# Newton s Law of Universal Gravitation

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1 12.1 Newton s Law of Universal Gravitation SECTION Explain Kepler s laws. Describe Newton s law of universal gravitation. Apply Newton s law of universal gravitation quantitatively. KEY TERMS OUTCOMES Tychonic system Kepler s laws law of universal gravitation In previous science courses, you learned about the Ptolemaic system for describing the motion of the planets and the Sun. The system developed by Ptolemy ( B.C.E.) was very complex because it was geocentric. That is, it placed Earth at the centre of the universe. In 1543, Nicholas Copernicus ( ) proposed a much simpler, heliocentric system for the universe in which Earth and all of the other planets revolved around the Sun. The Copernican system was rejected by the clergy, however, because the religious belief system at the time placed great importance on humans and Earth as being central to a physically perfect universe. You probably remember learning that the clergy put Galileo Galilei ( ) on trial for supporting the Copernican system. Have you ever heard of the Tychonic system? A famous Danish nobleman and astronomer, Tycho Brahe ( ), proposed a system, shown in Figure 12.1, that was intermediate between the Ptolemaic and Copernican systems. In Brahe s system, Earth is still and is the centre of the universe; the Sun and Moon revolve around Earth, but the other planets revolve around the Sun. Brahe s system captured the interest of many scientists, but never assumed the prominence of either the Ptolemaic or Copernican systems. Nevertheless, Tycho Brahe contributed a vast amount of detailed, accurate information to the field of astronomy. Figure 12.1 The Tychonic universe was acceptable to the clergy, because it maintained that Earth was the centre of the universe. The system was somewhat satisfying for scientists, because it was simpler than the Ptolemaic system. 574 MHR Unit 5 Force, Motion, Work, and Energy

2 Laying the Groundwork for Newton Astronomy began to come of age as an exact science with the detailed and accurate observations of Tycho Brahe. For more than 20 years, Brahe kept detailed records of the positions of the planets and stars. He catalogued more than 777 stars and, in 1572, discovered a new star that he named Nova. Brahe s star was one of very few supernovae ever found in the Milky Way galaxy. In 1577, Brahe discovered a comet and demonstrated that it was not an atmospheric phenomenon as some scientists had believed, but rather that its orbit lay beyond the Moon. In addition to making observations and collecting data, Brahe designed and built the most accurate astronomical instruments of the day (see Figure 12.2). In addition, he was the first astronomer to make corrections for the refraction of light by the atmosphere. History Link Tycho Brahe was a brilliant astronomer who led an unusual and tumultuous life. At age 19, he was involved in a duel with another student and part of his nose was cut off. For the rest of his life, Brahe wore an artificial metal nose. In 1600, Brahe invited Kepler to be one of his assistants. Brahe died suddenly the following year, leaving all of his detailed data to Kepler. With this wealth of astronomical data and his ability to perform meticulous mathematical analyses, Kepler discovered three empirical relationships that describe the motion of the planets. These relationships are known today as Kepler s laws. Figure 12.2 Brahe s observatory in Hveen, Denmark, contained gigantic instruments that, without magnification, were precise to 1/30 of a degree. KEPLER S LAWS 1. Planets move in elliptical orbits, with the Sun at one focus of the ellipse. 2. An imaginary line between the Sun and a planet sweeps out equal areas in equal time intervals. 3. The quotient of the square of the period of a planet s revolution around the Sun and the cube of the average distance from the Sun is constant and the same for all planets. r 3 T 2 = k or r 3 A T 2 A = r3 B TB 2, where A and B are two planets. Chapter 12 Universal Gravitation MHR 575

