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Why does the sound of an ambulance siren change as it passes?

admin — Thu, 24 Dec 2009 01:07:46 +0000

The Doppler effect, named for Austrian scientistChristopher Doppler, who described it in 1842, is a change in the frequency of a wave due
to motion of the source relative to the observer. You can detect the Doppler effect whenever an object that makes a constant sound is moving
relative to your position. Examples include the sound of a whistle on a passing train, a siren on a police car, and even a mosquito flying by.

You can picture the Doppler effect by thinking of sound waves at a specific wavelength as compressions in the air that occur at a constant
interval. As each compression enters your ear, it causes sensors to vibrate and send a message to your brain that is interpreted as sound. Shorter wavelengths have a greater frequency—that is, compressions occur at a greater rate. A higher frequency corresponds to a higher pitch. If the source of the sound is moving toward you, each compression starts at a point that is closer to you than the previous compression. As a result, succeeding waves reach your ear more frequently than they are emitted by the source, so the sound has a higher pitch. As the source moves away from you, the waves are spread out and reach you with less frequency. The pitch of the sound is then lower. The same thing would happen if the source of the sound were stationary and you were moving quickly toward or away from it.

Reference: The Science Of Everything – Steve Miller

Why do you see lightning before you hear thunder?

admin — Thu, 24 Dec 2009 01:03:40 +0000

Light travels through air at about 186,000 miles every second. Sound, however, is much slower. A sound wave travels through the air as vibrating molecules bump into their neighbors, transferring energy. This process takes time, so a sound wave moves about 1,000 feet in that same second (the speed varies a bit, depending on temperature and the amount of water vapor in the air). You can estimate the distance to the lightning strike based on this difference in the speed of light and sound. Count the seconds between the time you see the lightning and the time you hear the thunder. Divide the number of seconds by 5 to find the approximate distance, in miles, to the lightning strike.

Reference: The Science Of Everything – Steve Miller

Why do objects look different under different lighting?

admin — Thu, 24 Dec 2009 01:01:49 +0000

Picture a green leaf on a sunny day. Why does the leaf look green? When sunlight strikes the leaf, molecules inside the leaf absorb some of the energy of the light. Some wavelengths are absorbed more than others. This is the source of energy that a plant needs in order to grow. The wavelengths of sunlight that are not absorbed by the leaf are reflected. When you look at a leaf, you see the light that is reflected from its
surface and you perceive this light as green. What happens if the light that strikes the leaf is not sunlight, but rather light from a red stage light that produces only light in red wavelengths? In this case, the leaf absorbs all of the light that strikes it. When no light is reflected or emitted, an object appears to be black.

There are several kinds of artificial light used in buildings. Although the light they produce generally appears to be white, none of these light sources produce a spectrum of light that exactly matches sunlight in distribution and relative intensity of wavelengths. They also differ from one another. For example, incandescent light (light bulbs) tends to produce more light in red and yellow wavelengths compared to sunlight. Standard fluorescent tubes tend to produce light that has a greater proportion of violet and blue wavelengths. The light that is reflected from an object varies depending on the wavelengths of light that strike it. That is why a paint chip may appear different under different types of lighting. Some stores that sell paint offer a place where the colors can be observed under varying types of light.

What is mirage?

admin — Wed, 23 Dec 2009 17:06:53 +0000

Mirages occur when a sharp boundary forms between two layers of air with different temperatures. If the air just above the ground is much warmer than the air above it, light bends upward. The puddle that you see on the road ahead is actually light from the blue sky and clouds in the distance. Your mind, however, treats the image as if it occurred in a straight line from your eyes, not accounting for the bending of the light. It appears as though the light from the sky is reflecting from the surface of a puddle or lake. This type of mirage is usually seen on hot days when the sun heats the air above the pavement, but it can occur whenever the air near the ground is much warmer than the air above it. Sometimes you can see images of objects other than the sky in a refracted image. Try looking at the air above a dark-colored car that has been sitting in the sun on a hot day. You may see an inverted image of the storefront on the other side of the street.

Reference: The Science Of Everything – Steve Miller

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Why does a spoon in a glass of water seem to bend?

admin — Wed, 23 Dec 2009 16:53:01 +0000

The apparent bend in the spoon’s stem comes from this change in light speed as the light travels between the water and the air. You see the spoon because rays of light are reflected from its surface. When these rays of light pass from the water into the air, they travel at a different speed.

When parallel rays of light travel through the water, some reach the interface first and begin to move faster. Others take longer to reach the air, so their change in speed is delayed longer. As a result, the path that the light follows is bent. This bending of the light path causes the spoon to appear to bend. If you go fishing and see a large fish in the water, you cast your bait in front of the place where the fish appears to be swimming. Because of refraction, the fish seems farther away than it actually is.

Why is the sky blue?

admin — Wed, 23 Dec 2009 16:50:34 +0000

Light travels in waves of different lengths. The wavelengths of visible light are very short—about one ten-thousandth to one one-thousandth of a millimeter. The longest waves in the familiar spectrum of visible light are red and the shortest are violet. The sun produces light across the entire span of the spectrum. When all of the wavelengths of the visible spectrum are seen at once, the light appears white. That’s why the sun looks white to an observer on the space station.

When light reaches Earth, it has to pass through the atmosphere, the layer of gases that surrounds the solid and liquid parts of the planet. The atmosphere is full of dust particles and droplets of water vapor. When light hits these particles, it reflects and changes direction. As the light passes through the atmosphere, it is reflected over and over. That’s why sunlight can reach places that are not in a direct line to the sun.

