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SECTION 13.2 • Measuring the Gravitational Constant 393 13.2 Measuring the Gravitational Constant The universal gravitational constant G was measured in an important experiment by Example 13.1 Billiards, Anyone? Three 0.300-kg billiard balls are placed on a table at the cor- 1 ) re- sulting from the other two balls. Solution First we calculate separately the individual forces At the Interactive Worked Example link at http://www.pse6.com, you can move balls 2 and 3 to see the effect on the net gravitational force on ball 1. Quick Quiz 13.2 A planet has two moons of equal mass. Moon 1 is in a circu- lar orbit of radius r. Moon 2 is in a circular orbit of radius 2r. The magnitude of the Interactive 0.400 m m 2 0.500 m m 1 0.300 m m 3 F 21 F F 31 x y θ Figure 13.3 (Example 13.1) The resultant gravitational force acting on the cue ball is the vector sum F 21 & F 31 . The force exerted by m 2 on the cue ball is directed upward and is given by This result shows that the gravitational forces between everyday objects have extremely small magnitudes. The 3 on the cue ball is directed to the right: Therefore, the net gravitational force on the cue ball is and the magnitude of this force is From tan ' ! 3.75/6.67 ! 0.562, the direction of the net grav- ! 7.65 # 10 " 11 N F ! √ F 21
2 & F 31
2 ! √ (3.75) 2 & (6.67) 2 # 10 " 11 N (6.67ˆ i & 3.75ˆj) # 10 " 11 N F ! F 21 & F 31 ! ! 6.67 # 10 " 11
iˆ N ! (6.67 # 10 " 11 N$m 2 /kg 2 ) (0.300 kg)(0.300 kg) (0.300 m) 2 iˆ F 31 ! G
m
3 m
1 r 31
2
iˆ ! 3.75 # 10 " 11 jˆ N ! (6.67 # 10 " 11 N$m 2 /kg 2 ) (0.300 kg)(0.300 kg) (0.400 m) 2 jˆ F 21 ! G m 2 m 1 r 21
2
jˆ Mirror r m M Light source Figure 13.4 Cavendish apparatus for measuring G. The dashed line represents the original position of the rod. |