Downhill Race - Two cylinders that look the same may roll down a hill at different rates.

Downhill Race

 

Two cylinders that look the same may roll down a hill at different rates. 

 

Two objects with the same shape and the same mass may behave differently when they roll down a hill. How quickly an object accelerates depends partly on how its mass is distributed. A cylinder with a heavy hub accelerates more quickly than a cylinder with a heavy rim. 

 

 

 

• 2 identical round metal cookie tins (such as those from butter cookies).
• 10 large metal washers (about 1/4 pound [112 g] each).
• Double-sided foam stick-on tape (or adhesive-backed Velcro™).
• A ramp

(15 minutes or less)

Arrange five of the washers evenly around the outside rim of the bottom of one tin. Stack five washers in the middle of the bottom of the second tin. In both cases, secure the washers with tape or Velcro™.

(15 minutes or more)

Place both tins at the top of the ramp. Be sure the tops are on. Ask your friends to predict which tin will reach the bottom of the ramp first. Release the tins and let them roll down the ramp. The tin with the mass closer to the center will always reach the bottom first.

At the top of the ramp, both tins have identical potential energy, since both have the same mass and are at the same height. At the bottom of the ramp, each tin will have part of its original potential energy appearing as linear (or translational) kinetic energy and the rest appearing as rotational kinetic energy. Though both tins have the same total mass, each has this mass distributed differently. It is harder to get the tin with its mass distributed along the rim rotating than it is to get the tin with its mass concentrated at the center rotating. The tin with its mass at the rim will use a greater part of its original potential energy just to get rolling than will the tin with its mass concentrated at the center. Therefore the tin with its mass at the rim has less energy available to appear as translational kinetic energy, resulting in a lower linear speed. The tin with its mass concentrated around the rim will lose the race to the bottom of the ramp, and the tin with its mass concentrated at the center will win.

The use of lightweight "mag" wheels on cars is related to translational and rotational kinetic energy. Imagine that you had two cars of equal overall mass, but one had lightweight "mag" wheels and a heavy chassis, and the other had heavy steel wheels and a light chassis. Given the same energy input, the "mag" wheel car would accelerate more rapidly, since less of the energy supplied would be needed to get the wheels rotating, and more would therefore appear as straightline motion of the car as a whole.

It is interesting to experiment with rolling cans of soup down an inclined plane. Solid soups roll down the incline at a slower rate than liquid soups. The liquid does not have to rotate with the can, so the potential energy of the liquid soup can go into linear motion, not into rotation of the soup.

Read more »

Disappearing Glass Rods - You can make glass objects disappear.

      

Disappearing Glass Rods

 

You can make glass objects disappear. 

 

Glass objects are visible because they reflect some of the light that shines on them and bend or refract the light that shines through them. If you eliminate reflection from and refraction by a glass object, you can make that object disappear.

• Wesson™ oil. (Regular, not lite.)
• One or more Pyrex® stirring rods or other small, clear glass objects.
• A beaker.
• Optional:  glass eyedropper, glass magnifying lens.

(1 minute or less)
Pour some Wesson™ oil into the beaker.

(15 minutes or more)
Immerse a glass object in the oil. Notice that the object becomes more difficult to see. Only a ghostly image of the object remains. If you do this as a demonstration, keep your audience at a distance to make it harder for them to see the ghost object.

Experiment with a variety of glass objects, such as clear marbles, lenses, and odd glassware. Some will disappear in the oil more completely than others.

You can make an eyedropper vanish before your eyes by immersing it and then sucking oil up into the dropper.
Immerse the magnifying lens in the oil. Notice that it does not magnify images when it is submerged.

When light traveling through air encounters a glass surface at an angle, some of the light reflects. The rest of the light keeps going, but it bends or refracts as it moves from the air to the glass. You see a glass object because it both reflects and refracts light.

When light passes from air into glass, it slows down. It's this change in speed that causes the light to reflect and refract as it moves from one clear material (air) to another (glass). Every material has an index of refraction that is linked to the speed of light in the material. The higher a material's index of refraction, the slower light travels in that material.

