Physics 218: Mechanics

Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova

Lectures 38-39

0

2

4

6

8

10

12

14

16

18

20

1 2 3 4 5 6 7

Series1

9030 60 807050

Average 63High 100Low 1

Nu

mb

er o

f st

ud

ents

Score

Test 3

Physics 218 Challenge Exam will take place on December 1. Please come and participate!

DATE:  Monday, December 1, 2008TIME:  6:00 p.m.LOCATION:  202T ENPH

To All Physics 218 Students in All Sections:We are pleased to announce the fall 2008 competition for the Award for Exceptional

Performance in Physics 218. As many of you know, every semester there is a challenge exam given at the end of the semester to determine these awards. This exam is open to all students in all sections. It will cover the material from the first three exams in the regular Physics 218 sections, inclusively, and will in no way count towards your final class grade. It is, in some sense, simply for fun and a way to match your physics skills

against the very best here at A&M.This semester, the exam will be held at 6:00 p.m. on Monday, December 1, in classroom 202T of the Engineering Physics Building (ENPH). If you are interested in competing for this award (which includes a certificate, as well as a book and a monetary award), please contact Sandi Smith in ENPH Room 421, 845-7738, or [email protected], at your earliest convenience. Please include your name, instructor’s name, major, UIN, and an e-mail address where we can reach you. We would appreciate your response no later than

Friday, November 28, 2008.As an added incentive, all students who perform well on the challenge exam will be named Addison Wesley / Benjamin Cummings Mechanics Scholars and be invited to a catered lunch in room 205 of the MSC December 4 (reading day), from 12:00 to 1:30 p.m. in

their honor. In addition to lunch, we will have presentations by faculty members about potential careers for people with high levels of physics problem solving skills.

We hope to hear from you soon as only a limited number of seats for the exam exist.

Rotational Kinetic Energy

2

2

1 IKE

22

2

1

2

1 cmcm ImvKE

A primitive yo-yo is made by wrapping a string several times around a solid cylinder with mass M and radius R. You hold the end of the string stationary while releasing the cylinder with no initial motion. The string unwinds but does not slip or stretch as the cylinder drops and rotates. Use energy considerations to find the speed vcm of the center of mass of the solid cylinder after it has dropped a distance h.

Challenge Exam• Announcement of this semester’s 218 Challenge

Exam: Monday December 1st at 6:00PM– Not required (just for fun)– Does not (will not!!!) count as part of your final

grade

• Test your skills against the best Aggies from all the Physics 218 sections (not just this lecture) on Physics 218 material

• Students who perform well will be recognized as Mechanics Scholars, and honored at a banquet in their honor. Other prizes, including cash.

Ch. 17 Statics

00 extextF

There is a ladder against a wall. There is no friction at the wall and the coefficient of friction at the floor is . At what angle can the ladder be placed so that it would slip if the angle were decreased infinitesimally?

The advantage of a lever

GIVE ME A PLACE TO STAND AND I WILL MOVE THE EARTH                                                  

The advantage of a pulley

What minimum force F is needed to lift the piano of mass M?

2

MgF

Harmonic MotionSimple harmonic oscillator

Examples

Lots of stuff Vibrates or Oscillates:–Radio Waves–Guitar Strings–Atoms–Clocks, etc…

Why do we care?

Lots of engineering problems are oscillation problems– Buildings vibrating in the wind

– Motors vibrating when running

– Solids vibrating when struck

– Earthquakes

The forcesNo force

Force in –xdirection

Force in +xdirection

More Detail

Time

Some Terms

Amplitude: Max distance

Period: Time it takes to get back to here

A block with mass m is attached to the end of a spring, with spring constant k. The spring is stretched a distance L and let go at t=0

• Find the position of the mass at all times

• Where does the maximum speed occur?

• What is the maximum speed?

Resonance