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S E C T I O N 15 . 5 • The Pendulum 469 Physical Pendulum Suppose you balance a wire coat hanger so that the hook is supported by your ex- physical pendulum. Consider a rigid object pivoted at a point O that is a distance d from the center of mass (Fig. 15.18). The gravitational force provides a torque about an axis through O, The negative sign indicates that the torque about O tends to decrease .. That is, the $ . is valid, and the equation of motion reduces to (15.27) Because this equation is of the same form as Equation 15.3, the motion is simple har- max cos('t % (), where . max is the maximum angular position and ' ! √ mgd I d
2
. dt 2 ! " " mgd I # . ! "' 2
. " mgd
sin
. ! 2
d
2
. dt
2 Quick Quiz 15.7 A grandfather clock depends on the period of a pendulum to keep correct time. Suppose a grandfather clock is calibrated correctly and then a Quick Quiz 15.8 Suppose a grandfather clock is calibrated correctly at sea level and is then taken to the top of a very tall mountain. Does the grandfather clock Example 15.6 A Connection Between Length and Time Christian Huygens (1629–1695), the greatest clockmaker in Solution Solving Equation 15.26 for the length gives Thus, the meter’s length would be slightly less than one 0.248 m L ! T 2 g 4) 2 ! (1.00 s) 2 (9.80 m/s 2 ) 4) 2 ! What If? What if Huygens had been born on another planet? What would the value for g have to be on that planet Answer We solve Equation 15.26 for g : No planet in our solar system has an acceleration due to g ! 4) 2 L T 2 ! 4) 2 (1.00 m) (1.00 s) 2 ! 4) 2 m/s 2 ! 39.5 m/s 2 Pivot O θ θ d d sin CM m
g Figure 15.18 A physical pendu- lum pivoted at O. |