Day 6 - Rotational Inertia: Tops

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Day 6: Rotational Inertia: Tops

NOTE: You may wish to prepare in advance for this activity. For the activity, each student will need a 3-inch diameter paperboard circle. Poke a small starter hole, approximately 1/16 inch in diameter, in the center of each circle, and make pencil marks every 60 degrees. The older students could do this in class with the protractor and compass. For younger students, either make these in advance or make a few patterns for the students to use to make their own circles. Cut dowel rods to 2 1/2-inch lengths. They cut easily with wire cutters. With a pencil sharpener, sharpen each dowel rod to nearly a point to reduce surface friction. The sharpening must be well centered on the dowel rod.

Teacher information:

Tops are fun to play with because they keep spinning for some time after being set in motion. In this activity, students observe that the duration of the spin is affected by the amount of mass near the edge of the circle. Why? Once in motion, an object (such as a top ) that rotates about an axis (in this case, the dowel rod) tends to keep rotating about its axis. The resistance of an object to change its rotational state of motion is called rotational inertia. When the top is spun it tends to keep spinning because of this inertia. When more mass is added near the edge of the circle the rotational inertia is increased. The increased rotational inertia makes it harder to start the top tuning, but once it has started turning it will continue to spin linger. Friction eventually causes the top to slow down.

The rotational inertia depends not only on the mass of the spinning top but also on the location of the mass. The farther the mass is located from the axis of rotation the greater the rotational inertia. A top with six washers glued near the perimeter will have more rotational inertia than a top with the same mass but with the washers stacked near the center. Many real machines have the mass located far from the axis of rotation in order to increase the rotational inertia and rotational kinetic energy. For example, at the Ford Museum in Dearborn, Michigan there is a large display of old steam engines. Each one of these engines has a flywheel constructed as a wheel with spokes and most of the mass located near the perimeter.

Objectives:

1. The students will understand the concept of rotational inertia.
2. Students will see how mass affects rotational inertia.
3. Students will learn about the concept of variables.
4. They will collect and record data concerning the factors that affect length of spin.

Vocabulary:

Rotational inertial: The resistance of an object to change its rotational state of motion.

Variable: a change made in an experiment

Materials needed:

• a variety of tops per student:
• 2 1/2 inch piece of 1/4-diameter dowel rod
• scissors
• 1 3-inch square of paperboard (can be cut from cereal boxes)
• 6 pennies
• 6 small paper clips
• "Spinning Tops" Observation Sheet

per class:

• hot glue gun and glue stick

for older students, per group of 4

• stopwatch
• compass
• protractor