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Announcements. EXAM IV this THURSDAY! Homework for tomorrow… (Ch. 12, Probs 37, 40, & 42) 12.24: a) l = (-24 kg m 2 /s 3 t 2 )k hat b ) = (-48 kg m 2 /s 3 t )k hat c ) l ’ = (12 kg m 2 /s 3 t 2 )k hat d ) = (24 kg m 2 /s 3 t )k hat Office hours…

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**Announcements**• EXAM IV this THURSDAY! • Homework for tomorrow… (Ch. 12, Probs 37, 40, & 42) 12.24: a) l = (-24 kg m2/s3t2)k hat b) = (-48 kg m2/s3t)k hat c) l’ = (12 kg m2/s3t2)k hat d) = (24 kg m2/s3t)k hat • Office hours… M 3-4 pm TWR 9-10 am F 1-2 pm • Tutorial Learning Center – Houston Hall 113 MWR 8 am-6 pm T 8 am-7 pm F 8 am-5 pm**Timeline…**Today: Chapter 12, QQ’s + problems Tomorrow: REVIEW Thursday: EXAM IV**Chapter 12**Complex Rotations (Conservation of Rotational Momentum)**Last time…**• Total Rotational Momentum • Net external torque • Rotational Momentum (a.k.a Angular Momentum)**Quiz Question 1**A 1-kg rock is suspended by a massless string from one end of a 1-m measuring stick. What is the mass of the measuring stick if it is balanced by a support at the 0.25-m mark? • 0.25 kg • 0.5 kg • 1 kg • 2 kg • 4 kg • Impossible to determine**Problem 12.33**Three particles, each of mass m, are fastened to each other and to a rotation axis at O by three massless strings, each with length d as shown in Fig. 12-25. The combination rotates around the rotational axis with rotational velocity in such a way that the particles remain in a straight line. In terms of m, d, and , and relative to point O, what are the rotational inertia of the combination, the rotational momentum of the middle particle, and the total rotational momentum of the three particles?**Quiz Question 2**A force F is applied to a dumbbell for a time interval t, first as in (a) and then as in (b). In which case does the dumbbell acquire the greater center-of-mass speed? • (a) • (b) • no difference • The answer depends on the rotational inertia of the dumbbell**Quiz Question 3**A force F is applied to a dumbbell for a time interval t, first as in (a) and then as in (b). In which case does the dumbbell acquire the greater energy? • (a) • (b) • no difference • The answer depends on the rotational inertia of the dumbbell**Demo Model**Estimate the ratio of ω2/ω1? .2 m 1 kg 1 kg 82 kg .75 m**Demo**What happens to my angular velocity if I let go of the bags of rice?

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