Centrifugal Hoop

Introduction

In this demonstration, a centrifuge device will be used to demonstrate the flattening effect of a spinning object.

Concepts

• Centripetal force
• Centrifugal force

Materials

Safety Precautions

This demonstration is considered nonhazardous. Follow all normal classroom safety guidelines. Do not touch the motor axle while rotor is spinning. Remove battery from Bracken’s Demonstration Spinner when not in use and during storage.

Disposal

All materials may be saved for future use.

Procedure

1. Obtain the black plastic hoop assembly and Bracken’s Demonstration Spinner.
2. Use a candle or burner to heat a straightened paper clip or dissection needle. Use the hot metal paper clip to melt a small hole through the center of the bottom cap of the hoop assembly as shown in Figure 1. Be sure that the hole is directly in the middle of the cap. The hole should be just large enough to fit snugly over the axle of Bracken’s Demonstration Spinner.
   {11984_Procedure_Figure_1_Stationary position}
3. Place the hoop onto the axle of Bracken’s Demonstration Spinner.
4. Turn on the motor of Bracken’s Demonstration Spinner and observe the dramatic flattening effect (see Figure 2).
   {11984_Procedure_Figure_2_Flattening effect while spinning}
5. Slow the rate of rotation by disconnecting the clips of Bracken’s Demonstration Spinner to allow the spinning to slow gradually.
6. If desired, reattach the clips of Bracken’s Demonstration Spinner once again and observe the hoop as the rotational speed increases.

Teacher Tips

• Bracken’s Demonstration Spinner (Flinn Catalog No. AP6202) is required and sold separately.
• Make sure the plastic hoop is balanced and that the bands of the hoop aligned with one another perfectly before starting the motor.
• Ask students how the rate of rotation affects the flattening effect.

Discussion

The centrifugal hoop device illustrates centripetal force and inertia. All massive objects have inertia, which means that an object in motion will remain in motion in a straight line and an object at rest will remain at rest unless it is acted on by an outside force. Therefore, in order to make an object spin, there must be an outside force acting on it (because it is constantly changing direction). The force that holds an object in a circular path is known as a centripetal force. The inertia of the object wants to continue in a straight line so it appears that there is a force “throwing” the object out of the circular path. However, this is not a true force. In reality, it is only a property of inertia. This psuedo-force that arises due to inertia is referred to as the centrifugal force of the spinning object.

An example of these two “forces” can be felt when traveling around a sharp turn in a car. The car is able to make the turn because the friction between the tires and the road creates enough force to change the direction of the car. Thus, friction provides the centripetal force. However, as the car makes the turn, your body feels like it is being thrown out against the turn by an outside force. There is no real force on your body acting against the turn of the car. It is only the result of the property of inertia that wants to keep your body moving in a straight line.

The mass in a spinning planet “feels” the same pseudo-force—the force that makes a spinning planet bulge at its equator and flatten at the poles. When a planet rotates, the mass of the planet wants to move outward in a straight line, against the rotation, because of its inertia. Therefore, the mass tends to move away from the axis of rotation. The mass along the equator bulges the most because it is further from the axis of rotation and therefore will be spinning at the fastest rate. The expansion of the mass away from the axis causes the poles to squeeze closer to the center of the planet and they flatten slightly.

References

Flinn Scientific would like to thank Jeff Bracken, chemistry teacher at Westerville North High School in Westerville, Ohio, for sharing this original idea. Jeff would like to thank Matt Cocuzzi, his student laboratory assistant, for his help with the development of this classroom activity.

Heiden, D. The Physics Teacher. 2000, 38, 378.