The smaller the difference in speed between two clear materials, the less reflection will occur at the boundary and the less refraction will occur for the transmitted light. If a transparent object is surrounded by another material that has the same index of refraction, then the speed of light will not change as it enters the object. No reflection and no refraction will take place, and the object will be invisible.

Wesson™ oil has nearly the same index of refraction (n) as Pyrex® glass (n = 1.474). Different glasses have different indices of refraction. In Wesson™ oil, Pyrex® disappears, but other types of glass &emdash; such as crown glass or flint glass &emdash; remain visible. Fortunately for us, a great deal of laboratory glassware and home kitchen glassware is made from Pyrex® glass.

For most Pyrex® glass, the index matching with Wesson™ oil is not perfect. This is because the Pyrex® glass has internal strains that make its index of refraction vary at different places in the object. Even if you can match the index of refraction for one part of a Pyrex® stirring rod, the match will not be perfect for other parts of the rod. That's why a ghostly image of the rod remains even with the best index matching.

The index of refraction of the oil (and of the glass, too) is a function of temperature. This demonstration will work better on some days than others.

Index of refraction is sometimes called optical density, but optical density is not the same as mass density. Two materials can have different mass densities even when they have the same index of refraction. Though Pyrex® glass and Wesson™ oil have similar indices of refraction, Pyrex® sinks in Wesson™ oil because it has a higher mass density than the oil. Wesson™ oil has a higher index of refraction than water (n = 1.33), but it has a lower mass density and floats on water. The index of refraction depends not only on density, but also on the chemical composition of a material.

You can also make Pyrex® glass disappear by immersing it in mineral oil, which is available from pharmacies or chemical supply houses. However, mineral oil comes in light, medium, and heavy weights, and each variety has a different index of refraction. To match the index of refraction of Pyrex® glass, you'll need a mixture of mineral oils of different weights. To create the proper mixture, place a Pyrex® glass object into a large glass beaker and pour in enough heavy mineral oil to submerge it partially. Slowly add light mineral oil and stir. 

Watch the glass object as you pour. Most Pyrex® glass will disappear when the mixture is about 2 parts heavy mineral oil to 1 part light mineral oil. Notice the swirling refraction patterns as you mix the two oils.

Karo™ syrup is another material that has an index of refraction close to that of glass. Karo™ can be diluted with water to match some varieties of glass.

Read more »

Disappearing Act - If you want to stay hidden, you'd better stay still.

 

Disappearing Act

 

If you want to stay hidden, you'd better stay still. 

 

Some animals blend in with their surroundings so well that they're nearly impossible to see. Only when these animals move can you detect their presence and shape. With this Snack, you can compare what you see when a camouflaged figure remains still to what you see when the figure is moving. 

 

• 2 pieces of dark blue or black construction paper
• Liquid correction fluid.
• A piece of clear plastic the same size as the construction paper. )
• A partner.

(30 minutes or less)
Cut out an animal shape from one of the pieces of paper. Leave a projecting rectangle of paper to serve as a handle (see photo).

Use correction fluid to make a random pattern of dots on both the animal figure and the second piece of paper. The second piece of paper will act as the background for the figure.

Place the figure on the background and cover both pieces of paper with the plastic. The transparent covering keeps the edges of the animal flat against the background.

(5 minutes or more)
View the animal cutout against the background from an arm's length away. It should be very difficult, if not impossible, to detect the shape of the animal. If you can see the edges, move about 6 feet (2 m) away and have a friend hold the animal and the background.

Place the cutout so that you can use the handle to move the animal while it is under the glass or plastic. Notice that this movement makes it easy to detect the presence of the animal and to identify its shape.

By making several different shapes you can make a game of this. Can anyone identify the animal before it moves? Who can identify it first when it moves?