3 Math Link A circle is a special case of an ellipse. An ellipse is defined by two focuses and the relationship F 1 P + F 2 P = k, where k is a constant and is the same for every point on the ellipse. If the two focuses of an ellipse are brought closer and closer together until they are superimposed on each other, the ellipse becomes a circle. P F 1 F 2 Kepler s first law does not sound terribly profound, but he was contending not only with scientific observations of the day, but also with religious and philosophical views. For centuries, the perfection of celestial spheres was of extreme importance in religious beliefs. Ellipses were not considered to be perfect, so many astronomers resisted accepting any orbit other than a perfect circle that fit on the surface of a sphere. However, since Kepler published his laws, there has never been a case in which the data for the movement of a satellite, either natural or artificial, did not fit an ellipse. Kepler s second law is illustrated in Figure Each of the shaded sections of the ellipse has an equal area. According to Kepler s second law, therefore, the planet moves along the arc of each section in the same period of time. Since the arcs close to the Sun are longer than the arcs more distant from the Sun, the planet must be moving more rapidly when it is close to the Sun. v v Figure 12.3 According to Kepler s second law, the same length of time was required for a planet to move along each of the arcs at the ends of the segments of the ellipse. Kepler could not explain why planets moved faster when they were close to the Sun than when they were farther away. When Kepler published his third law, he had no way of knowing the significance of the constant in the mathematical expression r 3 /T 2 = k. All he knew was that the data fit the equation. Kepler suspected that the Sun was in some way influencing the motion of the planets, but he did not know how or why this would lead to the mathematical relationship. The numerical value of the constant in Kepler s third law and its relationship to the interaction between the Sun and the planets would take on significance only when Sir Isaac Newton ( ) presented his law of universal gravitation. 576 MHR Unit 5 Force, Motion, Work, and Energy

5 masses and inversely proportional to the square of the distance between their centres the law of universal gravitation. The mathematical equation for the law of universal gravitation is given in the following box. NEWTON S LAW OF UNIVERSAL GRAVITATION The force of gravity is proportional to the product of the two masses that are interacting and inversely proportional to the square of the distance between their centres. Quantity Symbol SI unit force of gravity F g N (newtons) first mass m 1 kg (kilograms) second mass m 2 kg (kilograms) distance between the centres of the masses r m (metres) universal gravitational constant Unit Analysis ( newton metre 2 newton = kilogram 2 ( N m 2 )( ) kg kg kg 2 m 2 = N F g = G m 1m 2 r 2 G N m 2 kg 2 )( ) kilogram kilogram metre 2 (newton metre squared per kilogram squared) Note: The value of the universal gravitational constant is G = N m 2 kg 2. Conceptual Problem You have used the equation F g = mg many times to calculate the weight of an object on Earth s surface. Now, you have learned that the weight of an object on Earth s surface is F g = G m Em o re-o 2, where m E is the mass of Earth, m o is the mass of the object, and r E-o is the distance between the centres of Earth and the object. Explain how the two equations are related. Express g in terms of the variables and constant in Newton s law of universal gravitation. 578 MHR Unit 5 Force, Motion, Work, and Energy

6 MODEL PROBLEM Weighing an Astronaut A 65.0 kg astronaut is walking on the surface of the Moon, which has a mean radius of km and a mass of kg. What is the weight of the astronaut? Frame the Problem The weight of the astronaut is the gravitational force on her. The relationship F g = mg, where g = 9.81 m 2, cannot be used in s this problem, since the astronaut is not on Earth s surface. The law of universal gravitation applies to this problem. Identify the Goal The gravitational force, F g, on the astronaut Variables and Constants Known Implied Unknown m M = kg m a = 65.0 kg G = N m 2 kg 2 F g r = km ( m) Strategy Apply the law of universal gravitation. Substitute the numerical values and solve. Calculations F g = G m 1m 2 r 2 F g = ( N m 2 kg 2 F g = N F g 105 N ) ( kg)(65.0 kg) ( m) 2 The weight of the astronaut is approximately 105 N. Validate Weight on the Moon is known to be much less than that on Earth. The astronaut s weight on the Moon is about one sixth of her weight on Earth (65.0 kg 9.81 m s 2 common knowledge. 638 N ), which is consistent with this continued Chapter 12 Universal Gravitation MHR 579

