Physics 121C Mechanics Review 3 Chapters 1113 December 1, 2004

1
Physics 121C - MechanicsReview 3Chapters
11-13December 1, 2004

• John G. Cramer
• Professor of Physics
• B451 PAB
• cramer_at_phys.washington.edu

2
Announcements

• Homework Assignment 7 has been posted on Tycho
  and is due today, Wednesday, December 1, at 9 PM
• Regrades from Exam 1 have been completed. The
  Exam 1 grades on Tycho have been updated, and
  the regraded papers are available from Laura
  Clement in room C136 PAB.
• Exam 3 will be held in this room on Friday,
  December 3. It will be administered by Prof.
  Gundlach. Check Tycho for your seat assignment
  before coming to the exam.
• There will be a lab question on Exam 3, and it
  will cover the last three labs.

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Lecture Schedule (Part 3)
You are here!
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Vector Cross Product
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Example Cross Product with Cartesian Unit Vectors
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Vector Torque andAngular Momentum
We can also define the torque t and the angular
momentum L as vectors
This is the rotational equivalent of
Newtons 2nd Law Fnet dp/dt ( ma)
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The Angular Momentumof a Rigid Body
The angular momentum L of an isolated system
(tnet0) is conserved. The final angular
momentum Lf is equal to the initial angular
momentum Li.
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Angular Momentumand Angular Velocity
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Example The Angular Momentum of the Earth
What is the angular momentum of the Earth as
it spins on its axis?
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ExampleTwo Interacting Disks
A 20 cm diameter 2.0 kg solid disk is
rotation at 200 rpm. A stationary 1.0 kg 20 cm
diameter circular loop is dropped straight down
onto the rotating disk. Friction causes the loop
to accelerate until it is riding on the disk.
What is the angular velocity of the combined
system?
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Chapter Summaries

• Chapters 11 - 13

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Chapter 11 Summary (1)
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Chapter 11 Summary (2)
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Chapter 11 Summary (3)
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ExampleUsing Work to Find Car Speed
A 1500 kg car accelerates from rest with a
net force that decreases linearly with position
from 10,000 N to zero, as shown. What is
the cars speed after traveling 200 m?
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ExampleThe Power Output of a Motor
A factory uses a motor and cable to drag a
300 kg machine into the proper place on the
factory floor. (mk0.60.) What power must
be supplied to drag the machine across the floor
at a speed of 0.50 m/s?
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Chapter 12 Summary (1)
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Chapter 12 Summary (2)
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Chapter 12 Summary (3)
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Example Escape Speed
A 1000 kg rocket is fired straight away from
the surface of the Earth. What speed does it
need to escape from the gravitational pull of
the Earth and never return? (Assume a
non-rotating Earth.)
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ExampleRaising a Satellite - LEO to Geo
How much work must be done in boosting a
1000 kg communication satellite from low Earth
orbit (h300 km) to geosynchronous orbit?
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Chapter 13 Summary (1)
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Chapter 13 Summary (2)
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Chapter 13 Summary (3)
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Example Rotating Rockets
Far out in space, a 100,000 kg rocket and a
200,000 kg rocket are docked at opposite ends of
a motionless 90 m long connecting tunnel. The
tunnel is rigid and its mass is much less than
that of either rocket. The rockets start their
engines simultaneously and fire in opposite
directions, each generating 50,000 N of thrust.
What is the angular velocity w of the structure
after 30 s?
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End of Review 3

• Homework Assignment 7 has been posted on Tycho
  and is due today, Wednesday, December 1, at 9 PM
• Regrades from Exam 1 have been completed. The
  Exam 1 grades on Tycho have been updated, and
  the regraded papers are available from Laura
  Clement in room C136 PAB.
• Exam 3 will be held in this room on Friday,
  December 3. It will be administered by Prof.
  Gundlach. Check Tycho for your seat assignment
  before coming to the exam.
• There will be a lab question on Exam 3, and it
  will cover the last three labs.