Many animals have patterns of color on their bodies that allow them to blend into the background. These animals are hard to detect when they're still. But when the animals move, you can easily pick them out. That's because humans, as well as many other animals, have specialized brain cells that detect motion. These cells receive information from the light-sensitive cells at the back of the eye.

What animals can you think of that use camouflage to blend into their environment?

Read more »

The Dipping Bird - The dipping bird seems to go forever but it's not perpetual motion!

The Dipping Bird

 

The dipping bird seems to go forever but it's not perpetual motion! 

 

A dipping bird is an example of a heat engine. It converts a difference in temperature (between the head cooled by evaporation and the bottom at room temperature) into cyclical motion. 

 

 

 

• Dipping bird (can be obtained from Edmund Scientific, catalog number 53617, $7.95 for a package of 2 birds. You can also try novelty or magic shops.)
• Cup or glass
• Water

 

Wet the bird's head thoroughly with water. Allow enough time for the fuzzy material on the head to absorb water (a few seconds should do it).

Fill a cup or glass with water and place it so that the bird's beak will dip into the water each time the bird tips. You may have to place pieces of wood or cardboard under the cup or glass if it's too short, or get a smaller glass if it's too tall.

Watch the bird go through its cycle. Notice what happens to the liquid inside the bird at different positions in the cycle.

When the bird is manufactured, most of the air is removed from the inside. The gas that remains is largely the vapor from the red liquid, which vaporizes very easily. When the fuzzy coating on the bird's head is wet, water evaporates and cools the vapor inside the bird's head. This condenses the vapor back to red liquid and reduces the pressure in the bird's head. When the fuzzy coating on the bird's head is wet, water evaporates and cools the vapor inside the bird's head. This condenses the vapor back to liquid and reduces the pressure in the bird's head. The bird's head keeps moving.

Since the pressure of the vapor in the bird's body is now higher than the pressure in its head, liquid is forced from the bottom up the tube toward the head. As the liquid moves up the tube, the center of gravity of the bird is raised, and the bird begins to tip around its fulcrum. When the bird finally dips into the water, a clear passage is opened between the head and the body, allowing the pressures to equalize and the liquid to fall back down to the body. The bird returns to the upright position and the whole process repeats.

Each time the bird's beak dips into the water, the fuzzy material absorbs a little water to replace any that has evaporated. This prevents the bird's head from drying out. The bird will continue its cycle until the head dries out, and evaporation can no longer cool it.

In summary, the steps in the cycle are as follows:

1. The bird's head dips and gets wet.
2. Water evaporates from the fuzzy head.
3. The vapor in the bird's head condenses into liquid.
4. Pressure in the bird's head is reduced because the liquid takes up less space than the vapor.
5. Liquid moves up the tube into the low- pressure area in the head; the cycle repeats.
6.  

An interesting extension is to paint the bottom chamber of the bird black. An essential requirement to make the bird dip is to get the head cooler than the body. Normally this is accomplished by evaporation of water from the head. By painting the body black and exposing the bird to a hot lamp or to sunlight, the body will become warmer than the head. In this way, you can either enhance the normal operation of the duck, or get it to operate without wetting the head at all.

Don rathjen has measured the power output of a dipping bird by attaching it to a windlass and using it to raise paper clips. He managed to extract a nanohorsepower of work from his dipping duck. (A nano-horsepower is about a microwatt.)

References

1. Mentzer, Robert, "The Drinking Bird - The Little Heat Engine That Could," The Physics Teacher, February 1993.

2. Bent, Harry, and Harold Teague, "The Hydro-Thermal-Dynamical Duck; A Sketch of His Uses in the Classroom and the Laboratory," Journal of College Science Teaching, September 1978.

3. Rathjen, Don, "Duckpower," Exploring, Winter 1994, pp. 7 - 8. ("Duckpower" shows how to use the dipping bird as a heat engine to lift a weight, and discusses the work and the horsepower involved. Exploring is the quarterly magazine of the Exploratorium.)

Read more »

Diffraction - Light can bend around edges

 

Diffraction

 

Light can bend around edges. 