7 continued from previous page PRACTICE PROBLEMS 1. Find the gravitational force between Earth and the Sun. (See Appendix B, Physical Constants and Data.) 2. Find the gravitational force between Earth and the Moon. (See Appendix B, Physical Constants and Data.) 3. How far apart would you have to place two 7.0 kg bowling balls so that the force of gravity between them would be N? Would it be possible to place them at this distance? Why or why not? 4. Find the gravitational force between the electron and the proton in a hydrogen atom if they are m apart. (See Appendix B, Physical Constants and Data.) 5. On Venus, a person with mass 68 kg would weigh 572 N. Find the mass of Venus from this data, given that the planet s radius is m. 6. In an experiment, an 8.0 kg lead sphere is brought close to a 1.5 kg mass. The gravitational force between the two objects is N. How far apart are the centres of the objects? 7. The radius of the planet Uranus is 4.3 times the radius of earth. The mass of Uranus is 14.7 times Earth s mass. How does the gravitational force on Uranus surface compare to that on Earth s surface? 8. Along a line connecting Earth and the Moon, at what distance from Earth s centre would an object have to be located so that the gravitational attractive force of Earth on the object was equal in magnitude and opposite in direction from the gravitational attractive force of the Moon on the object? Gravity and Kepler s Laws The numerical value of G, the universal gravitational constant, was not determined experimentally until more than 70 years after Newton s death. Nevertheless, Newton could work with concepts and proportionalities to verify his law. Newton had already shown that the inverse square relationship between gravitational force and the distance between masses was supported by Kepler s first law that planets follow elliptical paths. Kepler s second law showed that planets move more rapidly when they are close to the Sun and more slowly when they are farther from the Sun. The mathematics of elliptical orbits in combination with an inverse square relationship to yield the speed of the planets is somewhat complex. However, you can test the concepts graphically by completing the following investigation. 580 MHR Unit 5 Force, Motion, Work, and Energy

8 INVESTIGATION 12-A Orbital Speed of Planets TARGET SKILLS Modelling concepts Analyzing and interpreting Communicating results Can you show diagrammatically that a force directed along the line between the centres of the Sun and a planet would cause the planet s speed to increase as it approached the Sun and decrease as it moved away? If you can, you have demonstrated that Kepler s second law supports Newton s proposed law of universal gravitation. Problem How does a force that follows an inverse square relationship affect the orbital speed of a planet in an elliptical orbit? Equipment corkboard or large, thick piece of cardboard 2 pushpins blank paper 30 cm loop of string pencil ruler pull the string so that it is taut and draw an ellipse by pulling the string all the way around the pushpins. 3. Remove the string and pushpins and label one of the pinholes Sun. 4. Choose a direction around the elliptical orbit in which your planet will be moving. Make about four small arrowheads on the ellipse to indicate the direction of motion of the planet. 5. Make a dot for the planet at the point that is most distant from the Sun (the perihelion). Measure and record the distance on the paper from the perihelion to the Sun. From that point, draw a 1 cm vector directed straight toward the Sun. 6. This vector represents the force of gravity on the planet at that point: F g(per) = 1 unit. (F g(per) is the force of gravity when the planet is at perihelion.) 7. Select and label at least three more points on each side of the ellipse at which you will analyze the force and motion of the planet. 8. For each point, measure and record, on a separate piece of paper, the distance from the Sun to point P, as indicated in the diagram. Do not write on your diagram, because it will become too cluttered. P Procedure 1. Place the paper on the corkboard or cardboard. Insert two pushpins into the paper about 8 to 10 cm apart. 2. Loop the string around the pushpins, as shown in the illustration. With your pencil, 5.9 cm Sun 15.6 cm perihelion continued Chapter 12 Universal Gravitation MHR 581

9 9. Follow the steps in the table to see how to determine the length of the force vector at each point. Procedure Equation The masses of the Sun and planet remain the same, so the value Gm S m p is constant. Therefore, the expression F g r 2 for any point on F g = G m Sm p r 2 the orbit is equal to the same value. F g r 2 = Gm S m p Consequently, you can set the expression F g r 2 for any one point equal to F g r 2 for any other point. Use the values at perihelion as a reference and set F g(p) r 2 equal to F g(peri) r 2 peri. Then solve for the F g(p). You can now find the relative magnitude of the gravitational force on the planet at any point on the orbit by substituting the magnitudes of the radii into the above equation. For example, the magnitude of the force at point P in step 8 is 6.99 units. F g(peri) r 2 peri = F g(p) r 2 P F g(p) = F g(peri) r 2 peri r 2 P (1 unit)(15.6 cm)2 F g(p) = (5.9 cm) 2 F g(p) = 6.99 units 10. Calculate the length of the force vector from each of the points that you have selected on your orbit. 11. On your diagram, draw force vectors from each point directly toward the Sun, making the lengths of the vectors equal to the values that you calculated in step At each point at which you have a force vector, draw a very light pencil line tangent to the ellipse. Then, draw a line that is perpendicular (normal) to the tangent line. 13. Graphically draw components of the force vector along the tangent (F T ) and normal (F N ) lines, as shown in the diagram. normal P F g Sun Analyze and Conclude 1. The tangential component of the force vector (F T ) is parallel to the direction of the velocity F T F g tangent F N of the planet when it passes point P. What effect will the tangential component of force have on the velocity of the planet? 2. The normal component of the force vector (F N ) is perpendicular to the direction of the velocity of the planet when it passes point P. What effect will the normal component of force have on the velocity of the planet? 3. Analyze the change in the motion of the planet caused by the tangential and normal components of the gravitational force at each point where you have drawn force vectors. Be sure to note the direction of the velocity of the planet as you analyze the effect of the components of force at each point. 4. Summarize the changes in the velocity of the planet as it makes one complete orbit around the Sun. 5. The force vectors and components that you drew were predictions based on Newton s law of universal gravitation. How well do these predictions agree with Kepler s observations as summarized in his second law? Would you say that Kepler s data supports Newton s predictions? 582 MHR Unit 5 Force, Motion, Work, and Energy

