Physics

What is radioactive half-life?

The half-life of a substance is the time it takes for the number of radioactive nuclei to decrease to one half of the original number. The halflife of a given isotope is always the same, meaning it doesn’t matter how many you have at any given time. Read more…

Where do comets come from?

Mathematical theory suggests that most comets may come to the solar system from very far away, as far away as 100,000 Astronomical Units. In this picture, the solar system is buried deep within the cloud.

An Astronomical Unit (or AU) is the distance from the earth to the sun and is equivalent to about 93,000,000 miles. Mars is 1.5 AU from the sun, Jupiter is 5 AU from the sun, and Pluto is 39 AU from the sun. So comets come from very far away indeed.

Comets are observed to come to the solar system from all directions, therefore the place where the comets come from is thought to be a giant sphere surrounding the solar system. This sphere is called the Oort cloud after Jan Oort who suggested its existence in 1950.

But some comets may come to the solar system from closer in. The place where these comets come from is called the Kuiper Belt, which is located past the orbit of Pluto. Source: the University Corporation for Atmospheric Research.

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What is Doppler Radar?

What does Doppler radar mean and how is it different from other radar?

RADAR, an acronym for Radio Detection and Ranging, operates by transmitting a wave and recording the time it takes that wave to bounce off of an object and return to the source. Since we know the speed the transmitted wave is traveling, we can calculate the distance of the object.

Doppler radar operates on the same principle, but it also detects an objects motion by measuring the frequency shift between the outgoing wave and the returning wave. An object moving toward the radar would increase the returning wave’s frequency while an object moving away from the radar decreases the wave’s frequency. For weather purposes, this provides important information about the speed and direction of winds within thunderstorms.

Source: USA Today, Ask the Weather Experts. Posted by admin for the best selling toys of 2008 at Atomic Elephant Science & Toy Co.

Why does a curveball curve?

The principle factor affecting a moving (and spinning) baseball is air drag. Imagine a standard fastball thrown with a straight overhead motion. The ball will be spinning naturally on a horizontal axis (with the top of the ball rotating back toward the pitcher). This spin causes the magnitude of the drag vectors to be different near the top half of the ball than they are on the bottom half. Without the spin, the drag force would be equal on both hemispheres of the ball… Read more…

How does the thermodynamic drinking bird work?

For centuries, students and inventors alike have been intrigued by the idea of a perpetual motion machine. Alas, the second law of thermodynamics has held up to the test of time. It can be written in several forms but Rudolf Clausius may have said it best for our purposes: in an isolated system, a process will only occur if it increases the total entropy of the system. In other words, heat will not naturally flow from a body of lower temperature to one of higher. It will however, flow in the other direction.

So what does all this have to do with our classic drinking bird? The answer: plenty. Couple this law of thermodynamics with Boyle’s law stating the inversely proportional relationship of temperature and pressure relating to volume and you can begin to understand how this magical little bird can seemingly bob up and down forever.

Our thermodynamic, entropy-loving, pressure-, temperature-, and volume-driven machine (the bird) is quite a fascinating creature. Most machines– refrigerators, cars, nuclear reactors– produce work by creating this temperature and pressure differential. Perhaps by igniting a combustible gas, using an electric motor to compress a gas, or by splitting an atom. The bird creates this differential by dipping its beak in a glass of water. Not as intellectually exciting as smashing electrons and protons, but a temperature differential nonetheless.

Image from the Dept of Physics, the Chinese University of Hong Kong.

Just exactly how does our drinking bird do it? First, he’s made of two glass bulbs connected with a glass tube. The top bulb (the bird’s head) is a simple reservoir with the tube extending from that bulb down most of the way into the lower bulb (the bird’s belly). The system is partially filled with a liquid of low boiling point. When in an upright equilibrium position, the fluid is in the lower bulb and the vapor between the lower and upper bulbs is separated. In this position there is no temperature differential between the two bulbs.

But who wants a bulbous glass bird at perfect equilibrium? The trick is to change the temperature differential between the head and belly. This could be done by either warming the lower bulb (the body heat from your hand would do the trick) or cooling the top bulb. We’ll get our feathered friend started by wetting his head (cooling the top bulb).

Since his head and beak are covered with a thin felt that wicks the water around the bulb when he takes a drink, the subsequent evaporation cools the bulb and creates a temperature (and thus a pressure) difference between the bulbs. With a lower pressure in his head, the fluid starts rising from the lower bulb- there’s that second law of thermodynamics again with the system naturally tending toward an increase in entropy.

When enough fluid has collected in the top reservoir, the center of gravity has changed enough that the bird starts leaning forward. Right about the time he becomes horizontal, the tube in the lower bulb is no longer obstructed by the liquid in the lower bulb and the two pressure chambers equalize allowing the fluid to drain back down to the lower bulb. Now the trick that keeps it going is that when the bird was horizontal, it dipped its beak into the glass of water, wicking more fluid around the top bulb, causing it to cool again, and thus start the cycle over.

Do you think when Robert Boyle published his gas law in 1662 he had any idea it would help create this intriguing little toy that has fascinated folks for generations? Probably so. He was a pretty sharp scientist after all. When you think about it, there are several physical principles at work in this system. I can think of at least six without even straining my brain:

-The capillary action of the wicking felt
-The center of mass and torque around the pivot
-The ideal gas law (the relationship between gas particles and pressure)
-Boyle’s Law (the relationship between temperature and pressure)
-Maxwell-Boltzmann equation (molecules at a given temperature can exist in different phases)
-Latent heat of vaporization (heat transfers when a substance changes states

Can you think of any more principles at work here?

What is echolocation?

Echolocation is the ability to locate objects by bouncing sound waves off of them, and then measuring the time taken for an echo to return, and calculating the direction the echo came from.

These sound waves are very high-pitched, and most humans are unable to hear them. A device called a bat detector is able to pick up these sounds and convert them into sounds which we can hear as a series of clicks, pops and whistles.

Source: http://www.uksafari.com/bats4.htm.

What is Hubble’s Constant?

Hubble’s Constant is the rate ratio of the speed at which a galaxy is moving away from Earth divided by its distance from Earth. Note, this is obviously not our galaxy, but other galaxies in the Universe.