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Physics 211: Lecture 4, Pg 1
Physics 211: Lecture 4
Physics 211: Lecture 4
Today
Today
’s Agenda
s Agenda
z
Recap of centripetal acceleration
z
Newton’s 3 laws (these describe all of classical mechanics!)
¾
How and why do objects move?
¾
Dynamics
Dynamics
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Physics 211: Lecture 4, Pg 3
Dynamics
Dynamics
z
Isaac Newton (1643  1727)
published
Principia Mathematica
in 1687.
In this work, he proposed three “laws” of motion:
z
principia
Law 1
:
An object subject to no external forces is at rest or moves
with a constant velocity if viewed from an inertial reference
frame.
Law 2
:
For any object,
F
NET
=
Σ
F
= m
a
a
Law 3
:
Forces occur in pairs:
F
A ,B
= 
F
B ,A
(For every action there is an equal and opposite reaction.)
These are the postulates of mechanics
They are experimentally, not mathematically, justified.
They work, and
DEFINE
what we mean by “forces”.
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View Full Document Physics 211: Lecture 4, Pg 4
Newton
Newton
’s First Law
s First Law
z
An object subject to no external forces is at rest or moves
with a constant velocity if viewed from an
inertial reference
inertial reference
frame
frame
.
¾
If no forces act, there is no acceleration.
z
The following statements can be thought of as the definition
of
inertial reference frames.
¾
An
IRF
is a reference frame that is not accelerating (or
rotating) with respect to the “fixed stars”.
¾
If one
IRF
exists, infinitely many exist since they are
related by any arbitrary constant velocity vector!
¾
If you can eliminate all forces, then an
IRF
is a
reference frame in which a mass moves with a constant
velocity.
(alternative definition of
IRF
)
1. Dishes
2. Monkey
Physics 211: Lecture 4, Pg 5
Is Urbana a good IRF?
Is Urbana a good IRF?
z
Is Urbana accelerating?
z
YES!
¾
Urbana is on the Earth.
¾
The Earth is rotating.
z
What is the centripetal acceleration of Urbana?
¾
T = 1 day = 8.64 x 10
4
sec,
¾
R ~ R
E
= 6.4 x 10
6
meters .
z
Plug this in:
a
U
= .034 m/s
2
( ~ 1/300 g)
z
Close enough to
0
that we will ignore it.
z
Urbana is a pretty good IRF.
a
v
R
R
T
R
U
==
=
⎛
⎝
⎜
⎞
⎠
⎟
2
2
2
ω
π
2
Ice
puck
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View Full Document Physics 211: Lecture 4, Pg 6
Newton
Newton
’
’
s Second Law
s Second Law
z
For any object,
F
NET
=
Σ
F
= m
a
.
¾
The acceleration
a
of an object is proportional to the
net force
F
NET
acting on it.
¾
The constant of proportionality is called
“mass”
, denoted
m
.
»
This is the definition of
mass
and
force
.
»
The
mass
of an object is a constant property of that
object, and is independent of external influences.
»
The force is the external influence
»
The acceleration is a combination of these two things
z
Force has units of [
M
]x[
L / T
2
] =
kg m/s
2
=
N
(Newton)
Physics 211: Lecture 4, Pg 7
Newton
Newton
’s Second Law.
..
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This note was uploaded on 02/10/2009 for the course PHYS 211 taught by Professor Tonyliss during the Spring '09 term at University of Illinois, Urbana Champaign.
 Spring '09
 TonyLiss
 Acceleration

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