10 Kepler s third law simply states that the ratio r 3 /T 2 is constant and the same for each planet orbiting the Sun. At first glance, it would appear to have little relationship to Newton s law of universal gravitation, but a mathematical analysis will yield a relationship. To keep the mathematics simple, you will consider only circular orbits. The final result obtained by considering elliptical orbits is the same, although the math is more complex. Follow the steps below to see how Newton s law of universal gravitation yields the same ratio as given by Kepler s third law. Write Newton s law of universal gravitation, using m S for the mass of the Sun and m p for the mass of a planet. Since the force of gravity must provide a centripetal force for the planets, set the gravitational force equal to the required centripetal force. Simplify the equation. Since Kepler s third law includes the period, T, as a variable, find an expression for the velocity, v, of the planet in terms of its period. A planet travels a distance equal to the circumference of the orbit during a time interval equal to its period. Substitute the expression for the velocity of the planet into the above equation. Multiply each side of the equation by r 4π 2. F g = G m Sm p r 2 G m Sm p r 2 = m pv 2 r G m S = v 2 r v = d t d = 2πr t = T v = 2πr T G m S r G m S r ( G m S r r 3 T 2 ( 2πr = T = 4π2 r 2 T 2 )( r 4π 2 ) = = Gm S 4π 2 ) 2 ( 4π 2 r 2 T 2 )( r 4π 2 ) As you can see, Newton s law of universal gravitation indicates not only that the ratio r3 2 is constant, but also that the constant T is Gm S 2. All of Kepler s laws, developed prior to the time when 4π Newton did his work, support Newton s law of universal gravitation. Kepler had focussed only on the Sun and planets, but Chapter 12 Universal Gravitation MHR 583

11 Newton proposed that the laws applied to all types of orbital motion, such as moons around planets. Today, we know that all of the artificial satellites orbiting Earth, as well as the Moon, follow Kepler s laws. History Link Henry Cavendish was a very wealthy and brilliant man, but he also was very reclusive. He was rarely seen in public places, other than at scientific meetings. His work was meticulous, yet he published only a very small part of it. After his death, other scientists discovered his notebooks and finally published his results. Cavendish had performed the same experiments and obtained the same results for some experiments that were later done by Coulomb, Faraday, and Ohm, who received the credit for the work. PHYSICS FILE The value of G shows that the force of gravity is extremely small. For example, use unit amounts of each of the variables and substitute them into Newton s law of universal gravitation. You will find that the mutual attractive force between two 1 kg masses that are 1 m apart is N. Mass of the Sun and Planets Have you ever looked at tables that contain data for the mass of the Sun and planets and wondered how anyone could weigh the Sun and planets or determine their masses? English physicist and chemist Henry Cavendish ( ) realized that if he could determine the universal gravitational constant, G, he could use the mathematical relationship in Kepler s third law to calculate the mass of the Sun. A brilliant experimentalist, Cavendish designed a torsion balance, similar to the system in Figure 12.4, that allowed him to measure G. mirror support light source A torsion balance can measure extremely small amounts of the rotation of a wire. First, the torsion balance must be calibrated to determine the amount of force that causes the wire to twist by a specific amount. Then, the large spheres are positioned so that the bar supporting them is perpendicular to the rod supporting the small spheres. In this position, the large spheres are exerting equal gravitational attractive forces on each of the small spheres. The system is in equilibrium and the scale can be set to zero. The large spheres are then moved close to the small spheres and the amount of twisting of the wire is determined. From the amount of twisting and the calibration, the mutual attractive force between the large and small spheres is calculated. Using his torsion balance, Cavendish calculated the value of G to be N m 2 /kg 2. The best-known figure today is N m 2 /kg 2. Cavendish s measurement was within approximately 1% of the correct value. As Cavendish did, you can now calculate the mass of the Sun and other celestial bodies Figure 12.4 In the torsion balance that Cavendish designed and used, the spheres were made of lead. The small spheres were about 5 cm in diameter and were attached by a thin but rigid rod about 1.83 m long. The large spheres were about 20 cm in diameter MHR Unit 5 Force, Motion, Work, and Energy

