Center of Gravity - How to balance a checkbook using the physics method !

Center of Gravity

 

How to balance a checkbook using the physics method.

Here is an easy way to find the center of gravity of a long, thin object, even if the object's weight is unevenly distributed. 

 

• A meterstick, cane, or any stick of similar length.
• Clay or weight.
• Masking tape.

 

(5 minutes or less)

First try the experiment with just the stick itself. Then tape the clay or weight somewhere on the stick and try again.

(5 minutes or more)

Support the stick by resting each of its ends on a finger. Slowly slide your fingers together until they meet. Your fingers will meet under the stick's center of gravity. Attach the weight or a piece of clay to some point on the stick. Again support the stick on two fingers, and then slide your fingers together to locate the new center of gravity. Move the weight or piece of clay to some new place on the stick. Repeat the experiment. Your fingers will always meet right under the center of gravity.

The stick's center of gravity is the place where you could balance the stick on just one finger. When you first support the stick with two fingers, in general one finger (the one that is closer to the center of gravity) will be holding a little more of the weight than the other. When you try to move your fingers closer together, the one that is carrying less weight will slide more easily. This finger will continue to slide more easily until it gets closer to the center of gravity than the other finger, at which point the situation will reverse and the other finger will begin to slide faster. Your left and right fingers simply alternate moving until they meet at the center of gravity, where both fingers support equal weight.

Read more »

Bubble Tray - Create giant bubbles!

  

Bubble Tray

 

Create giant bubbles

Bubbles are fascinating. What gives them their shape? What makes them break or last? What causes the colors and patterns in the soap film, and why do they change? 

 

 

• Measuring cups and spoons.
• Dawn™ or other dishwashing liquid.
• Glycerine (available at drugstores).
• Tap water.
• A wire coat hanger.
• A shallow tub or tray about 18 inches (45 cm) in diameter such as a potted-plant drain dish, a pizza pan, or a catering tray).
• Optional: Yarn.

 

(30 minutes or less)

Mix up a bubble solution of 2/3 cup (160 ml) Dawn™ dishwashing liquid and 1 tablespoon (15 ml) glycerine in one gallon (3.8 l) of water. We have found that more durable bubbles form if you let this solution age for at least a day, preferably for a week.

Bend the coat hanger into a flat hoop with the hook sticking up at an angle to serve as a handle. Bubbles will form more consistently when the hoop is as circular as possible. If you wrap yarn tightly around the wire of the hoop, the yarn will absorb the bubble solution, which will make the hoop easier to use.

If you prefer a more elegant apparatus, a bubble tray complete with a bubble hoop is available at the Exploratorium Store for about $20. 
 

(15 minutes or more)

Fill the shallow tray with bubble solution and submerge the hoop in the solution. Then tilt the hoop toward you until it is almost vertical, and lift it from the tray. You should have a bubble film extending across the hoop. Swing the hoop through the air to make a giant bubble. When you have a big bubble, twist the hoop to seal it off at the end.

What shapes do the bubbles take once they are free of the hoop? What roles do convection and air currents play in the bubble's movement? Look for patterns and colors in the bubbles. Dip the hoop in the solution and hold it up to the light without forming a bubble. What patterns (and changes in patterns) do you observe? 
 

The strong mutual attraction of water molecules for each other is known as surface tension. Normally, surface tension makes it impossible to stretch the water out to make a thin film. Soap reduces the surface tension and allows a film to form.

Because of surface tension, a soap film always pulls in as tightly as it can, just like a stretched balloon. A soap film makes the smallest possible surface area for the volume it contains. If the bubble is floating in the air and makes no contact with other objects, it will form a sphere, because a sphere is the shape that has the smallest surface area compared to its volume. (Wind or vibration may distort the sphere.)

The patterns of different colors in a so
ap bubble are caused by interference. Light waves reflected from the inner and outer surfaces of the soap film interfere with each other constructively or destructively, depending on the thickness of the bubble and the wavelength (that is, the color) of the light. For example, if the soap film is thick enough to cause waves of red light to interfere destructively with each other, the red light is eliminated, leaving only blue and green to reach your eyes.

You can make other devices to create large bubbles. One of the easiest is a length of string (or, still better, fuzzy yarn) threaded through two drinking straws, with the ends tied to make a loop any size you want. Not only will this device make large bubbles, but you can twist the straws to make film surfaces with different shapes.

