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The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


HOPPER POPPER

Figure I. Figure II. Figure III.

Description

Equipment: Racquetball, cut in half and then shaved a little so that a little less than half of the racquetball remains. The total mass is approximately 20 grams.

Step 1: The audience is asked how high half a racquetball can bounce, and if it's possible for it to bounce up higher than it is dropped. The racquetball half is then dropped normally from about waist height and bounces up only a little bit, hardly bouncing at all.

Step 2: The racquetball half is dropped a second time, but this time it bounces up much higher, noticeably higher than its original height.

Description

Basic Ideas: Energy can be neither created nor destroyed, but it can be converted from one form to another and it can be transferred from one object to another.

During a collision, energy is usually transfered from one object to another.

Energy can be stored in a spring. The potential energy of a spring depends on what the spring is made of and how far back the spring is pulled. The stiffer the spring, the more energy can be stored, and the further it is pulled back, the more energy is stored.

Step 1: During this step, nothing exciting happens. The racquetball half is dropped from a certain height, and collides with the floor. During this collision, very little of the racquetball's energy is transferred back to kinetic energy. Nearly all the racquetball's energy is transferred to the floor, lost to heat, or lost to vibrations. The key to this step is that the racquetball half is not turned inside out. (See Figure I.)

Step 2: Before the racquetball half is dropped a second time, the demonstrators secretly turn the half inside out. (See Figure II. and Figure III.) By pressing on the top of the racquetball half, the half flips inside out. The energy used to press the racquetball inside out is stored in the racquetball half. Although a completely accurate discussion of exactly how this energy is stored is very complex, the racquetball half is storing energy much like a spring. When the top of the half is pushed in slightly, potential energy is stored as it would be in a spring. When the racquetball half collides with the floor this time, the potential energy is released. The potential energy is now converted to kinetic energy, bouncing the half up high in the air.

Description

Basic Ideas: Energy can be neither created nor destroyed, but it can be converted from one form to another and it can be transferred from one object to another.

During a collision, energy is usually transfered from one object to another.

U = mgh, where U = gravitational potential energy, m = mass, g = gravitational constant, and h = height.

U = 1/2 kx2, where U = potential energy of a spring, k = spring constant of the material, and x = displacement.

To perform the calculations, we have estimated that the displacement of the racquetball half is 1 inch or 0.025 meters, and that the spring constant of a racquetball is approximately 1000 Newtons/meter.

Step 1: During this step, nothing exciting happens. The racquetball half is dropped from a certain height, and collides with the floor. During this collision, very little of the racquetball's energy is transferred back to kinetic energy. Nearly all the racquetball's energy is transferred to the floor, lost to heat, or lost to vibrations. The key to this step is that the racquetball half is not turned inside out. (See Figure I.)

Step 2: Before the racquetball half is dropped a second time, the demonstrators secretly turn the half inside out. (See Figure II. and Figure III.) By pressing on the top of the racquetball half, the half flips inside out. The energy used to press the racquetball inside out is stored in the racquetball half. Although a completely accurate discussion of exactly how this energy is stored is very complex, the racquetball half is storing energy much like a spring. When the top of the half is pushed in slightly, potential energy is stored as it would be in a spring. When the racquetball half collides with the floor this time, the potential energy is released. Using our estimations for the displacement and the spring constant, we can calculate that the energy stored in the racquetball is approximately 0.31 Joules. (See calculations below) This amount of energy will send the racquetball half to a calculated height of approximately 5.2 feet.

Utotal = 0.5 * 1000 (N/m) * {0.025 (m)}2 = 0.31 (Joules)
hfinal = 0.31 (Joules) / {20 (g) * 9.8 (m/s2)} = 1.6 (m) = 5.2 (ft)


Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia