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Physics 211: Lecture 4, Pg 1
Physics 211: week 3
Physics 211: week 3
Today’s Agenda
Today’s Agenda
●
Recap of centripetal acceleration
●
Newton’s 3 laws (these describe all of classical mechanics!)
How and why do objects move?
Dynamics
Dynamics
Original: Mats A. Selen
Modified: W.Geist
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Physics 211: Lecture 4, Pg 3
Forces?
Forces?
We will consider 2 kinds of forces:
1.Contact forces
(normal force, force of string on ball F
B,S
etc.
These forces are identified by the contact between the
object under consideration (the ball) and the objects that are in
contact with this object (the string).
2.
Long range forces: The only long range force in this class is
the force of gravity. Gravitational force of the earth on the
student.
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View Full Document Physics 211: Lecture 4, Pg 4
Dynamics
Dynamics
●
Isaac Newton (1643  1727)
published
Principia Mathematica
in 1687.
In this work, he proposed three “laws” of motion:
●
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
(here we compare
(here we compare
two different concepts and combine them with an equation)
two different concepts and combine them with an equation)
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”.
Physics 211: Lecture 4, Pg 5
Newton’s First Law
Newton’s First Law
●
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
.
If no forces act, there is no acceleration.
●
The following statements can be thought of as the
definition of
inertial reference frames.
An
IRF
is a reference frame that is not accelerating (or
rotating) with respect to the “fixed stars”.
If one
IRF
exists, infinitely many exist since they are
related by any arbitrary constant velocity vector!
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
)
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Is Atlanta a good IRF?
Is Atlanta a good IRF?
●
Is Atlanta accelerating?
●
YES!
Atlanta is on the Earth.
The Earth is rotating.
●
What is the centripetal acceleration of Atlanta?
T = 1 day = 8.64 x 10
4
sec,
R ~ R
E
= 6.4 x 10
6
meters .
●
Plug this in:
a
U
= .034 m/s
2
( ~ 1/300 g)
●
Close enough to
0
that we will ignore it.
●
Atlanta is a pretty good IRF.
a
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This note was uploaded on 12/05/2009 for the course PHYSICS PHYS taught by Professor Phys during the Spring '09 term at Abu Dhabi University.
 Spring '09
 PHYS
 Acceleration

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