Ch. 5: Newton’s Laws of Motion
DYNAMICS
Force
Force: “A push or a pull”. F is a VECTOR!
Vector Addition is needed vector to add Forces!
Examples of Forces
“Contact” Forces
“Field” Forces
“Pulling”
Forces
(Physics II):
“Pushing”
Force
Classes of Forces
• Contact forces involve physical
contact between two objects
– Examples (in pictures):
spring force, pulling force, pushing force
• Field forces act through empty space.
– No physical contact is required.
– Examples (in pictures):
gravitation, electrostatic, magnetic
Measurement of Forces:
Spring Scale
Vector
addition
to add
Forces!

Fundamental Forces of Nature
• Gravitational Forces
– Between objects
• Electromagnetic Forces
– Between electric charges
• Nuclear Weak Forces
– Arise in certain radioactive decay
processes
• Nuclear Strong Forces
– Between subatomic particles
Note: These are all field forces!
The 4 Fundamental Forces of Nature
The Sources of these forces: In order of decreasing strength
• Strong Nuclear Force:
– Binds nuclei together. Still being researched.
• Electromagnetic Force:
– E&M phenomena. Chemical forces. Most everyday
forces. Maxwell, Coulomb, Ampere, Faraday, ...
• Weak Nuclear Force:
– Nuclear decay. Fermi, Bethe & others. Still being
researched.
• Gravitational Force:
– Newton (“classical” mechanics)
– Einstein (general relativity)
The 4 Fundamental Forces of Nature
The Sources of these forces: In order of decreasing strength
The 4 Fundamental Forces of Nature
• “Electro-Weak” Force:
– Since ~ the late 1960’s, in some sense have
reduced the 4 fundamental forces to 3!
– The Electromagnetic Force & the Weak
Nuclear Force were combined into one
theory.
– S. Weinberg & A. Salaam:
The 1972 Nobel Prize in Physics!
Sir Isaac Newton
• 1642 – 1727
• Formulated the
Basic Laws of Mechanics
• Discovered the
Law of Universal
Gravitation
• Invented a form of
Calculus
• Made many observations dealing
with Light
and Optics
Newton’s Laws of Motion
• The ancient (& wrong!) view (of Artistotle):
A common
– Need a force to keep an object in motion.
MISCONCEPTION
– The “natural” state of an object is at rest. in the 21st Century!
• The CORRECT VIEW (of Galileo & Newton):
– It’s just as natural for an object to be in motion at constant
speed in a straight line as to be at rest.
Proven by
Galileo
– At first, imagine the case of NO FRICTION
in the 1620’s!

– Experiment: If NO FORCE is applied to an object moving at a
constant speed in straight line, it will continue moving at the same
speed in a straight line! If I succeed in having you overcome the
wrong ancient misconception & understand the correct view of this,
A MAJOR GOAL of the COURSE WILL
HAVE BEEN ACHIEVED!
Reference Frames
Inertial Reference Frame
(As defined by Newton)
≡ A reference frame (coordinate system)
which is moving with constant velocity
(no acceleration!) with respect to the
“fixed stars”.
– Clearly, an idealization!
Rigorously, Newton’s Laws are
ONLY valid in an Inertial
Reference Frame
Newton’s Laws
• Galileo laid the ground work for Newton’s Laws.
Galileo
Galilei 
in middle
age
• Newton: Built on Galileo’s work
• Newton’s 3 Laws: One at a time
 Sir Isaac
Newton
as a young
man
Newton’s First Law
• 1st Law: (“Law of Inertia”): “In the absence of
external forces and when viewed from an inertial
reference frame, an object at rest remains at rest
and an object in motion remains in motion with a
constant velocity (constant speed in a straight line).”
Sir Isaac
Newton 
as an older man
Newton was
born the same
year Galileo
died!
• Newton’s 1st Law: (Alternate Language):
1. “When no net force (∑F = 0) acts on an object,
the acceleration of the object is zero.”
∑ = a math symbol meaning sum (capital sigma)
2. “If an object does not interact with other
objects, it is always possible to identify a
reference frame (an inertial frame) in which the
object has zero acceleration.”
• From the 1st Law: Can define a Force as
“An action which causes a change in the
motion of an object.”
• Newton’s 1st Law: First stated by Galileo!
Newton’s First Law
A Mathematical Statement of Newton’s 1st Law:
If v = constant, ∑F = 0 OR if v ≠ constant, ∑F ≠ 0
Conceptual Example
Newton’s First Law
A school bus comes to a sudden stop,
and all of the backpacks on the floor
start to slide forward. What force
causes them to do that?
Newton’s First Law
Alternative Statement
• In the absence of external forces, when
viewed from an inertial reference frame,
an object at rest remains at rest & an object
in motion continues in motion with a
constant velocity.
– Newton’s 1st Law describes what happens in
the absence of a net force.
– It also tells us that when no force acts on an
object, the acceleration of the object is zero.