Read more »

Bubble Suspension - Soap bubbles float on a cushion of carbon dioxide gas

   

Bubble Suspension

 

Soap bubbles float on a cushion of carbon dioxide gas

This beautiful experiment illustrates the principles of buoyancy, semipermeability, and interference 

 

• A small aquarium
• Dry ice
• Bubble solution You can use a commercial solution like Wonder Bubbles™, or use the Exploratorium's recipe: 2/3 Cup (160 ml) Dawn™ dishwashing liquid and 1 tablespoon (15 ml) glycerine (available at most drugstores) in 1 gallon (3.8 l) of water. Aging the solution for at least a day before use significantly increases the lifetime of the bubbles.
• Gloves
• Adult help

 

(5 minutes or less)

Place a slab of dry ice flat in the bottom of the aquarium. (CAUTION: Use gloves when handling the dry ice; do not touch it with bare skin.) Allow a few minutes for a layer of carbon dioxide gas to accumulate. 
 

(15 minutes or more)

Blow bubbles so they float down into the aquarium. The bubbles will descend and then hover on the denser layer of carbon dioxide gas. After a few minutes, notice that the bubbles begin to expand and sink. Notice the color bands on the bubbles. Notice how some of the bubbles freeze on the dry ice. 
 

As dry ice turns from a solid to a vapor, or sublimes, it produces carbon dioxide gas. Carbon dioxide is denser than air. (Carbon dioxide molecules have an atomic mass of 44 amu [atomic mass units]. Air is made up of nitrogen, 28 amu, and oxygen, 32 amu.) The denser carbon dioxide gas forms a layer on the bottom of the aquarium.

A bubble is full of air. It floats on the carbon dioxide layer just like a helium balloon floating in the air. You might expect that the air in the bubble would cool and contract near the dry ice, but the bubble actually expands slightly. The soapy wall of the bubble allows carbon dioxide to pass through but does not allow air molecules to pass through. Initially, the concentration of carbon dioxide gas is low inside the bubble and high outside the bubble.

The gas gradually diffuses into the bubble, a process called osmosis. The bubble film is a semipermeable membrane--a surface that allows some substances to pass through while preventing others from passing through at all. The cells in your body have the same property. Water, oxygen, and carbon dioxide easily enter some cells, whereas other molecules do not. The added carbon dioxide makes the bubble denser, causing it to gradually sink. The carbon dioxide at the bottom of the tank is cold enough to freeze the bubble.

You can do many experiments with these bubbles.

What happens when bubbles of different sizes collide? Sometimes they make a single larger bubble, other times they join as two bubbles with a flat or bulging wall between them. If the two bubbles are the same size, the wall is flat between them, since the pressure is equal on both sides. If the two bubbles are of different sizes, the wall will bulge away from the smaller of two bubbles, since the smaller bubble will have a higher pressure inside.

How does a bubble respond to a comb that has been charged by rubbing it with a wool cloth? The neutral bubble is electrically polarized by, and attracted to, the charged comb.

Read more »

Bronx Cheer Bulb

Bronx Cheer Bulb

 

Some light sources may appear to wiggle and flash when you give them the raspberry, but the only thing wiggling is you

Some light sources flash on and off many times a second. When you give them the "Bronx cheer," you can see the their hidden flickering. 

 

• A digital radio or clock radio that uses light-emitting diodes (LEDs) These have red numbers. Or
• A circuit tester with an LED on it Or
• A neon glow lamp available from your local hardware store, such as a GE Guide lamp, and an extension cord. (Any nightlight labeled "1/4 watt" has a neon glow lamp in it)

 

(5 minutes or less)

No assembly is required for the digital radio, circuit tester, or neon glow lamp; just plug them in and observe them from a few feet away.

A simple source for a neon glow lamp is a button-type nightlight. These are small orange night-lights advertised as 1/4 watt bulbs. They do not have a regular replaceable small lighibulb. Plug the nightlight into the wall or into an extension cord that is plugged in. 
 

(5 minutes or more)

Observe the light source from 3 to 10 feet (90 to 300 cm) away and give it the "Bronx cheer." (A Bronx cheer, also known as a "raspberry," is a rude noise made by blowing air through your lips in a way that makes them vibrate.) Notice that the light seems to wiggle back and forth and flicker. Try shaking your head rapidly and notice whether the light still flickers. See if you can find other body motions that make the light flicker. Try the Bronx cheer on other light sources, such as incandescent lightbulbs. Notice whether the light flickers. 
 

No part of the LEDs or the neon glow tube move when you give the Bronx cheer. Instead, your whole body is vibrating, including your eyes. you can feel this vibration by putting your hand on your head as you blow. The LEDs flash on and off sixty times a second (a neon glow tube glows on and off 120 times a second). This flashing is so fast that your eyes normally can't separate the "blinks." But when your body is vibrating, your eyes are in a different position each time the bulb flashes. As the image of the bulb traces a path across your eyes, it looks like the bulb is moving and flickering. An incandescent bulb won't flicker when you give the Bronx cheer, because the bulb doesn't flash on and off. Incandescent bulbs give a steady glow.