 

Light bends when it passes around an edge or through a slit. This bending is called diffraction. You can easily demonstrate diffraction using a candle or a small bright flashlight bulb and a slit made with two pencils. The diffraction pattern, the pattern of dark and light created when light bends around an edge or edges, shows that light has wavelike properties.

 

 

 

• 2 clean new pencils.
• A piece of transparent tape. (Any thin tape will do.)
• A candle.

OR

• a Mini-Maglite® flashlight (available for under $15 in many hardware stores). Do not substitute other flashlights.

OR

• A flashlight bulb for a Mini-Maglite®, two AA batteries, a battery holder (available from Radio Shack), and two clip leads.
• Optional: pieces of cloth, a feather, plastic diffraction grating, a metal screen, a human hair.
•  

(5 minutes or less) Light the candle or, if you are using a Mini-Maglite®, unscrew the top of the flashlight. The tiny lamp will come on and shine brightly. You can also make your own bright point source of light by attaching the Mini-Maglite® flashlight bulb to the battery holder with the clip leads. Be sure you put two AA batteries in the battery holder.
Wrap one layer of tape around the top of one of the pencils, just below the eraser.

(15 minutes or more) If you measure distances on the diffraction pattern, you can calculate the wavelength of light emitted by the candle or bulb.

Place the light at least one arm-length away from you.
Hold the two pencils vertically, side by side, with the erasers at the top. The tape wrapped around one pencil should keep the pencils slightly apart, forming a thin slit between them, just below the tape. Hold the pencils close to one eye (about 1 inch [2.5 cm] away) and look at the light source through the slit between the pencils. Squeeze the pencils together, making the slit smaller. Notice that there is a line of light perpendicular to the slit. While looking through the slit, rotate the pencils until they are horizontal, and notice that the line of light becomes vertical.

If you look closely you may see that the line is composed of tiny blobs of light. As you squeeze the slit together, the blobs of light grow larger and spread apart, moving away from the central light source and becoming easier to see. Notice that the blobs have blue and red edges and that the blue edges are closer to the light source.

Stretch a hair tight and hold it about 1 inch (2.5 cm) from your eye. Move the hair until it is between your eye and the light source, and notice that the light is spread into a line of blobs by the hair, just as it was by the slit. Rotate the hair and watch the line of blobs rotate.

Look at the light through a piece of cloth, a feather, a diffraction grating, or a piece of metal screen. Rotate each object while you look through it.

The black bands between the blobs of light show that there is a wave associated with the light. The light waves that go through the slit spread out, overlap, and add together, interacting in complex ways to produce the diffraction pattern that you see. Where the crest of one wave overlaps with the crest of another wave, the two waves combine to make a bigger wave, and you see a bright blob of light. Where the trough of one wave overlaps with the crest of another wave, the waves cancel one another out, and you see a dark band.

The angle at which the light bends is proportional to the wavelength of the light. Red light, for instance, has a longer wavelength than blue light, and so it bends more than blue light does. This different amount of bending gives the blobs their colored edges: blue on the inside, red on the outside.

The narrower the slit, the more the light spreads out. In fact, the angle between two adjacent dark bands in the diffraction pattern is inversely proportional to the width of the slit.

Thin objects, such as a strand of hair, also diffract light. Light that passes around the hair spreads out, overlaps, and produces a diffraction pattern. A piece of cloth or a feather, which are both made up of many smaller, thinner parts, produce complicated diffraction patterns.

In a dimly lit room, look at a Mini-Maglite® bulb with one eye (a candle will not work). Notice the lines of light radiating out from the light source, like the seeds radiating out from the center of a dandelion. Propose experiments to find the origin of these lines. For example, rotate the light source, and notice that the lines of light do not rotate. Rotate your head, and notice that the lines do rotate. Hold your hand or an index card in front of your eye so that it doesn't quite block your view of the light source. Notice that you still see a full circle of lines radiating out from the light source. The lines of light are spread out onto your retina by imperfections in the tissues of your cornea.

Read more »