Transcript Lab 8: Rotational Motion II: The Rotating Bar

Lab 8: Rotational Motion
II: The Rotating Bar
University of Michigan
Physics Department
Mechanics and Sound Intro Lab
Welcome back to the second part of your three-week examination of
rotational motion. In the inclined plane experiment, you learned the
subtle differences between mass and moment of inertia, and you also
became familiar with several of the rotational analogs to linear
quantities. In this lab, you will learn about and explore two more
important rotational quantities: torque and angular momentum. You
will also begin to explore how these quantities affect one another and
cause the rotational motion of an object to change. This background
will prove invaluable in your study of gyroscopes in next week’s lab.
 The rotating bar apparatus used
in today’s lab.
Torque is the rotational analog of force, and it can be a somewhat
tricky concept when you first approach it. However, just keeping a
few things in mind can make torques easy to work with. First, a
torque, like a force, is a vector. Like the moment of inertia, torque
also takes into account the distance from the center of rotation, and
the direction of the torque can be found using the right hand rule.
However, finding the direction of a torque will be more important next
week than it will be in today’s lab.
 For the best results, you
will need to level your
rotating bar before
releasing it. You should
use the levels provided for
this purpose.
The other important concept in this lab is angular momentum. You
will explore both of these concepts using the rotating bar. You will
notice that your bar has a photogate at the top of its stand. This
photogate records the angular velocity of the spinning bar. The bar
slows over time due to friction, so you will use LoggerPro®’s linear fit
tool to find the initial angular velocity of the bar. The first part of this
lab requires you to place balls into various pockets along the bar and
observe the effect of the torque due to the ball’s weight. In the ball
drop portion of the lab, it is important that you only use trials for
which you observe a clean bounce.
 A typical plot of angular
velocity vs. angular
displacement. You will use
the intercept of the graph for
the initial angular velocity.
The final portion of today’s lab involves observing what happens when
a person flips a spinning gyroscope while sitting on a spinning stool.
Whenever you spin up a gyroscope using the Dremel tool, be sure to
wear safety glasses. Also make sure to keep your fingers clear of the
spinning rotor.
 The gyroscope and swiveling
chair that you will use in the final
portion of today’s lab.
Safety glasses
This brief introduction to the use of gyroscopes will be very helpful for
your next lab. After today’s lab, you will have a better understanding
of torque and angular momentum, and next week you will see how all
of these concepts come together to cause precession.
 You should now be ready to use
the concepts that you have learned
thus far to study the fascinating
motion exhibited by gyroscopes.