Drawing Board - A pendulum moving in two directions creates beautiful designs.

Drawing Board

 

A pendulum moving in two directions creates beautiful designs. 

 

The Drawing Board consists of a marking pen that remains stationary and a platform that swings beneath the pen, acting as a pendulum. As the platform swings, the pen marks a sheet of paper that is fastened to the platform, generating beautiful repetitive patterns, which grow smaller with each repetition. These colorful designs contain hidden lessons in physics. 

 

Make Your Own Version:

• A ruler.
• One 4 x 4 x 12 inch (10 x 10 x 30 cm) wood post.
• A nonskid base.
• A drill.
• A pivot bolt.
• 2 metersticks or yardsticks.
• Washers.
• Wire.
• Rubber bands.
• 3 large tables or 4 evenly spaced hooks in a ceiling.
• A large board.
• 4 large hook eyes.
• Rope.
• Duct tape.
• Bricks.
• A large sheet of plain paper.
• Masking tape.
• Marking pens.
• Optional: String.
• Adult help.

(5 minutes or less)

Set up the Ellipto™ or Pendul-Art™ according to the manufacturer's instructions. If you want to build your own Drawing Board, see the information here.

(15 minutes or more)

Once the Drawing Board is adjusted, you can create wonderfully intricate designs. Try drawing one to four patterns on the same paper using pens of different colors, changing the direction and force of the push with each new color.

When the platform is displaced from its rest position, the four suspending strings exert forces on it to bring it back. You can think of these forces as acting in two directions perpendicular to each other: "north-south" and "east-west," for example. The combination of these two simultaneous motions can produce a variety of curved forms, in the same way that proper simultaneous manipulation of the two knobs on an Etch-a-Sketch™ toy allows you to draw curves.

The diminishing size of each successive repetition of the pattern is a graphic demonstration of how friction steadily dissipates the energy of a moving object.

Make Your Own Drawing Board

(1 hour or less) One of our teachers put together a large-scale version of the Drawing Board that was dramatic. Rather than attempting to give detailed instructions for assembling this device, we have chosen instead to supply some labeled drawings (see below) and helpful hints. The rest is left to the dedicated experimenter. The penholder must be counterbalanced so that the pen exerts minimum pressure on the moving board while maintaining constant contact with the writing surface. You will have to adjust the length of the suspension ropes, since they stretch with time. Try using a knot called a slip hitch, shown in the diagram. The placement of the weight on the board is critical. Experiment with various positions.         One person pushes the board to start rotational as well as translational motion. Another person controls the penholder, lowering the pen to start drawing and raising it to stop. The Drawing Board should produce a pattern that repeats the same basic shape over and over again, with each cycle getting smaller. If the pattern is not consistent from one cycle to the next, try moving the weight on the board or adjusting the counterbalance weight on the penholder. Also make sure that the penholder is not shifting on the floor.

Some of the shapes you will produce with the Drawing Board are known as harmonograms or Lissajous figures. An oscilloscope can easily produce these figures, since the pattern on the scope face is generated by a single electron beam simultaneously moving vertically and horizontally on the screen. An oscilloscope can be thought of as an electronic Etch-a-Sketch™. One of our teachers had this Snack set up and running during an aftershock of the 1989 Loma Prieta earthquake. The pen traced the pattern of motion generated by the aftershock. The operating principle behind the Drawing Board - a pen directly attached to the earth with a paper only loosely attached to the earth - is the operating principle behind a seismograph.

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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.

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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.

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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?

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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.)

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