Chapter 4: Forces and Laws
of Motion
Physics 1-2
Richwoods High School
Mr. Chumbley
Section 1: Changes in Motion
Force
 Forces describe the interactions between an object and its
environment
 A force is an action exerted on an object that may change the
object’s state of rest or motion
 Forces cause a change in motion in three ways:
 Start moving
 Stop moving
 Change direction
Measuring Force
 Forces are measured in different ways depending on the
system of measure
 The SI unit of measure is the newton (N)
System
SI
Mass
kg
Acceleration
m/s2
cgs
g
cm/s2
Avoirdupois
slug
ft/s2
Force
kg·m/s2= N
g·m/s2= dyne
lb =slug·ft/s2
How Forces Act
 Contact Forces
 Field Forces
 Pushes or pulls that result
 Pushes or pulls that do not
from direct physical
contact between objects
 Typically called mechanical
forces
require direct physical
contact between objects
 Examples:
 Electric Force
 Magnetic Force
 Gravitation Force
Force Diagrams
 Force is a vector quantity, so it can be depicted graphically by
drawing vectors
 To help identify the comparative effect of forces acting on an
object, drawing a diagram is beneficial
 A free-body diagram is a graphical representation of a body
depicting all forces being exerted on that body
Drawing Free-Body Diagrams
 Only the forces acting on the object are included
 Any forces exerted by the object are excluded
 When drawing a force vector in a free-body diagram, label it
clearly with either a symbol or the magnitude of the force
 Free-body diagrams are often very simplified pictures
showing
Sample Problem A
The picture below shows a little girl pulling a dog in a sled. Draw a
free-body diagram for this sled. Assume the magnitudes of the forces
acting on the sled are 60N by the string, 130 N by the earth, and 90 N
upward by the ground.
 The string exerts 60N in the direction the
string pulls

Earth exerts a downward force of 130 N on
the sled

The ground exerts an upward force of 90 N
on the sled
Fground
Fstring
Fgravity
Homework
 P. 122
 Practice A
 #1-2
 Section 1 Formative Assessment #5
Section 2: Newton’s First Law
Newton’s Laws of Motion
 In 1687, Isaac Newton
published a work entitled
Philosophiæ Naturalis
Principia Mathematica
 In Principia, Newton
outlined three laws of
motion that became the
foundation for describing
motion
Newton’s First Law
 Lex I: Corpus omne perseverare in statu suo quiescendi
vel movendi uniformiter in directum, nisi quatenus a
viribus impressis cogitur statum illum mutare.
 An object at rest will remain at rest unless acted upon by an
external and unbalanced force . An object in motion will
remain in motion unless acted upon by an external and
unbalanced force.
Inertia
 This law is often times referred to as the Law of Inertia
 Inertia is the tendency of an object to resist being moved or,
if the object is moving, to resist a change in speed or
direction
 Mass is a measure of inertia
 The more massive an object, the more inertia it posesses
Net Force
 When looking at all of the forces acting on objects, it can be
difficult to determine which one is affecting the motion
 The reality is that all of the forces acting on an object affect its
motion, but we can typically simplify all of the forces into a single
summative force
 Net force is a single force whose external effects on a body are the
same as the effects of several actual forces acting on the body
 Net force is the vector sum of all forces acting on an object
Sample Problem
 Four forces act on a hot air balloon, as shown below. Find
the magnitude and direction of the net force.
5120 N
1520 N
950 N
4050 N
Homework
 Practice B (p. 126)
 #1-3
Equilibrium
 Equilibrium is the state in which the net force on an object
is zero
Section 3: Newton’s Second and
Third Laws
Newton’s Second Law
 Lex II: Mutationem motus proportionalem esse vi motrici
impressae, et fieri secundum lineam rectam qua vis illa
imprimitur.
 The acceleration of an object is directly proportional to the
net force acting on an object and inversely proportional to
the object’s mass.
𝐧𝐞𝐭 𝐟𝐨𝐫𝐜𝐞 = 𝐦𝐚𝐬𝐬 × 𝐚𝐜𝐜𝐞𝐥𝐞𝐫𝐚𝐭𝐢𝐨𝐧
Σ𝐅 = 𝑚𝐚
Sample Problem C (p.129)
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 force acting on the book is 1.6 N
to the right, what is the book’s acceleration?
Σ𝐅 = 1.6 N
𝑚 = 2.2 kg
Σ𝐅 = 𝑚𝐚
ΣF
1.6 N
Σ𝐅
𝐚=
𝑚
1.6 N
𝐚=
2.2 kg
𝐚 = 0.73 m s2 to the right
Newton’s Third Law
 Lex III: Actioni contrariam semper et æqualem esse
reactionem: sive corporum duorum actiones in se mutuo
semper esse æquales et in partes contrarias dirigi.
 If two objects interact, the magnitude of the force exerted on
the first object by the second is equal in magnitude and
opposite in direction of the force simultaneously exerted on
the second object by the first
Action-Reaction Pairs
 When examining action-reaction pairs, there are a couple
key points to understand
 Action-reaction forces occur simultaneously, not sequentially
 Action-reaction forces act on different objects
 All forces exist in pairs
Homework:
 Practice C (p. 130)
 #1-5
Sample Problem
 Four forces act on a hot air balloon, as shown below. Find
the acceleration of the hot air balloon.
5120 N
1520 N
950 N
4050 N
Section 4: Everyday Forces
Weight and Normal Force
 Weight (mg) is a measure of the gravitational force exerted
on an object by another much larger object
 Mass is an inherent property of matter, weight is not
 Normal force (FN) is a force that acts on a surface in a
direction perpendicular to the surface
 Normal force is often times the reactive force to weight, but
it is not
Friction
 In general, any force that opposes motion due to interaction
with the environment can be considered friction
 Static friction is the force that resists the initiation of sliding
motion between two objects that are in contact and at rest
 Kinetic friction is the force that opposes the movement of
two surfaces that are in contact and sliding over each other
Friction
 A coefficient of friction is a 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
𝐹𝑓 = 𝜇𝐹𝑁
 For static friction, μs is used
𝐹𝑓 ≥ 𝜇𝑠𝐹𝑁
 For kinetic friction, μs is used
𝐹𝑓 = 𝜇𝑘𝐹𝑁
Sample Problem D (p.137)
A 24 kg crate initially at rest on a horizontal floor requires 75 N of
horizontal force to set it in motion. Find the coefficient of static friction
between the crate and the floor.
FN = mg
𝐹𝑓 = 𝜇𝑠𝐹𝑁
𝐹𝑓
𝜇𝑠 =
𝐹𝑁
𝜇𝑠 =
75 N
= 0.32
2
(24 kg)(9.8 m s )
Ff = 75 N
Fa = 75 N
mg
Sample Problem E (p.138)
A student attaches a rope to a 20.0 kg box of books. He pulls with a
force of 90.0 N at an angle of 30.0˚ above the horizontal. The
coefficient of kinetic friction between the box and the sidewalk is
0.500. Find the acceleration of the box.
FN
Fa = 90 N
30.0˚
Ff
mg
Homework:
 Practice D (p. 137)
 #1-2
 Practice E (p. 139)
 #1, 4
Sample Problem B (p.125)
Derek leaves his physics book on top of a drafting table that is inclined
at a 35˚ angle. If the book has a mass of 2.25 kg, what is the
coefficient of friction if it remains at rest?
FN
y
Ff
x
Fg