Thursday, September 8, 2016
Gravitation Newtons Theory And Early Ideas About Gravitation
Gravitation Newtons Theory And Early Ideas About Gravitation
Gravitation,
the force of attraction between all objects that tends to pull them toward one another. It is a universal force, affecting the largest and smallest objects, all forms of matter, and energy. Gravitation governs the motion of astronomical bodies. It keeps the moon in orbit around the earth and keeps the earth and the other planets of the solar system in orbit around the sun. On a larger scale, it governs the motion of stars and slows the outward expansion of the entire universe because of the inward attraction of galaxies to other galaxies. Typically the term gravitation refers to the force in general, and the term gravity refers to the earths gravitational pull.
Gravitation is one of the four fundamental forces of nature, along with electromagnetism and the weak and strong nuclear forces, which hold together the particles that make up atoms. Gravitation is by far the weakest of these forces and, as a result, is not important in the interactions of atoms and nuclear particles or even of moderate-sized objects, such as people or cars. Gravitation is important only when very large objects, such as planets, are involved. This is true for several reasons. First, the force of gravitation reaches great distances, while nuclear forces operate only over extremely short distances and decrease in strength very rapidly as distance increases. Second, gravitation is always attractive. In contrast, electromagnetic forces between particles can be repulsive or attractive depending on whether the particles both have a positive or negative electrical charge, or they have opposite electrical charges (see Electricity). These attractive and repulsive forces tend to cancel each other out, leaving only a weak net force. Gravitation has no repulsive force and, therefore, no such cancellation or weakening.
The gravitational attraction of objects for one another is the easiest fundamental force to observe and was the first fundamental force to be described with a complete mathematical theory by the English physicist and mathematician Sir Isaac Newton. A more accurate theory called general relativity was formulated early in the 20th century by the German-born American physicist Albert Einstein. Scientists recognize that even this theory is not correct for describing how gravitation works in certain circumstances, and they continue to search for an improved theory.
II EARTHS GRAVITATION
The gravitational attraction of every bit of matter in the earth for every other bit of matter amounts to an inward pull that holds the earth together against the pressure forces tending to push it outward. Similarly, the inward pull of gravitation holds stars together. When a stars fuel nears depletion, the processes producing the outward pressure weaken and the inward pull of gravitation eventually compresses the star to a very compact size (see Star, Black Hole).
A Acceleration
A bowling ball that weighs 16 lb is actually being pulled toward the earth with a force of 16 lb. In the metric system, the bowling ball is pulled toward the earth with a force of 71 newtons (a metric unit of force abbreviated N). The bowling ball also pulls on the earth with a force of 16 lb (71 N), but the earth is so massive that it does not move appreciably. In order to hold the bowling ball up and keep it from falling, a person must exert an upward force of 16 lb (71 N) on the ball. This upward force acts to oppose the 16 lb (71 N) downward weight force, leaving a total force of zero. The total force on an object determines the objects acceleration.
If the pull of gravity is the only force acting on an object, then all objects, regardless of their weight, size, or shape, will accelerate in the same manner. At the same place on the earth, the 16 lb (71 N) bowling ball and a 500 lb (2200 N) boulder will fall with the same rate of acceleration. As each second passes, each object will increase its downward speed by about 9.8 m/sec (32 ft/sec), resulting in an acceleration of 9.8 m/sec/sec (32 ft/sec/sec). In principle, a rock and a feather both would fall with this acceleration if there were no other forces acting. In practice, however, air friction exerts a greater upward force on the falling feather than on the rock and makes the feather fall more slowly than the rock.
The mass of an object does not change as it is moved from place to place, but the acceleration due to gravity, and therefore the objects weight, will change because the strength of the earths gravitational pull is not the same everywhere. The earths pull and the acceleration due to gravity decrease as an object moves farther away from the center of the earth. At an altitude of 4000 miles (6400 km) above the earths surface, for instance, the bowling ball that weighed 16 lb (71 N) at the surface would weigh only about 4 lb (18 N). Because of the reduced weight force, the rate of acceleration of the bowling ball at that altitude would be only one quarter of the acceleration rate at the surface of the earth. The pull of gravity on an object also changes slightly with latitude. Because the earth is not perfectly spherical, but bulges at the equator, the pull of gravity is about 0.5 percent stronger at the earths poles than at the equator.
