# Newton's Laws

Figure 2.17(1642–1727). Isaac Newton(The Granger Collection) |

Kepler's three laws, which so simplified the solar system, were discovered

*empirically.*In other words, they resulted solely from the analysis of observational data and were not derived from any theory or mathematical model. Indeed, Kepler did not have any appreciation for the physics underlying his laws. Nor did Copernicus understand the basic reasons*why*his heliocentric model of the solar system worked. Even Galileo, often called the father of modern physics, failed to understand why the planets orbit the Sun (although Galileo's work laid vital groundwork for Newton's theories—see*More Precisely 2-2*).What prevents the planets from flying off into space or from falling into the Sun? What causes them to revolve about the Sun, apparently endlessly? To be sure, the motions of the planets obey Kepler's three laws, but only by considering something more fundamental than those laws can we really understand these motions. The heliocentric system was secured when, in the seventeenth century, the British mathematician Isaac Newton (Figure 2.17) developed a deeper understanding of the way

*all*objects move and interact with one another as they do.#### THE LAWS OF MOTION

Isaac Newton was born in Lincolnshire, England, on Christmas Day in 1642, the year Galileo died. Newton studied at Trinity College of Cambridge University, but when the bubonic plague reached Cambridge in 1665, he returned to the relative safety of his home for two years. During that time he made probably the most famous of his discoveries, the law of gravity (although it is but one of the many major scientific advances for which Newton was responsible). However, either because he regarded the theory as incomplete or possibly because he was afraid that he would be attacked or plagiarized by his colleagues, he did not tell anyone of his monumental achievement for almost 20 years. It was not until 1684, when Newton was discussing with Edmund Halley (of Halley's comet fame) the leading astronomical problem of the day—Why do the planets move according to Kepler's laws?—that he astounded his companion by revealing that he had solved the problem in its entirety nearly two decades before!.

Prompted by Halley, Newton published his theories in perhaps the most influential physics book ever written:

*Philosophiae Naturalis Principia Mathematica*(*The Mathematical Principles of Natural Philosophy*—what we would today call "science"), usually known simply as Newton's*Principia.*The ideas expressed in that work form the basis for what is now known as**Newtonian mechanics**. Three basic laws of motion, the law of gravity, and a little calculus (which Newton also developed) are sufficient to explain and quantify virtually all the complex dynamic behavior we see on Earth and throughout the universe.Figure 2.18 illustratesNewton'sfirst law of motion:

I. Every body continues in a state of rest or in a state of uniform motion in a straight line unless it is compelled to change that state by a force acting on it.

Figure 2.18 An object at rest will remain at rest (a) until some force acts on it (b). It will then remain in that state of uniform motion until another force acts on it. The arrow in (c) shows a second force acting at a direction different from the first, which causes the object to change its direction of motion. Newton's First Law |

The first law simply states that a moving object will move forever in a straight line unless some external

**force**—a push or a pull—changes its direction of motion. For example, the object might glance off a brick wall or be hit with a baseball bat; in either case, a force changes the original motion of the object. The tendency of an object to keep moving at the same speed and in the same direction unless acted upon by a force is known as**inertia**. A familiar measure of an object's inertia is its**mass**—loosely speaking, the total amount of matter it contains. The greater an object's mass, the more inertia it has, and the greater is the force needed to change its state of motion.Newton's first law implies that it requires no force to maintain motion in a straight line with

*constant velocity*—that is, motion with constant speed and constant direction in space. This contrasts sharply with the view of Aristotle, who maintained (incorrectly) that the natural state of an object was to be*at rest*—most probably an opinion based on Aristotle's observations of the effect of friction. In our discussion we will neglect friction—the force that slows balls rolling along the ground, blocks sliding across tabletops, and baseballs moving through the air. In any case, it is not an issue for the planets because there is no appreciable friction in outer space. The fallacy in Aristotle's argument was first realized and exposed by Galileo, who conceived of the notion of inertia long before Newton formalized it into a law.The rate of change of the velocity of an object—speeding up, slowing down, or simply changing direction—is called its

**acceleration**, and is the subject of*Newton's second law,*which states that the acceleration of an object is directly proportional to the applied force and inversely proportional to its mass:II. When a forceFacts on a body of massm,it produces in it an acceleration a equal to the force divided by the mass. Thus,a=F/m,orF=ma.

