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Text 1 The discovery of Universal Gravitation

Gravity is the attractive force exerted by all objects on all other objects. By the early seventeenth century, many forces had been identified: friction, gravity, air resistance, electrical, forces people exerted, etc. Newton’s mathematical concept of gravity was the first step in joining these seemingly different forces into a single, unified concept.

An apple fell; people had weight; the moon orbited Earth—all for the same reason. Newton’s law of gravity was a giant, simplifying concept. Newton’s concept of, and equations for, gravity stand as one of the most used concepts in all science. Most of our physics has been built upon Newton’s concept of universal gravitation and his idea that gravity is a fundamental property of all matter.

How Was It Discovered?

In 1666, Isaac Newton was a 23-year-old junior fellow at Trinity College in Cambridge. With his fair complexion and long blond hair, many thought he still looked more like a boy. His intense eyes and seemingly permanent scowl pushed people away.

In London, the bubonic plague ravaged a terrified population. Universities were closed, and eager academics like Isaac Newton had to bide their time in safe country estates waiting for the plague to loosen its death grip on the city. It was a frightening time. In his isolation, Newton was obsessed with a question: What held the moon circling the earth, and what held the earth in a captive orbit around the sun? Why didn’t the moon fall down to the earth? Why didn’t the earth fall down to the sun?

In later years Newton swore that this story actually happened. As he sat in the orchard at his sister’s estate, he heard the familiar soft “thunk” of an apple falling to the grass-carpeted ground, and turned in time to see a second apple fall from an overhanging branch and bounce once before settling gently into the spring grass. It was certainly not the first apple Isaac Newton had ever seen fall to the ground, nor was there anything at all unusual about its short fall. However, while it offered no answers to the perplexed young scientist, the falling apple did present Isaac with an important new question, “The apple falls to Earth while the moon does not. What’s the difference between the apple and the moon?

Next morning, under a clearing sky, Newton saw his young nephew playing with a ball. The ball was tied to a string the boy held tight in his fist. He swung the ball, slowly at first, and then faster and faster until it stretched straight out.

With a start Newton realized that the ball was exactly like the moon. Two forces acted on the ball—its motion (driving it outward) and the pull of a string (holding it in). Twoforces acted on the moon. Its motion and the pull of gravity—the same pull (force) that made the apple fall.

For the first time, Newton considered the possibility that gravity was a universal attractive force instead of a force that applied only to planets and stars. His deep belief in alchemy and its concept of the attraction of matter led him to postulate that gravitational attraction force did not just apply to heavenly objects, but to allobjects with any mass.

Gravity pulled apples to earth, made rain fall, and held planets in their orbits around the sun. Newton’s discovery of the concept of universal gravitation was a major blow to the prevalent belief that the laws of nature on Earth were different from those that ruled the heavens. Newton showed that the machinery that ruled the universe and nature is simple.

Newton developed universal gravitation as a property of all matter, not just of planets and stars. Universal gravitation and its mathematical expression lie at the foundation of all modern physics as one of the most important principles in all science.

Fun Facts:The Flower of Kent is a large green variety of apple. According to the story, this is the apple Isaac Newton saw falling to ground from its tree, inspiring his discovery of universal gravitation.

Text 2

Aerodynamics and Birds

Birds have provided humans with much information about heavier-than-air vehicle design. About 8,800 species of birds make up most living organisms capable of flight.

Most birds, however, fly well, and humans learned a lot about heavier-than-air vehicle design from observing them. Birds differ from heavier-than-air aircraft primarily in that their wings are movable, or flappable. Most aircraft have fixed wings, which do not move.

Birds’ bodies are specially engineered for flight. Their skeletons are light, often weighing less than their feathers. Feathers combine the qualities of lightness, strength, and flexibility; a feather, bent double, quickly regains its shape upon release. Made of keratin, feathers also keep birds warm, dry, and protected from injury.

Bird lungs and hearts are designed for the high metabolic rates needed to produce the huge amounts of energy required by all flying machines, biologic or manufactured.

