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SECTION 13.5 • The Gravitational Field 401 Figure 13.9 (Example 13.5) A satellite of mass m moving around the Earth in a circular orbit of radius r with constant speed v. The only force acting on the satellite is the gravitational force F g . (Not drawn to scale.) h R E m v F g r 13.5 The Gravitational Field When Newton published his theory of universal gravitation, it was considered a success An approach to describing interactions between objects that are not in contact came well after Newton’s death, and it enables us to look at the gravitational interac- gravitational field that exists at every point in space. When a particle of mass m is placed at a point where the gravitational Solving for v and remembering that the distance r from the E & h, we obtain (1) (B) If the satellite is to be geosynchronous (that is, appearing to remain over a fixed position on the Earth), how fast is it Solution In order to appear to remain over a fixed position S : M E , we find the radius of the orbit: Substituting numerical values and noting that the period is To find the speed of the satellite, we use Equation (1): ! To finalize this problem, it is interesting to note that the 3.07 # 10 3 m/s ! √ (6.67 # 10 " 11 N$m 2 /kg 2 )(5.98 # 10 24 kg) 4.23 # 10 7 m v ! √ GM E r ! 4.23 # 10 7 m r ! √ 3 (6.67 # 10 " 11 N$m 2 /kg 2 )(5.98 # 10 24 kg)(86 400 s) 2 4% 2 r ! √ 3 GM E T
2 4%
2 T 2 ! ! 4%
2 GM E " r
3 √ GM E R E & h v ! √ GM E r ! lite above the surface of the Earth of almost 36 000 km. What If? What if the satellite motion in part (A) were taking place at height h above the surface of another planet more Answer If the planet pulls downward on the satellite with You can adjust the altitude of the satellite at the Interactive Worked Example link at http://www.pse6.com. |