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Announcements

Announcements. Homework for tomorrow… (Ch. 11, Probs 41, 47, & 54) 11.10: a. t = 30 s b .  = 1800 rad 11.20: a) ω = 3.49 rad/s b ) v = 0.52 m/s c ) v = 0.26 m/s Office hours… M 3-4 pm TWR 9-10 am F 1-2 pm

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Announcements

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  1. Announcements • Homework for tomorrow… (Ch. 11, Probs 41, 47, & 54) 11.10: a. t= 30 sb.  = 1800 rad 11.20: a) ω = 3.49 rad/sb) v = 0.52 m/sc) v = 0.26 m/s • 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

  2. Chapter 11 Rotation (Torque)

  3. Last time… • Kinetic Energy of Rotation • Rotational Inertia for a collection of particles • Rotational Inertia for a continuous body

  4. Quiz Question 1 To increase the rotational inertia of a solid disk about its axis without changing its mass: • drill holes near the rim and put the material near the axis • drill holes near the axis and put the material near the rim • drill holes at points on a circle near the rim and put the material at points between the holes • drill holes at points on a circle near the axis and put the material at points between the holes • do none of the above (the rotational inertia cannot be changed without changing the mass)

  5. Demo • Guesstimate KEr ofthe bicycle wheel..

  6. Quiz Question 2 Two spheres, one hollow and one solid, are rotating with the same angular speed around an axis through their centers. Both spheres have the same mass and radius. Which sphere, if either, has the higher rotational kinetic energy? • The hollow sphere • The solid sphere • They have the same kinetic energy.  

  7. Section 11.7:Torque •  depends on the chosen axis of rotation • What are the SI units? ‘Tail-to-Tail’ Rotation Axis

  8. Quiz Question 3 You are trying to open a door that is stuck by pulling on the door knob in a direction perpendicular to the door. If you instead tie a rope to the door knob and then pull with the same force, is the torque you exert increased? • Yes • No

  9. Section 11.8: Newton’s Second Law for Rotation Newton’s 2nd Law… • for Translations: • for Rotations: “Moment of Inertia”

  10. Quiz Question 4 A constant net torque is applied to an object. Which one of the following will not be constant? • angular acceleration • angular velocity • moment of inertia • center of gravity

  11. Example A 150-kg merry-go-round in the shape of a uniform, solid, horizontal disk of radius 1.50 m is set in motion by wrapping a rope about the rim of the disk and pulling on the rope. What constant force must be exerted on the rope to bring the merry-go-round from rest to an angular speed of 0.500 rev/s in 2.00 s?

  12. Touchstone Example 11.5: Figure 11-20a shows a uniform disk, with mass M = 2.5 kg and radius R = 20cm, mounted on a fixed horizontal axle. A block with mass m = 1.2 kg hangs from a massless cord that is wrapped around the rim of the disk. Find the acceleration of the disk, and the tension in the cord. The cord does not slip, and there is no friction at the axle.

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