Figure 2.19 A ball thrown up from the surface of a massive object such as a planet is pulled continuously by the gravity of that planet (and, conversely, the gravity of the ball continuously pulls the planet). Gravity |

Thus, the greater the force acting on the object, or the smaller the mass of the object, the greater its acceleration. If two objects are pulled with the same force, the more massive one will accelerate less; if two identical objects are pulled with different forces, the one experiencing the greater force will accelerate more. In honor of Newton, the SI unit of force is named after him. By definition, 1 newton (N) is the force required to cause a mass of 1 kilogram to accelerate at a rate of one meter per second every second (1 m/s

^{2}). One newton is approximately 0.22 pounds.Finally,

*Newton's third law*simply tells us that forces cannot occur in isolation:III. To every action there is an equal and opposite reaction.

In other words, if body A exerts a force on body B, then body B necessarily exerts a force on body A that is equal in magnitude, but oppositely directed.

Only in extreme circumstances—when velocities approach the speed of light—do Newton's laws break down, and this fact was not realized until the twentieth century, when Albert Einstein's theories of relativity once again revolutionized our view of the universe (see Chapter 22). Most of the time, however, Newtonian mechanics provides an excellent description of the motion of planets, stars, and galaxies through the cosmos.

#### GRAVITY

Forces may act

*instantaneously*or*continuously.*The force from a baseball bat that hits a home run can be thought of as being instantaneous. A good example of a continuous force is the one that prevents the baseball from zooming off into space—**gravity**, the phenomenon that started Newton on the path to the discovery of his laws. Newton hypothesized that any object having mass always exerts an attractive*gravitational force*on all other massive objects. The more massive an object, the stronger its gravitational pull.Consider a baseball thrown upward from Earth's surface, as illustrated in Figure 2.19. In accordance with Newton's first law, the downward force of Earth's gravity continuously modifies the baseball's velocity, slowing the initial upward motion and eventually causing the ball to fall back to the ground. Of course, the baseball, having some mass of its own, also exerts a gravitational pull on Earth. By Newton's third law, this force is equal and opposite to the

*weight*of the ball (the force with which Earth attracts it). But, by Newton's second law, Earth has a much greater effect on the light baseball than the baseball has on the much more massive Earth. The ball and Earth feel the same gravitational force, but Earth's*acceleration*is much smaller.Now consider the trajectory of the same baseball batted from the surface of the Moon. The pull of gravity is about one-sixth as great on the Moon as on Earth, so the baseball's velocity changes more slowly—a typical home run in a ballpark on Earth would travel nearly half a mile on the Moon. The Moon, less massive than Earth, has less gravitational influence on the baseball. The magnitude of the gravitational force, then, depends on the

*masses*of the attracting bodies. In fact, the force is*directly proportional*to the product of the two masses.Studying the motions of the planets reveals a second aspect of the gravitational force. At locations equidistant from the Sun's center, the gravitational force has the same strength, and it is always directed toward the Sun. Furthermore, detailed calculation of the planets' accelerations as they orbit the Sun reveals that the strength of the Sun's gravitational pull decreases in proportion to the

*square*of the distance from the Sun. (See also*More Precisely 2-2*for the line of reasoning that originally led Newton to this conclusion.) The force of gravity is said to obey an**inverse-square law**. As shown in Figure 2.20(b), inverse-square forces decrease rapidly with distance from their source. For example, tripling the distance makes the force 3^{2}= 9 times weaker, while multiplying the distance by five results in a force that is 5^{2}= 25 times weaker. Despite this rapid decrease, the force never quite reaches zero. The gravitational pull of an object having some mass can never be completely extinguished.We can combine the preceding statements about mass and distance to form a law of gravity that dictates the way in which

*all*material objects attract one another:Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them.