Water droplets are larger than the light waves so they tend to reflect every wavelength. That explains why a cloud looks white—the droplets reflect all colors of light and the combination looks white. Most of the atmosphere consists of molecules of nitrogen and oxygen, which are much smaller than the wavelength of light. When light strikes these molecules, it does not reflect in the same way that it reflects from water droplets. However, when light strikes them, the molecules occasionally absorb the energy of the light wave. After a while (a very tiny fraction of a second, generally), the molecules emit light at the same wavelength. While all of the light from the sun is traveling in the same direction, the light emitted by a gas molecule can travel in any direction in a straight line away from the molecule. The light is scattered in all directions. This is called
Rayleigh scattering, named after Lord John Rayleigh, who first described it.

So how does this explain the color of the sky? It turns out that the shorter the wavelength of the light, the more likely it is to be scattered by Rayleigh scattering. Long wavelengths, such as red and yellow, are much less likely to be scattered than short wavelengths, such as violet and blue. If there were no atmosphere the sky would look black, just as it does in space. What you see when you look at a blue sky is blue sunlight
that has bounced around in the air until it is coming at you from all directions.

What is the difference between DC and AC electric current?

admin — Wed, 23 Dec 2009 16:32:08 +0000

In direct current (DC), electrons flow from one place to another in a single direction. For example, when you turn on a flashlight, electrons flow from the negative pole of the battery, through the light bulb, and then to the positive terminal. As they pass through the bulb, the electrons lose energy and light is emitted. In an alternating current (AC), the electrons don’t flow from point to point. Instead, they move back and
forth inside the wire, changing direction in a cycle of 60 times each second. When alternating current flows in a light bulb, the electrons move back and forth within the element, losing energy due to resistance to their motion.

Early in the history of electric power, both types of current were used to transmit electricity. The first power plants used direct current. However, alternating current is able to transmit energy over much greater distances without losing energy. In addition, AC voltage can be changed very easily using a transformer. It is more efficient to transport the power over long distances at very high voltage, but for use inside homes and businesses, a lower voltage is necessary for safety. A series of transformers in the transmission and delivery system reduces the voltage from production to use.

Reference: Science Of Everything – Steve Miller

Why can’t we cool the kitchen by opening the refrigerator?

admin — Wed, 23 Dec 2009 16:25:37 +0000

Specifically, the laws of thermodynamics say that there is no way for you to get something for nothing. So, if you make one place cooler, you have to make another place warmer. As you cool your perishable foods, you heat up the kitchen. A refrigerator is a kind of heat pump. It
removes heat from the inside of a box and then it has to dump the heat somewhere else, outside the box. In order to move heat, it compresses a gas. The gas gets warm and then it is cooled by a fan. The heat goes out of the back of the fridge. Place your hand behind your refrigerator when it is running and you should feel a flow of warm air. The cooled gas is then allowed to expand inside coils on the cool side of the box. When gas expands, it gets even cooler than the air around it. Heat flows from the air to the coils. Then the gas is again compressed and cooled, starting the cycle again.

So what does this all mean for kitchen cooling? Initially, you will dump cool air from the refrigerator into the kitchen, dropping the average temperature of the room by some amount. Immediately, however, the thermostat in the refrigerator detects that the temperature inside has risen. The motor kicks on, the compressor kicks on, the system starts cooling the air in the box but it is quickly warmed by air coming
through the open door. The final result, thanks to the inefficiency of the heat pump and the dumping of the same heat that has been removed, is that the room heats by about 21⁄2 degrees for every degree that it is cooled.

Reference: The Science Of Everything – Steve Miller

Greenhouse design tips: sizing, style

How does the space station stay in orbit?

admin — Wed, 23 Dec 2009 16:18:29 +0000

Remember, due to inertia, a moving object continues moving in a straight line unlessa force acts on it. There is a force that acts on every object near Earth—the force of gravity. If you throw a ball, it does not continue moving indefinitely in a straight line because gravity causes it to move toward the center of the Earth. In a few seconds, the ball collides with Earth and the force of friction stops its motion.

The space station in its orbit also has inertia that causes it to tend to move in a straightline away from Earth. However, it is also subject to the force of gravity. It is pulled toward the center of the planet. This force is at a right angle to its inertial motion so it does not change the forward motion. It does pull the space station downward, just like any other falling object. While gravity is pulling downward, the tendency to move in
a straight line would carry it away from Earth. As a result, the space station is always falling but always the same distance from the surface. Gravity is the centripetal force on the satellite, pulling it toward the center of its orbit.

If some force were to slow the forward motion of the space station, gravity would pull it to Earth. Satellite orbits must be located above the
atmosphere. Otherwise, the force of friction with the air would slow the satellite, causing it to fall. If the force of gravity were suddenly to disappear, the space station would head away in a straight line.

Reference: The Science Of Everything – Steve Miller

What keeps riders in their seats on a roller coaster

admin — Wed, 23 Dec 2009 16:10:54 +0000

When you get into a roller coaster that loops upside down, the attendant makes sure that your safety belt or safety bar is in place. These devices are not what keeps you in place, though. Physics takes care of that.

Acceleration is a change in direction or speed. When something moves in a circle, it is constantly changing direction, so it is constantly accelerating, even if its speed remains the same. The force that causes the acceleration toward the center of a circle is called centripetal force. Think about what happens if you swing an object attached to a string in a circle. The string remains tight and its pull forces the object to move
in a circle. If you let go of the string, the object flies away. You can feel the constant force at the other end of the string.

The same thing happens when you ride a roller coaster through a loop. Because you are constantly changing direction, you are accelerating the whole time. The track is constantly pushing the car into a new direction. (It may seem strange that a stationary object can push, but in physics, forces come in pairs. The car pushes against the track and the track pushes against the car.) As a result, the seat of the car pushes you in the same direction.

Reference: The Science Of Everything – Steve Miller