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Angular Momentum and its
The total angular momentum of a
rotating body remains constant if the
net torque acting on it is zero.
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?
1. Why is it possible for a high diver to execute more front
somersaults in the tuck position that in the layout
2. Why do figure skaters spin faster when they pull in their
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-inchdiameter wheels but was otherwise the same size as a
normal bicycle. How would the small wheels affect the
bicycle’s stability?
Classical Relativity
A Reference System
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.
Motion Viewed in Different
Reference System
Galileo asked if observers could decide
whose description was “correct”. He
concluded that they couldn’t. In fact,
each observer’s description was correct!
We define an inertial reference system
as one in which first Newton’s law is
Galilean Principle of Relativity
The laws of physics are the same for all
inertial reference systems.
Comparing Velocities
Comparing Energies
Accelerating Reference System
Forces that arise in accelerating
reference systems are called inertial
Another common label for these forces
is fictitious forces.
Realistic Inertial Forces
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 Alice’s reference system and from the reference
system of someone sitting on the shelf.
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?
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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