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Angular Momentum and its Conservation

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... of a rotating body remains constant if the net torque acting on it is zero. ... The other end of the string passes through a hole in the table. ... – PowerPoint PPT presentation

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Title: Angular Momentum and its Conservation


1
Angular Momentum and its Conservation
2
The total angular momentum of a rotating body
remains constant if the net torque acting on it
is zero.
3
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4
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5
Object rotating on a String of Changing Length
A mass m attached to the end of a string revolves
in a circle on a frictionless tabletop. The other
end of the string passes through a hole in the
table. Initially, the mass revolves with a speed
v1 2.4 m/s in a circle of radius r1 0.80 m.
What is the speed, v2, of the mass when the
radius is 0.40 m?
6
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7
  1. Why is it possible for a high diver to execute
    more front somersaults in the tuck position that
    in the layout position?
  2. Why do figure skaters spin faster when they pull
    in their arms?

8
  1. If you look down the inside of barrel of a rifle,
    you see long spiral grooves. When the bullet
    travels down the barrel, these grooves cause the
    bullet to spin. Why would we want the bullet to
    spin?
  2. Bicycle wheels are normally about 2 feet in
    diameter. Imagine that a bicycle was constructed
    with 4-inch-diameter wheels but was otherwise the
    same size as a normal bicycle. How would the
    small wheels affect the bicycles stability?

9
Classical Relativity
10
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11
A Reference System
12
Reference system the set of objects that are
not moving relative to each other and that can be
therefore used as the basis for detecting and
describing motion.
13
Motion Viewed in Different Reference System
14
Galileo asked if observers could decide whose
description was correct. He concluded that they
couldnt. In fact, each observers description
was correct!
15
We define an inertial reference system as one in
which first Newtons law is valid.
16
Galilean Principle of Relativity
The laws of physics are the same for all inertial
reference systems.
17
Comparing Velocities
18
Comparing Energies
19
Accelerating Reference System
20
Forces that arise in accelerating reference
systems are called inertial forces. Another
common label for these forces is fictitious
forces.
21
Realistic Inertial Forces
22
Questions
1. Alice in Wonderland begins with Alice falling
down a deep, deep rabbit hole. As she falls, she
notices that the hole is lined with shelves and
grabs a jar of orange marmalade. She is afraid to
drop the jar for it might hit somebody on the
head. What would really happen to the jar if
Alice had dropped it? Describe its motion from
Alices reference system and from the reference
system of someone sitting on the shelf.
23
2. Imagine riding in a class-walled elevator that
goes up the outside of a tall building at a
constant speed of 20 m/s. As you pass a window
washer, he throws a ball upward at a speed of 20
m/s. Assume that you drop a ball out of window at
the same instant. Describe the motion of each
ball from the point of view of window washer and
your point of view. 3. You wake up in a
windowless room on a train, which rides along
particularly smooth tracks. Imagine that you have
a collection of objects and measuring devices in
your room. What experiment could you do to
determine whether the train is stopped at the
train station or moving horizontally at a
constant velocity?
24
Assume that you are riding in a windowless train
on perfectly smooth tracks. Imagine that you have
a collection of objects and measuring devices in
the train. What experiment could you do to
determine whether the train is moving
horizontally at a constant velocity or is
speeding up?
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