Physics 116
General Physics:
ELECTROMAGNETISM AND
OSCILLATORY MOTION
Lecture 1
Course Introduction
Periodic motion and oscillations
Sept 29, 2011
R. J. Wilkes
Email: ph116@u.washington.edu
First: I’m not Wilkes!
Your lecturer today: Victor Polinger
Substituting for R. J. Wilkes
(He will return on Monday)
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Announcements
(we’ll have a slide like this one most days)
- PHYS 116 Course home page:
Visit frequently for updated course info!
http://faculty.washington.edu/wilkes/116/
- Physics Dept’s general-information page for
100 courses:
http://www.phys.washington.edu/1xx/
- Webassign page (for homework and grades):
http://webassign.net/washington/login.html
WebAssign: Where Do I Log In ?
„
Go to the Login page at
http://webassign.net/washington/login.html
„
Click the LOG IN button and log in with your
UW Net ID.
All HW Assignments and Grades on WebAssign
Homework solutions are submitted online.
No work is accepted past due date.
Click on assignment name to begin working.
Grades for homework are stored in lecture section.
NOTE: Lab grades stored separately, under their own sections.
Example: First page of HW1
5 problems assigned,
total 20 pts
Other Class Resources
• Your Fellow Students
– You’re encouraged to study and discuss class together!
• Physics Study Center (A-wing Mezzanine, just under
our lecture room)
– Meeting place / work space for students
– Get guidance and help from TAs outside 116 office hours
– Look over other textbooks on the same subjects
• Physics Library (6th floor of C-wing)
– Best view from a comfy chair on campus!
• Class discussion board
–
https://catalyst.uw.edu/gopost/board/wilkes/23253/
• Your TA
– Kyle Armour (take a bow, Kyle)
– He will post office hours next week
– Email questions to him via class email, ph116@uw.edu
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“Clickers” are required
iCue / H-ITT Clickers (TX-3100)
• Required to enter answers in quizzes
– Be sure to get radio (RF), not infrared (IR)
• We will not use them this week
• We’ll practice using them next week, and begin using
them for pop quizzes thereafter
– Bring your clicker to class every day from Oct 3 onward
• Why must we torture you with pop quizzes?
– Motivation to attend class (physically and mentally)!
– Quizzes are designed to be easy IF you are paying attention
• Questions will be about something we just discussed!
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Course grade will be based on:
• Exams: 60%
– 3 midterm exams, each on a specified range of text sections
• Oct 17, Nov 7, Nov 29
– In class, formula sheet provided, closed book/notes
– Only your best exam scores (2 out of 3) will be used, so no
makeup exams
– Final exam Monday Dec 11, 2:30-4:20 pm, this room
– ALL exams will be multiple choice, you MUST bring your own
mark-sense sheet (available at physics café) and pencil
• Homework assignments (online): 25%
– Best 6 out of 8 scores for full set (NOT best 60 out of 80
individual problem scores)
• Pop quiz scores: 15%
– Only your best 7 (of 10 or more) grades will be used, so no
makeup quizzes
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Lecture Schedule
(up to exam 1)
Today
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Periodic motion and oscillations
Terms you have heard before:
• Stable equilibrium: restoring force acts to oppose displacement
– Unstable equilibrium: net force acts to increase displacement
•
Restoring force: F = - k x
– Minus sign means: force points opposite direction of displacement
• What if it were + ?
•
Result: oscillations around the equilibrium point (where F=0)
– Periodic motion = oscillation repeated with some regular cycle time
– For Simple Harmonic Motion (SHM)
• k = a constant: force is proportional to displacement
• So k = F / x - units for k are force per unit distance (N per m)
•
Examples everywhere in the physical world around us
–
–
–
–
Orbits of planets around Sun (or other stars)
Guitar string’s motion
Pendulum, mass on a spring, twisting of a tire hung by a rope…
Water molecules’ vibrations
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Oscillation terminology
•
•
•
Period T = length of one full cycle (time between peaks)
Frequency f = number of cycles per unit time (units = 1 / time)
Amplitude A = maximum displacement (length)
•
Graph of oscillating object’s position vs time for T= 1 sec, f = 1 Hz, A = 2 m
– We could also say A is "4 meters peak-to-peak "
T = 1 sec
T and f are
alternate ways to
describe the
same thing - so
2
1
f=1/T
Unit of
frequency =
1 cycle/sec
= 1 hertz (Hz)
(Heinrich Hertz,
discovered radio
waves c. 1880)
0
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
A=2m
-1
-2
Distance,
meters
time, seconds
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Mathematical review
Recall high school math: right triangles
• Pythagoras says c2 = a2 + b2
a
φ
• Sine(θ)= a / c
• Cosine(θ)= b / c
• Tangent(θ) = a / b = sinθ / cosθ
• φ = 90ο − θ because sum of all angles = 18
Circles are related to right triangles:
if c is radius of circle, then a and b
are coordinates of a point on circle
at angle from the x axis, so
x=Rcos θ y=Rsinθ
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c
θ
b
R
θ
y
x
13
Sines and cosines
•
Physical oscillations can be modeled with trigonometry functions:
– Here is a graph of the sine and cosine functions, from x=0 to x=2π:
1
0.5
0
0
1.57
3.14
4.71
6.28
sin(x)
cos(x)
-0.5
-1
x
(or t)
– They are “waves” with A=1 and T=2π (x could be radians, sec, m…)
– We can describe all waves in terms of these models
• Deep thought: we can reproduce any oscillation by adding up sine and
cosine waves of different frequencies (Fourier Analysis)
– Jargon: We refer to position along the wave as the phase of the wave:
( t / T ) = (φ / 2π) so φ = 2π ( t / T ) , or φ = 360° ( t / T )
• We say sine and cosine are “identical, but 90° out of phase”
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