As a proportionality, Newton's law of gravity is

The quantity

6.67 x 10

*G*is known as the*gravitational constant,*or often simply as Newton's constant. It is one of the fundamental constants of the universe. Its value has been measured in extremely delicate laboratory experiments as6.67 x 10

^{-11}newton meter^{2}/kilogram^{2}(N m^{2}/kg^{2}).To Newton, gravity was a force that acted at a distance, with no obvious way in which it was actually transmitted from place to place. Newton was not satisfied with this explanation, but he had none better. To appreciate the modern view of gravity, consider any piece of matter having some mass—it could be smaller than an atom or larger than a galaxy. Extending outward from this object in all directions is a

**gravitational field**produced by the matter. We now regard such a field as a property of space itself—a property that determines the influence of one massive object on another. All other matter "feels" the field as a gravitational force.#### PLANETARY MOTION

The mutual gravitational attraction between the Sun and the planets, as expressed by Newton's law of gravity, is responsible for the observed planetary orbits. As depicted in Figure 2.21, this gravitational force continuously pulls each planet toward the Sun, deflecting its forward motion into a curved orbital path. Because the Sun is much more massive than any of the planets, it dominates the interaction. We might say that the Sun "controls" the planets, not the other way around.

The Sun–planet interaction sketched here is analogous to what occurs when you whirl a rock at the end of a string above your head. The Sun's gravitational field is your hand and the string, and the planet is the rock at the end of that string. The tension in the string provides the force necessary for the rock to move in a circular path. If you were suddenly to release the string—which would be like eliminating the Sun's gravity—the rock would fly away along a tangent to the circle, in accordance with Newton's first law.

In the solar system, at this very moment, Earth is moving under the combined influence of these two effects: the competition between gravity and inertia. The net result is a stable orbit, despite our continuous rapid motion through space. (In fact, Earth orbits the Sun at a speed of about 30 km/s, or about 70,000 mph. Verify this for yourself by calculating how fast Earth must move to complete a circle of radius 1 A.U.—and hence of circumference 2π A.U., or 940 million km—in 1 year, or 3.2 x 10

^{7}seconds. The answer is 9.4 x 10^{8}km/3.2 x 10^{7 }s, or 29.4 km/s.)*More Precisely 2-3*describes how astronomers can use Newtonian mechanics and the law of gravity to quantify planetary motion and measure the masses of Earth, the Sun, and many other astronomical objects by studying the orbits of objects near them.#### KEPLER'S LAWS RECONSIDERED

Newton's laws of motion and the law of universal gravitation provided a theoretical explanation for Kepler's empirical laws of planetary motion. Kepler's three laws follow directly from Newtonian mechanics, as solutions to the equations describing the motion of a body moving in response to an inverse-square force. However, just as Kepler modified Copernican model by introducing ellipses rather than circles, so too did Newton make corrections to Kepler's first and third laws. It turns out that a planet does not orbit the exact center of the Sun. Instead, both the planet and the Sun orbit their common

**center of mass**. Because the Sun and the planet feel equal and opposite gravitational forces (by Newton's third law), the Sun must also move (by Newton's first law), driven by the gravitational influence of the planet. The Sun is so much more massive than any planet that the center of mass of the planet–Sun system is very close to the center of the Sun, which is why Kepler's laws are so accurate. Thus, Kepler's first law becomesI. The orbit of a planet around the Sun is an ellipse, with thecenter of mass of the planet–Sunsystem at one focus.

