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SECTION 10.4 • Rotational Kinetic Energy 301 Quick Quiz 10.7 A section of hollow pipe and a solid cylinder have the same radius, mass, and length. They both rotate about their long central axes with Example 10.3 The Oxygen Molecule Consider an oxygen molecule (O 2 ) rotating in the xy plane about the z axis. The rotation axis passes through the center % 26 kg, and at room temperature the average separation between the two atoms is % 10 m. (The atoms are modeled as particles.) (A) Calculate the moment of inertia of the molecule about the z axis. Solution This is a straightforward application of the defini- " (2.66 + 10 % 26 kg)(1.21 + 10 % 10 m) 2 2 I " % i
m i
r i
2 " m # d $ 2 ) m # d $ 2 " md
2 2 " This is a very small number, consistent with the minuscule (B) If the angular speed of the molecule about the z axis is 4.60 + 10 12 rad/s, what is its rotational kinetic energy? Solution We apply the result we just calculated for the mo- R : " 2.06 + 10 % 21 J " 1 2 (1.95 + 10 % 46 kg,m 2 )(4.60 + 10 12 rad/s) 2 K R " 1 2
I& 2 1.95 + 10 % 46 kg,m 2 Example 10.4 Four Rotating Objects Four tiny spheres are fastened to the ends of two rods of (A) If the system rotates about the y axis (Fig. 10.8a) with an angular speed &, find the moment of inertia and the rota- Solution First, note that the two spheres of mass m, which y (that is, r i " 0 for these spheres about this axis). Applying Equation 10.15, we Therefore, the rotational kinetic energy about the y axis is The fact that the two spheres of mass m do not enter into x " 2mb 2 with a rotational kinetic en- ergy about that axis of K R " mb 2 & 2 . Ma 2 & 2 K R " 1 2
I y & 2 " 1 2 (2Ma 2 )& 2 " 2Ma 2 I y " % i
m i
r i
2 " Ma 2 ) Ma 2 " (B) Suppose the system rotates in the xy plane about an axis (the z axis) through O (Fig. 10.8b). Calculate the mo- Solution Because r i in Equation 10.15 is the distance be- tween a sphere and the axis of rotation, we obtain " Comparing the results for parts (A) and (B), we con- clude that the moment of inertia and therefore the rota- (Ma 2 ) mb 2 )& 2 K R " 1 2
I z & 2 " 1 2 (2Ma 2 ) 2mb 2 )& 2 " 2Ma 2 ) 2mb 2 I z " % i
m i
r i
2 " Ma 2 ) Ma 2 ) mb 2 ) mb 2 |