Who was Robespierre?

Maximilien Robespierre was an important leader during the French Revolution. He was part of the Commune of Paris that overthrew Louis XVI. Under his orders, protesters were paraded and guillotined in public. On 28 July 1794, Robespierre himself was guillotined without A trial.

Which mammals fly?

There is only one mammal that flies: the bat. Most bats are nocturnal, which means they sleep during the day and are most active at dawn, dusk, or nighttime. During the day, they sleep by hanging upside down in groups called roots. Most bats, called microbats, eat flying insects, such as moths and flies, but others eat small mammals, like mice. Some insect-eating bats can land on the ground and chase insects that live in leaf litter or dirt. One of these bats, the pallid bat, feeds on scorpions and large centipedes. Others eat fish or live on cow’s blood. The largest bats are megabats, which feed mostly on fruit.

What do A.M. and P.M. stand for?

The initials A.M. stand for ante meridian, which is Latin for "before noon." The initials P.M. stand for post meridian, which is Latin for "after noon."

How do you get static out of hair?

If you put a conditioner on your hair, it will attract enough moisture to allow static charge to dissipate.

Why do we say “Goodnight sleep tight”?

Sometime during the sixteenth century, British farmers moved from sleeping on the ground to sleeping in beds. These beds were little more than straw-filled mattress tied to wooden frames with ropes. To secure the mattress before sleeping, you pulled on the ropes to tighten them, and that’s when they began saying, “Goodnight, sleep tight.”

What happens to the current when it "stops"?

Current refers to moving charged particles. In most solids, the particles that do the moving are negatively charged electrons that move in the opposite direction from the way we say that current is flowing. These charged particles are the components of atoms and molecules, so they are always there inside a wire or the filament of a light bulb, even if they are not moving. Thus when the current "stops", these electrically charged particles simply stop moving. You can imagine a pipe full of water. The water can be flowing to the right or left (a current) or it can be standing still (no current). The water itself, like the charged particles, doesn't disappear when the flow stops.

I have read articles about research into anti-gravity. Do you think it is really possible?

No, I don't think that anti-gravity is possible. The interpretation of gravity found in Einstein's General Theory of Relativity is as a curvature of space-time around a concentration of mass/energy. That curvature has a specific sign, leading to what can be viewed as an attractive force. There is no mechanism for reversing the sign of the curvature and creating a repulsive force—anti-gravity. I know of only one case, involving a collision between two rapidly spinning black holes, in which two objects repel one another through gravitational effects. But that bizarre case is hardly the anti-gravity that people would hope to find.

How do black holes work?

As you assemble more and more mass together in a small volume, the gravity there becomes stronger and stronger. At first, it becomes more and more difficult to throw a ball upward hard enough to make it sail away from the mass into space. Eventually, you need a cannon to get the ball to leave. And by the time you get enough mass together, the gravity becomes so strong that light itself begins to have trouble escaping. Light falls in gravity, just like anything else. But it travels so fast that you barely notice it falling. However when the gravity becomes strong enough, light falls enough to cause some weird effects. A black hole forms when the gravity is so strong the even light is unable to escape from the mass.

Why do only certain orbitals exist in an atom?

Because the electrons in an atom move about as waves, they can follow only certain allowed orbits that we call orbitals. This limitation is equivalent to the case of a violin string—it can only vibrate at certain frequencies. If you try to make a violin string vibrate at the wrong frequency, it won't do it. That's because the string vibrates in a wave-like manner and only certain waves fit properly along the strong. Similarly, the electron in an atom "vibrates" in a wave-like manner and only certain waves fit properly around the nucleus.

Why is the Ming dynasty so famous?

The Mongols, who ruled China for 89 years, were overthrown by Hung-Wu in AD 1368. That marked the start of the Ming dynasty, which lasted until 1644. The Ming dynasty is remembered for making the administration strong, widespread and fair. Judges had to take a test to get a job. Chinese literature, art and philosophy reached new heights during Ming rule. Some of the best Chinese porcelain was manufactured at Jingdezhen. The yellow imperial bowls, red vases and highly decorated painted ceramics became popular. For the first time, both cotton and silk production did very well.

How hot is the Sun?

The Sun is extremely hot. The surface of the Sun (or its outer visible layer, called the photosphere) is about 10,000 degrees Fahrenheit (5,537 degrees Celsius)— about 50 times the temperature required to boil water. The core of the Sun, where solar energy is created, reaches 27 million degrees Fahrenheit (15 million degrees Celsius). It is so intense that nuclear reactions take place there.

Is there a simple rhyme to help recall how many days are in each month?

Yes. Many children learn this simple poem to help them remember how many days are in each month. Although the origin of the lyrics to "Thirty Days Hath September" is unclear, the use of old English dates this poem to at least the sixteenth century: Thirty days hath September, April, June and November All the rest have thirty-one, excepting only February Which hath but twenty eight-days clear And twenty-nine in each leap year.

Will the dinosaurs ever come back?

