1) Moment of inertia is the rotational analog of mass and describes an object's resistance to changes in its rotation. It depends on the object's mass and how it is distributed.
2) Newton's second law of rotation states that the torque applied to an object produces angular acceleration proportional to the torque and inversely proportional to the object's moment of inertia.
3) The work-energy theorem states that the work done on an object equals its change in kinetic energy. This applies to both linear and rotational motion, with rotational kinetic energy depending on the moment of inertia and angular velocity.
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2. Inertia of Rotation Consider Newtonβs first law for the inertia of rotation to be patterned after the law for translation. Moment of Inertia or Rotational Inertia is the rotational analog of mass. Units of I: kg-m2 ; g-cm2 ; slug-ft2
4. Example 2: A circular hoop and a disk each have a mass of 3 kg and a radius of 30 cm. Compare their rotational inertias. I = 0.3 kg m2 I = 0.1 kg m2 hoop R R Disk
5. Inertia of Rotation Ξ£F = 20 N a = 4m/s2 F = 20 N R = 0.5 m a = 2 rad/s2 Consider Newtonβs second law for the inertia of rotation to be patterned after the law for translation. Linear Inertia, m Rotational Inertia, I Force does for translation while torque does for rotation
6. Newtonβs 2nd Law for Rotation V f t w wo = 50 rad/s t = 40 N m R 4 kg Important Analogies For many problems involving rotation, there is an analogy to be drawn from linear motion. I A resultant torque t produces angular acceleration a of disk with rotational inertia I. A resultant force F produces negative acceleration a for a mass m.
7. F wo = 50 rad/s R = 0.20 m F = 40 N w R 4 kg Newtonβs 2nd Law for Rotation How many revolutions are required to stop?
8. Seatwork: Β½ sheet of pad paper (crosswise) Time allotted: 40 minutes A disk of mass π=6.00Β kgΒ and radius π =0.500Β mΒ is used to draw water from a well (Fig. 1). A bucket of mass π=2.00Β kg is attached to a cord that is wrapped around the disk.a. What is the tension T in the cord and acceleration a of the bucket?b. If the bucket starts from rest at the top of the well and falls for 3.00 s before hitting the water, how far does it fall? c. About how many revolutions are made by the disk in the situation stated in (b)?Note: πΌπππ π=12ππ 2 Β Fig. 1
9. Work and Power for Rotation βπ Β βπ Β F F βπ =π βπ Β π=πΉβπ Β since Β π Β πΉπ =π Β π=πΉπ βπ Β since βπ =π βπ Β ππππ‘=πβπ Β ππππ‘=ππππ‘βπ‘ Β ππππ‘=πβπβπ‘ Β βπβπ‘=π Β since
10. Example:The rotating disk has a radius of 40 cm and a mass of 6 kg. Find the work and power needed in lifting the 2-kg mass 20 m above the ground in 4 s in uniform motion. s q F 6 kg 2 kg F=W s = 20 m
11. Rotational Kinetic Energy v = wR m m4 w m3 m1 m2 axis Object rotating at constant w. Consider tiny mass m: πΎ=12ππ£2 Β π£=ππ Β since πΎ=12πππ 2 Β πΎ=12ππ 2π2 Β To find the total kinetic energy: ππ 2=πΌ Β since π²πππ=πππ°ππ Β (Β½w2 same for all m )
12. Recall for linear motion that the work done is equal to the change in linear kinetic energy: Using angular analogies, we find the rotational work is equal to the change in rotational kinetic energy: The Work-Energy Theorem
13. F wo = 60 rad/s R = 0.30 m F = 40 N w R 4 kg Example:Applying the Work-Energy Theorem: What work is needed to stop wheel rotating in the figure below?