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1
Fall 2007
PHYS 172: Modern Mechanics
Lecture 5 –
Fundamental Forces
Read 3.1 – 3.8
Announcements:
• All iClickers should be registered by now. Anyone
using an unregistered iClicker will not receive points for
any quizzes done before registration is completed.
• Exam 1 is next week (7:00 pm Thurs Sep 13 in Elliott).
Students needing special accommodations must notify
their instructor in advance.
• Exam from last semester is posted on home page,
under “Exams”.
• All HW assignments are due on either Tues or Thurs at
1 minute before midnight, local time. (WebAssign
recently moved us from Central to Eastern time zone,
causing some confusion.)
CLICKER QUESTION #1
Reading Question (Sections 3.1 – 3.8)
(This is a closedbook quiz, no consulting with neighbors, etc.)
Which of the following is true?
A. gravitational force between two objects is proportional to
the sum of their masses
B. electric forces are always attractive
C. the spring force is in general a constant force
D. iteratively predicting motion is pretty accurate for any time
step
E. gravitational force between two objects is proportional to
product of the masses.
Today’s Lecture
• Recap a few items…
• Fundamental forces (
Gravity specifically)
• “Composite” Forces (
Spring force)
• Introduce Iterative Prediction of Motion
Last week
’
s Recitation
:
From Newton’s first law, we conclude that
any object in circular motion must be
experiencing a net force. By using the Moon
as an example, you showed that for uniform
circular motion this force has the form:
!
F
=
F
=
!
!
p
!
t
=
mv
2
R
You got this by applying the momentum principle:
!
!
p
=
!
F
net
!
t
For the Moon, the only force that matters is gravity. However, we also
considered your own daily orbital motion, which is also uniform circular
motion. Here, the net force is not just gravity. It is vector sum of gravity,
normal, and frictional forces that constrains you to your daily orbit of the
Earth.
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This note was uploaded on 04/21/2008 for the course PHYS 172 taught by Professor ? during the Fall '08 term at Purdue UniversityWest Lafayette.
 Fall '08
 ?
 mechanics

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