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SECTION 12.2 • More on the Center of Gravity 365 wise, the point of application of F 2 relative to O& is r 2 ' r&, and so forth. Therefore, the torque about an axis through O& is Because the net force is assumed to be zero (given that the object is in translational if an object is in transla- tional equilibrium and the net torque is zero about one axis, then the net torque 12.2 More on the Center of Gravity We have seen that the point at which a force is applied can be critical in determining Whenever we deal with a rigid object, one of the forces we must consider is the gravitational force acting on it, and we must know the point of application of this g acting at the center of gravity of the object. How do we find this special point? As we mentioned in Section 9.5, if we assume that g is uniform over the object, then the center of gravity of the object coincides with 1 , m 2 , m 3 , . . . having coordinates (x 1 , y 1 ), (x 2 , y 2 ), (x 3 , y 3 ), . . . . In Equation 9.28 we defined the x coordinate of the center of mass of such an object to be We use a similar equation to define the y coordinate of the center of mass, replacing Let us now examine the situation from another point of view by considering the gravitational force exerted on each particle, as shown in Figure 12.6. Each particle con- 1 g 1 is m 1 g 1 x 1 , where g 1 is the value of the gravitational acceleration at the position of the particle of mass m 1 . We wish to locate the center of gravity, the point at which appli- cation of the single gravitational force M g (where M ! m 1 $ m 2 $ m 3 $ % % % is the total mass of the object) has the same effect on rotation as does the combined effect of all i g i . Equating the torque resulting from M g acting at the center of gravity to the sum of the torques acting on the individual particles gives This expression accounts for the fact that the value of g can in general vary over the (m 1 g 1 $ m 2 g 2 $ m 3 g 3 $ % % % )x CG ! m 1 g 1 x 1 $ m 2 g 2 x 2 $ m 3 g 3 x 3 $ % % % x CM ! m 1 x 1 $ m 2 x 2 $ m 3 x 3 $ % % % m 1 $ m 2 $ m 3 $ % % % ! ! i
m i x i ! i
m i ! r 1 " F 1 $ r 2 " F 2 $ r 3 " F 3 $ % % % ' r& " (F 1 $ F 2 $ F 3 $ % % % ) !
! O& ! ( r 1 ' r&) " F 1 $ ( r 2 ' r&) " F 2 $ ( r 3 ' r&) " (F 3 $ % % % Figure 12.4 Construction showing that if the net torque is zero about origin O, it is also zero about any other origin, such as O&. Figure 12.6 The center of gravity of an object is located at the center of mass if g is constant over the object. Figure 12.5 An object can be di- vided into many small particles each having a specific mass and specific coordinates. These parti- cles can be used to locate the cen- ter of mass. F 2 F 1 F 3 F 4 r
1 r
1 – r
′ r
′ O O ′ x 1 ,y 1 y x
2 ,y
2 x
3 ,y
3 m 1 m 2 m
3 CM O x × m 3 g m 2 g x 1 ,y 1 y x
2 ,y
2 x
3 ,y
3 m 1 g CG O x × F g = Mg |