases, static friction also increases
 Fs = -Fapplied
 Fsmax is when the force of static friction reaches its
max value – the applied force is as great as it can be
without causing the jug to move
 Kinetic friction is less than static friction (Fsmax)
 The net external force (F) is equal to the
difference between Fapp and Fk (F-Fk)
Friction
 The force of friction is
proportional to the normal
force (due to mass of the
object)
 Friction depends on the
surfaces in contact
 Objects that appear smooth
really aren’t at the microscopic
level
The Coefficient of friction
 The force of friction also depends on the
composition and qualities of the surfaces
in contact.
 The coefficient of friction (µ) is a ratio of
forces acting between two objects
 Coefficient of kinetic friction is the ratio of
kinetic friction to normal force µk=Fk/Fn
 Coefficient of static friction max is the ration
of static friction to the normal force
µs=Fsmax/Fn
Sample 4C
 A 24 kg crate initially at rest on a horizontal
floor requires a 75N horizontal force to set it
in motion. Find the coefficient of static
friction between the crate and the floor.
 Do Practice 4C