Plug a commercial neon night-light into an extension cord. Tape it firmly in place. Twirl the light around in a circle. (Be careful not to let it hit anything.) Notice that you can see the light flashing. Since the light is moving, it's in a new position each time it flashes. The light traces a path across your eye, and its flashes become spread out and visible. Find an oscilloscope and set it up so that the beam goes straight across the middle of the screen in about 1/1ooth of a second. Ask a couple of friends to stand back a few yards from the scope. Tell them that the oscilloscope is an eating detector. Have your friends watch the scope at the same time. Have one of them eat a peanut and the other one not eat. The person eating the peanut will see the beam jump up and down. Eating causes vibrations of your skull, including vibrations of your eyes. If your eyes are moving, the dot of light scanning across the oscilloscope shines on different parts of your eyes and appears to jump around.

Read more »

Bridge Light - A thin layer of air trapped between two pieces of Plexiglas™ produces rainbow-colored interference patterns

   

Bridge Light

 

A thin layer of air trapped between two pieces of Plexiglas™ produces rainbow-colored interference patterns

When light hits two slightly separated transparent surfaces, part of the light will be reflected from each surface. If the distance between the surfaces is a multiple of half or whole wavelengths of the light, constructive and destructive interference will occur, producing an interference pattern. 

 

• 2 sheets of Plexiglas™, 1/4 or 1/8 inch (.64 or .33 cm) thick and approximately 1 foot (30 cm) square. (Size is not critical.)
• 1 piece of dark construction paper
• One 3 x 5 inch (8 x 13 cm) piece of transparent red plastic
• Electrical or duct tape
• A light source, such as a desk lamp

 

(15 minutes or less)

Peel the paper from the Plexiglas™ and smooth off all edges with sandpaper if necessary. Be careful not to scratch the surfaces. Clean the top and bottom surfaces with alcohol and a soft cloth. Press the plates tightly together and tape around the edges to hold them in place. Tape a sheet of dark construction paper to one plate to make the interference patterns more visible. 
 

(15 minutes or more)

Hold the plates, with the dark-paper side on the bottom, in any strong source of white light. Observe the rainbow-colored interference patterns. The patterns will change as you bend, twist, or press on the plates. Notice that the patterns strongly resemble the contour lines on a topographic map.
Place the red plastic between the light source and the plates. Notice that the patterns are now just red and black. 
 

Light waves reflect from the surfaces of two plastic sheets separated by a thin air gap. These light waves meet after reflecting from the two surfaces. When two waves meet, they can add together, cancel each other, or partially cancel each other. This adding and canceling of light waves, called constructive interference and destructive interference, creates the rainbow-colored patterns that you see.

White light is made up of all different colors mixed together. When light waves of a particular color meet and cancel each other, that color is subtracted from white light. For example, if the blue light waves cancel, you see what is left of white light after the blue has been removed--yellow (the complementary color of blue).

The thickness of the gap between the plates determines which colors of light cancel out at any one point. For example, if the separation of the plates is roughly equal to one-half the wavelength of blue light (or some multiple of it), the crests of waves of blue light reflected from the top surface of the air gap will match up with the troughs of waves reflected from the bottom surface, causing the blue light to cancel out.

This is what happens: Imagine that the distance between the two plates is one-half the wavelength of blue light. When a wave hits the top of the air layer, part reflects and part continues on. Compared to the part that reflects from the top of the air layer, the part that continues on and reflects from the bottom travels an extra wavelength through the air layer (half a wavelength down and half a wavelength back). In addition, the wave that reflects from the bottom is inverted. The net effect is that the blue light waves reflected from the two surfaces recombine trough-to-peak, and cancel each other out.

Because the interference pattern depends on the amount of separation between the plates, what you're actually seeing is a topographical map of the distance between plates.
When you place a red filter in front of the light source, only red and black fringes will appear. Where destructive interference takes place, there is no red light left to reach your eyes, so you see black. Where the waves constructively interfere, you see red.

If you can find a sodium-vapor lamp (a yellow street lamp, for example), try placing the plates under its light. The sodium vapor gives off sodium's predominant fingerprint: a very pure yellow light.

The beautiful rainbow colors you see in soap bubbles and on pieces of metal heated to high temperatures are produced in the same way: by light reflecting from the top and bottom of a thin transparent layer.

When you open a package of new, clean microscope slides, you can often see colored interference patterns created by the thin air space between the glass slides

Read more »