Lab 8: Rotational Motion II: The Rotating Bar

University of MichiganPhysics Department

Mechanics and Sound Intro Lab

Rotating Bar Experiment

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.

See you in the lab!

You should now be ready to use the concepts that you have learned thus far to study the fascinating motion exhibited by gyroscopes.