When the dinosaurs became extinct, the world’s environment was already changing. It was stressed by natural events, such as falling sea levels and volcanic eruptions, leading to the decline and disappearance of many plants and animals. Our modern world has continued to create many environmental changes and stresses— most man-made, including pollution and global warming—and our fragile ecosystem could not support the return of dinosaurs. (Imagine a large dinosaur roaming New York City!) However, some scientists argue that not all dinosaurs became extinct. The striking similarities between modern birds and some kinds of dinosaurs have led some people to believe that birds are living descendants of dinosaurs. Although not all dinosaurs were similar to modern birds, some did have features such as bony tails, claws on the fingers, beaks, and feathers.

How do I speak?

Human beings have vocal chords inside the larynx which produce sound. When air passes through a gap between the chords, these chords vibrate and produce a sound. All animals that can produce a sound have vocal chords, except birds which produce sound through a bony ring, called a syrinx.

How does the horizontal sync signal work?

The brightness information comes to the television as a steady stream. While the television knows that this information should control the brightness of adjacent spots on the screen, from left to right, it needs to be told when each horizontal line begins and when each vertical sweep begins. It knows that a new line is coming when the brightness information contains a "blacker-than-black" level. This level seems to say that the electron gun should not only stop sending electrons at the screen, it should send less than no electrons at the screen! Actually, this level is an instruction to the television's electronics, telling the television to bring its electron beam back to the left side of the screen to begin a new horizontal line. A long "blacker-than-black" level is an instruction to the television to begin a new vertical scan down the screen.

What is drag force?

A drag force is a force that opposes an object's motion through a fluid. Like sliding friction, drag always pushes the object in the direction opposite its motion though the fluid. Air resistance is really a drag force. You feel drag pushing you backward when you ride a bicycle fast. You also feel drag when you hold your hand out the window of a fast-moving car—it pushes your hand toward the back of the car and in the direction opposite your hand's motion through the air. If you were to fall downward, you would feel a drag force upward, in the direction opposite your motion through the air. And leaves experience a drag force when wind blows on them—pushing them downwind and in the direction opposite their motion through the air (they are moving upwind through the air, so it pushes them downwind). Incidentally, the object pushes back on the fluid with drag force, too, and this force on the fluid pushes the fluid in the direction opposite its motion past the object. This force tends to stop moving fluids and to turn their kinetic energies into thermal energy.

Why do we say that a political candidate on a speaking tour is “on the stump”?

When early European settlers were moving west and clearing the land, every farm had an abundance of tree stumps in their fields. “Barnstorming” politicians who looked for a place of prominence to be seen and heard by the gathered electorate would invariably find a large tree stump to stand on from which he would make his pitch. This gave us the expression “on the stump,” which is still used to describe a politician seeking election.

Did different ancient civilizations have their own art?

Each ancient civilization, including Egypt, Mesopotamia, China, Greece, Rome and India, had its unique art style. Art was used for worship, but it also came to be seen as a form of beauty. Paintings of Egyptian gods and goddesses covered the insides of pyramids. The Mesopotamians covered the walls of their temples with paintings. The Greeks studied the human body closely, and their paintings and sculptures include details of muscles and proportion. The Chinese were among the first to paint on paper and used painting to develop the script for the Chinese language. Indian artists were highly skilled at carving and painting on walls.

What is a magnetic field?

The area around a permanent magnet has a force that can affect other magnets or magnetic materials that come near it. This area is called a magnetic field. Even the Earth has a magnetic field. It is like a huge bar magnet. Even if you cut a huge bar magnet into tiny pieces, each piece is still a magnet with its own small magnetic field.

How do helicopters fly with such small wings without them breaking off?

As you suggest, the blades of a helicopter are really rotating wings. But unlike the wings of a normal airplane, the helicopter blades are always moving through the air, even when the helicopter's body is not. That's why a helicopter can obtain an upward "lift" force from the air while it's hovering motionless—the wings keep moving and obtaining that lift force. A second difference between a helicopter's rotating blades and the wings of a normal aircraft is that a helicopter's blades are under enormous tension. Were it not for this tension, the end of each blade would naturally travel in a straight line at constant speed, a behavior that we associate with inertia—objects that are free of outside forces travel at constant velocity (they follow straightline paths at constant speeds). To make the end of its blade travel in a circle (which is certainly not a straight line), the helicopter must pull the end of the blade toward the pivot about which the blade is turning. Thus as the blades turn, each blade experiences an enormous tension pulls the parts of the blade toward the pivot. This tension is what stiffens the blade, just as tension stiffens the strings of a guitar or a violin. Just as it's hard to break a guitar string by bending it, it's hard to break a helicopter blade by bending it. However, both guitar strings and helicopter blades will snap if they're exposed to more tension than they can tolerate. The manufacturers of the blades work hard to make each blade strong enough to withstand the enormous tension it experiences in use. As long as the blades can tolerate this tension, they won't break and will have no trouble supporting the body of the helicopter.

Do neon lights have glass that is not colored, but has phosphors that emit a particular color?

A true neon light tube has completely clear (no color, no phosphor) glass surrounding a thin gas of neon atoms. When current runs through that gas, the neon atoms emit red light. In "neon tubes" that emit colors other than red (green, pink, orange, yellow, etc.), there is a layer of phosphor on the inside surface of the glass and mercury vapor inside the tube. These fluorescent tubes probably don't contain any neon at all. You can see the light coming from the phosphor coating. In a true neon tube, you can see the light coming from the gas itself, well inside the glass tube.