12 MODEL PROBLEM The Mass of the Sun Find the mass of the Sun, using Earth s orbital radius and period of revolution. Frame the Problem Kepler s third law, combined with Newton s law of universal gravitation, yields an equation that relates the period and orbital radius of a satellite to the mass of the body around which the satellite is orbiting. Earth orbits the Sun once per year. Let R E represent the radius of Earth s orbit around the Sun. This value can be found in Appendix B, Physical Constants and Data. Identify the Goal The mass of the Sun, m S Variables and Constants Known Implied Unknown Sun G = N m 2 kg 2 T = days R E(orbit) = m m S Strategy Write Kepler s third law, using the constant derived from Newton s law of universal gravitation. Solve for the mass of the Sun. Convert the period into SI units. Substitute the numerical values into the equation and solve. Calculations r 3 T 2 = Gm S 4π 2 The mass of the Sun is approximately kg. ( r 3 )( 4π 2 ) ( )( GmS 4π 2 ) T 2 = G 4π 2 G ( 4π 2 )( r 3 ) m S = G T 2 ( )( )( ) 24 h 60 min 60 s days = s day h min ( 4π m S = 2 ) ( m) 3 11 N m ( s) 2 kg 2 m S = kg m S kg Validate The Sun is much more massive than any of the planets. The value sounds reasonable. ( )( 1 m 3 ) ( kg 2 )( m 3 ) ( kg Check the units: N m 2 s 2 = N m 2 s 2 = 2 )( m 3 ) kg m kg m 2 s 2 = kg. 2 s 2 continued Chapter 12 Universal Gravitation MHR 585

13 continued from previous page PRACTICE PROBLEMS 9. Jupiter s moon Io orbits Jupiter once every days. Its average orbital radius is m. What is Jupiter s mass? 10. Charon, the only known moon of the planet Pluto, has an orbital period of days at an average distance of m from Pluto. Use Newton s form of Kepler s third law to find the mass of Pluto from this data. 11. Some weather satellites orbit Earth every 90.0 min. How far above Earth s surface is their orbit? (Hint: Remember that the centre of the orbit is the centre of Earth.) 12. How fast is the moon moving as it orbits Earth at a distance of km? 13. On each of the Apollo lunar missions, the command module was placed in a very low, approximately circular orbit above the Moon. Assume that the average height was 60.0 km above the surface and that the Moon s radius is 7738 km. (a) What was the command module s orbital period? (b) How fast was the command module moving in its orbit? 14. A star at the edge of the Andromeda galaxy appears to be orbiting the centre of that galaxy at a speed of about km/s. The star is about AU from the centre of the galaxy. Calculate a rough estimate of the mass of the Andromeda galaxy. Earth s orbital radius (1 AU) is km. Newton s law of universal gravitation has stood the test of time and the extended limits of space. As far into space as astronomers can observe, celestial bodies move according to Newton s law. As well, the astronauts of the crippled Apollo 13 spacecraft owe their lives to the dependability and predictability of the Moon s gravity. Although Albert Einstein ( ) took a different approach in describing gravity in his general theory of relativity, most calculations that need to be made can use Newton s law of universal gravitation and make accurate predictions Section Review 1. K/U Explain the meaning of the term empirical as it applies to empirical equations. 2. K/U What did Tycho Brahe contribute to the development of the law of universal gravitation? 3. K/U Describe how Newton used each of the following phenomena to support the law of universal gravitation. (a) the orbit of the moon (b) Kepler s third law 4. K/U How did Newton s concepts about gravity and his development of the law of universal gravitation differ from the ideas of other scientists and astronomers who were attempting to find a relationship that could explain the motion of the planets? 5. K/U Describe the objective, apparatus, and results of the Cavendish experiment. 6. C Explain how you can weigh a planet. 7. I Suppose the distance between two objects is doubled and the mass of one is tripled. What effect does this have on the gravitational force between the objects? 586 MHR Unit 5 Force, Motion, Work, and Energy

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