As shown in Figure 2.22, however, the center of mass for two objects of comparable mass does not lie within either object. For identical masses (Figure 2.22a), the orbits are identical ellipses, with a common focus located midway between the two objects. For unequal masses (as in Figure 2.22b), the elliptical orbits still share a focus and both have the same eccentricity, but the more massive object moves more slowly and on a tighter orbit. (Note that Kepler's second law, as stated earlier, continues to apply without modification to each orbit separately, but the

*rates*at which the two orbits sweep out area are different.) In the extreme case of a planet orbiting the much more massive Sun (see Figure 2.22c), the path traced out by the Sun's center lies entirely within the Sun itself.The change to Kepler's third law is also small in the case of a planet orbiting the Sun, but very important in other circumstances, such as the orbital motion of two stars that are gravitationally bound to each other. Following through the mathematics of Newton's theory, we find that the true relationship between the semi-major axis

*a*(measured in astronomical units) of the planet's orbit relative to the Sun and its orbital period*P*(in Earth years) iswhere

*M*

_{total}is the

*combined*mass of the two objects. Notice that Newton's restatement of Kepler's third law preserves the proportionality between

*P*

^{2}and

*a*

^{3}, but now the proportionality includes

*M*

_{total}, so it is

*not*quite the same for all the planets. The Sun's mass is so great, however, that the differences in

*M*

_{total}among the various combinations of the Sun and the other planets are almost unnoticeable, so Kepler's third law, as originally stated, is a very good approximation. This modified form of Kepler's third law is true in all circumstances, inside or outside the solar system.

The law of gravity that describes the orbits of planets around the Sun applies equally well to natural moons and artificial satellites orbiting any planet. All our Earth-orbiting, human-made satellites move along paths governed by a combination of the inward pull of Earth's gravity and the forward motion gained during the rocket launch. If the rocket initially imparts enough speed to the satellite, it can go into orbit. Satellites not given enough speed at launch (such as intercontinental ballistic missiles, ICBMs) fail to achieve orbit and fall back to Earth (see Figure 2.23). (Technically, ICBMs actually do orbit Earth's attracting center, but their orbits intersect Earth's surface.)

Some space vehicles, such as the robot probes that visit other planets, attain enough speed to escape our planet's gravitational field and move away from Earth forever. This speed, known as the

Some space vehicles, such as the robot probes that visit other planets, attain enough speed to escape our planet's gravitational field and move away from Earth forever. This speed, known as the

**escape speed**, is about 41 percent greater (actually, = 1.414… times greater) than the speed of a circular orbit at any given radius.* At less than the escape speed, the old adage "what goes up must come down" (or at least stay in orbit) still applies. At more than the escape speed, however, a spacecraft will leave Earth for good. Planets, stars, galaxies—all gravitating bodies—have escape speeds. No matter how massive the body, gravity decreases with distance. As a result, the escape speed diminishes with increasing separation. The farther we go from Earth (or any gravitating body), the easier it becomes to escape.The speed of a satellite in a circular orbit just above Earth's atmosphere is 7.9 km/s (roughly 18,000 mph). The satellite would have to travel at 11.2 km/s (about 25,000 mph) to escape from Earth altogether. If an object exceeds the escape speed, its motion is said to be

**unbound**, and the orbit is no longer an ellipse. In fact, the path of the spacecraft relative to Earth is a related geometric figure called a*hyperbola.*If we simply change the word*ellipse*to*hyperbola,*the modified version of Kepler's first law still applies, as does Kepler's second law. (Kepler's third law does not extend to unbound orbits because it doesn't make sense to talk about a period in those cases.)Newton's laws explain the paths of objects moving at any point in space near any gravitating body. These laws provide a firm physical and mathematical foundation for Copernicus's heliocentric model of the solar system and for Kepler's laws of planetary motions. But they do much more than that. Newtonian gravitation governs not only the planets, moons, and satellites in their elliptical orbits but also the stars and galaxies in their motion throughout our universe—as well as apples falling to the ground.

*In terms of the formula presented in More Precisely 2-3, the escape speed is given by v*

_{escape}=
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