III EARLY IDEAS ABOUT GRAVITATION
At the beginning of the 17th century, the Italian physicist and astronomer Galileo discovered that all objects fall toward the earth with the same acceleration, regardless of their weight, size, or shape, when gravity is the only force acting on them. Galileo also had a theory about the universe, which he based on the ideas of the Polish astronomer Nicolaus Copernicus. In the mid-16th century, Copernicus had proposed a heliocentric, or sun-centered system, in which the planets moved in circles around the sun, and Galileo agreed with this cosmology. However, Galileo believed that the planets moved in circles because this motion was the natural path of a body with no forces acting on it. Like the Greek philosophers, he saw no connection between the force behind planetary motion and gravitation on earth.
In the late 16th and early 17th centuries the heliocentric model of the universe gained support from observations by the Danish astronomer Tycho Brahe, and his student, the German astronomer Johannes Kepler. These observations, made without telescopes, were accurate enough to determine that the planets did not move in circles, as Copernicus had suggested. Kepler calculated that the orbits had to be ellipses (slightly elongated circles). The invention of the telescope made even more precise observations possible, and Galileo was one of the first to use a telescope to study astronomy. In 1609 Galileo observed that moons orbited the planet Jupiter, a fact that could not reasonably fit into an earth-centered model of the heavens.
The new heliocentric theory changed scientists views about the earths place in the universe and opened the way for new ideas about the forces behind planetary motion. However, it was not until the late 17th century that Isaac Newton developed a theory of gravitation that encompassed both the attraction of objects on the earth and planetary motion.
IV NEWTONS THEORY OF GRAVITATION
To develop his theory of gravitation, Newton first had to develop the science of forces and motion called mechanics. Newton proposed that the natural motion of an object is motion at a constant speed on a straight line, and that it takes a force to slow down, speed up, or change the path of an object. Newton also invented calculus, a new branch of mathematics that became an important tool in the calculations of his theory of gravitation.
Newton proposed his law of gravitation in 1687 and stated that every particle in the universe attracts every other particle in the universe with a force that depends on the product of the two particles masses divided by the square of the distance between them. The gravitational force between two objects can be expressed by the following equation: F= GMm/d2 where F is the gravitational force, G is a constant known as the universal constant of gravitation, M and m are the masses of each object, and d is the distance between them. Newton considered a particle to be an object with a mass that was concentrated in a small point. If the mass of one or both particles increases, then the attraction between the two particles increases. For instance, if the mass of one particle is doubled, the force of attraction between the two particles is doubled. If the distance between the particles increases, then the attraction decreases as the square of the distance between them. Doubling the distance between two particles, for instance, will make the force of attraction one quarter as great as it was.
According to Newton, the force acts along a line between the two particles. In the case of two spheres, it acts along the line between their centers. The attraction between objects with irregular shapes is more complicated. Every bit of matter in the irregular object attracts every bit of matter in the other object. A simpler description is possible near the surface of the earth where the pull of gravity is approximately uniform in strength and direction. In this case there is a point in an object (even an irregular object) called the center of gravity, at which all the force of gravity can be considered to be acting.
Newtons law affects all objects in the universe, from raindrops in the sky to the planets in the solar system. It is therefore known as the universal law of gravitation. In order to know the strength of gravitational forces in general, however, it became necessary to find the value of G, the universal constant of gravitation. Scientists needed to perform an experiment, but gravitational forces are very weak between objects found in a common laboratory and thus hard to observe. In 1798 the English chemist and physicist Henry Cavendish finally measured G with a very sensitive experiment in which he nearly eliminated the effects of friction and other forces. The value he found was 6.754 x 10-11 N-m2/kg2—close to the currently accepted value of 6.670 x 10-11 N-m2/kg2 (a decimal point followed by 10 zeros and then the number 6670). This value is so small that the force of gravitation between two objects with a mass of 1 metric ton each, 1 meter from each other, is about 67 millionths of a newton, or about 15 millionths of a pound.
Gravitation may also be described in a completely different way. A massive object, such as the earth, may be thought of as producing a condition in space around it called a gravitational field. This field causes objects in space to experience a force. The gravitational field around the earth, for instance, produces a downward force on objects near the earth surface. The field viewpoint is an alternative to the viewpoint that objects can affect each other across distance. This way of thinking about interactions has proved to be very important in the development of modern physics.
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