July 1998 (Volume 7, Number 7)

# Physics and Toys: Fun for Everyone

By Raymond C. Turner

All too often when the word physics is mentioned, people have a very negative reaction. Fortunately, the basic principles of physics can often be demonstrated, as well as made fun, by using ordinary children's toys. By understanding how these toys work the observers can better understand the world around them. Toys can also be used with students of all ages. I have successfully used toys in physics presentations to pre-kindergarten classes, and elder hostel groups, as well as all ages in between. The detailed analysis of the toys differs depending on the background of the observers, but in every case the physics is the same, and the toys allow for a nonthreatening and fun presentation.

There are any number of toys which can be used to illustrate various physics principles of mechanics. A Hot Wheels race car and track can be used to demonstrate velocity and acceleration, or be analyzed using energy concepts, while a water rocket can be used to demonstrate the principle of momentum. For now, let us discuss several toys that deal with magnetism and light.

For example, a toy periscope can be used to demonstrate reflection of light and image formation with flat mirrors. A particularly interesting version is the rotating periscope. While a periscope is usually used to look at an object in front of you, the top of this periscope can be rotated about the vertical axis so that you can look behind you. When you rotate the top mirror in this way, the image is inverted, such that the image is in the normal upright position when looking straight ahead, but inverted when you look behind you. (Now you know why submarine commanders in the movies always turned with their periscopes.) Examination of the toy periscope shows that it contains two mirrors, one in the top and one in the bottom, each of which is at an angle of 45 degrees to the vertical. You should be able to quickly sketch two light rays from the object to your eye in order to analyze the upright and inverted images. This is exactly what I have my students do in order to have them begin to understand image formation using light rays.

A more complex toy that became popular several years ago is called laser tag, consisting of a "laser" gun and a target. The gun emits an invisible beam of radiation, and the target is a sensor which emits an audible signal when the wearer is "tagged." What are the properties of this radiation? There are any number of specific questions that can be asked and then answered by investigation. Does the beam travel in straight lines, or does it bend around corners? Can the beam be reflected, and if so, does it behave like light when it is reflected? Can the beam be refracted? Can it be diffracted with a diffraction grating? Is the beam transmitted through a sheet of paper?

If you perform some of these experiments, you find, for example, that the beam can be reflected from a mirror or sheet of metal, and that it obeys the law of reflection. The beam is also found to refract when it is sent through plexiglass and is found to obey Snell's law of refraction. Perhaps surprisingly, it is found that the beam will transmit through a sheet of paper. The particular version of this toy I prefer to use is called the infrared blaster, emitting a beam with an infrared wavelength of about 930 nm. This can be used to illustrate how a physicist goes about investigating the properties of an unknown, obtaining good experimental results in the process.

Another toy - perhaps better termed a novelty - is a flicker light, commonly found in toy stores and gift shops. This is an electric light bulb in which the filament vibrates in an erratic fashion that is supposed to simulate a flickering candle. What makes its filament vibrate? Examination of the bulb shows that the wire filament is free to move somewhat, and there is a magnet near the filament. When a current flows in the filament, there is a magnetic force on it, due to the field of the permanent magnet. This magnetic force is in a direction perpendicular to both the field and the current. Since the light is operating on the usual 60 Hz ac line, the direction of the current reverses 60 times per second. Thus, the direction of the force on the filament changes at the same rate, causing the filament to vibrate back and forth, at a rate too rapid to be followed by our eyes. So why does the light appear to flicker?

While the magnet for most of these lights is mounted inside the bulb, I was fortunate enough to obtain a magnetic wild flicker bulb which had the magnet mounted on the outside. This allowed me to move the magnet away from the bulb, so that I could see the effect of reducing the magnetic field at the filament. As you would expect, the further the magnet was moved away, the smaller the amplitude of the vibration. But this permits another aspect of the vibration to be observed.

When the magnetic field is small, the vibration of the filament is regular (60 Hz) and is observed only as an apparent broadening of the filament. As the magnet is moved closer to the bulb, the amplitude of this smooth vibration increases until suddenly, the filament begins to wildly flicker. This flicker is at a much lower frequency than 60 Hz and its rate is presumably determined by the mechanical resonant frequencies of the filament. The onset of this flicker is most likely a demonstration of chaotic motion that occurs when the amplitude of the motion is large enough to cause significant nonlinear effects. This simple toy thus demonstrates the magnetic force on a current carrying wire, as well as serving as a means for introducing the concept of chaos.

These are just a few of the many toys that can be used to illustrate basic physics principles. Only your imagination and ingenuity limit you in your application of the fundamental laws of physics to ordinary objects, even toys. Physics can be fun not only for students, but also for the teacher. By using toys, physics can be fun for everyone.

Raymond Turner is an Alumni Distinguished Professor of Physics at Clemson University in South Carolina, and is the recipient of the first AAPT Award for Excellence in Undergraduate Teaching. This article first appeared in the Fall 1997 issue of the APS Forum on Education Newsletter and was the basis of an invited paper presented at the APS/AAPT April Meeting in Columbus, Ohio. Other physics toys in action may be observed at the following Web site: www.clemson.edu/phys-car.

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