To understand flight requirements, a background in aerodynamics, a branch of fluid dynamics that studies movement of bodies, such as birds or aircraft, through gases such as air, is essential. For example, the fifteenth century Italian artist and engineer Leonardo da Vinci studied bird flight and proposed to enable human beings to fly with flappable wings. His ideas failed because da Vinci knew nothing about aerodynamics, a science which did not exist then.

Any heavier-than-air flying vehicle must conquer gravity before it can climb into the air in controlled flight. Three main forces, exclusive of weight, are involved. The first is thrust, which birds produce by flapping their wings. Flapping merely enables a bird to move forward as long as its design allows enough thrust to exceed the drag caused by the viscosity of the air through which the bird moves. Drag diminishes the speed of moving objects due to air resistance. In vehicle design, thrust-to-drag ratios can be increased by streamlining to minimize drag. The third aerodynamic force, lift, is the key to flight. Lift, enabling an object’s rise into the air, operates upward perpendicular to the direction of forward motion, and is supplied in both birds and aircraft by wings and tails (airfoils).

Bird wings are designed so the angle at which they meet air passing them causes it to flow much more rapidly past the upper airfoil surface than past its lower surface.

This design lowers air pressure above the airfoil compared to that under it and engenders the lift that raises a bird into flight. In birds, this unsymmetrical airflow is produced by muscle movement that changes both the positions of wing feathers and the angle at which wings meet the air, known as the angle of attack.

Wing Design and Flight

Birds create lift with down strokes of their wings, attached by flight muscles to a large breastbone. Birds contract flight muscles to cause this down stroke, during which long primary and secondary flight feathers spread out to provide the maximum possible surface area to push against air below. The downstroke is followed by an upstroke in which the feathers fold to minimize air resistance while positioning the wings for the next downstroke. Bird wings have a short upper arm bone that moves up and down during flapping.

There are four basic types of bird flight. In skimming flight, birds such as albatrosses use winds to stay aloft. In soaring flight, birds such as eagles, hawks, and vultures can remain aloft for long periods of time, seeking prey below.

In active flight, birds such as swallows fly all day, flapping their wings continuously. Finally, game birds such as quail conceal themselves and, when endangered, burst into the sky. They pick up speed quickly and fly short distances before landing and hiding again. There is a wing shape most efficient for each flight type. Skimming birds have wings that are long, slender, and ribbon-shaped, with parallel edges and many secondary feathers. Skimming wings are the most highly developed, helping such birds ride the winds. Soaring birds have wings that are large, broad, almost square, and rich in primary feathers. Swallows and other birds engaging in active flight have long, tapering, pointy wings with broad bases and slender tips. Finally, game birds have short wings that beat rapidly, enabling to get to speed quickly. However, these wings are not useful in long flights. No bird has wings designed entirely for one type of flying.

However, in gliding, birds use gravity as thrust to overcome drag and move forward, as their wings produce lift to hold them up. Drag slows down a gliding bird and causes it to sink earthward.

Body Design and Flight

A second group of characteristics enabling bird flight is the design of the bird’s body. Body weight is important to flight: The heavier an object is, the larger its wings need to be to enable liftoff and maintain flight. In birds this problem is met by their relatively small, light bodies. For example, hawks and eagles have cat- or even dog-sized bodies, but they weigh only 25 to 35 percent as much as the earthbound mammals. This special anatomy, combined with wings that engender appropriate amounts of lift, allows birds to fly. Depending on their wing size and shape, birds can fly, soar, or skim.


Energy Needs

To meet the energy needs of flight, birds must eat a relatively large amount of food each day. For their muscles to work well, birds need efficient blood circulation to quickly supply fuel and oxygen and to remove wastes. Bird heartbeat rates are also much faster than those of mammals, usually from 200 to 1,000 beats per minute, compared to 80 in humans. Thus, with its wings; its small, light body; its superbly useful feathers; and its high-capacity heart and lungs, a bird is superbly designed to be airborne.

Text 3 Sergei Korolev

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