Slide 1

Slide 2 Chapter 8 Rotational Motion Slide 3 Rotational Inertia n An object rotating about an axis tends to remain rotating unless interfered with by some external influence. n This influence is called torque. n Rotation adds stability to linear motion. Examples: spinning football bicycle tires Frisbee Slide 4 n The greater the distance between the bulk of an object's mass and its axis of rotation, the greater the rotational inertia. n Examples: Tightrope walker Inertia Bars Ring and Disk on an Incline Metronome Slide 5 Torque n Torque is the product of the force and lever- arm distance, which tends to produce rotation. Torque = force lever arm Torque = force lever arm Examples: wrenches see-saws Slide 6 Center of Mass n The center of mass of an object is the average position of mass. n Objects tend to rotate about their center of mass. n Examples: Meter stick Map of Texas Rotating Hammer Slide 7 Stability n For stability center of gravity must be over area of support. n Examples: Tower of Pisa Touching toes with back to wall Meter stick over the edge Rolling Double-Cone Slide 8 n n What is that force that throws you to the right if you turn to the left in your car? Its a center-fleeing force called centrifugal force. n n What is that force that keeps you in your seat when you turn left in your car? Its a center-seeking force called centripetal force. Slide 9 Direction of Motion Centrifugal Force Centripetal Force Slide 10 Centripetal Force n is applied by some object. n Centripetal means "center seeking". Centrifugal Force n results from a natural tendency. n Centrifugal means "center fleeing". Slide 11 Examples n water in bucket n moon and earth n car on circular path n coin on a hanger n jogging in a space station Centripetal Force n Bucket n Earths gravity n Road Friction n Hanger n Space Station Floor Centrifugal Force n Nature Slide 12 Conservation of Angular Momentum angular momentum = rotational inertia angular momentum = rotational inertia rotational velocity rotational velocity L = I L = I n Newton's first law for rotating systems: A body will maintain its state of angular momentum unless acted upon by an unbalanced external torque. Slide 13 n Examples: 1. ice skater spin 2. cat dropped on back 3. Diving 4. Collapsing Stars (neutron stars) Slide 14 End of Chapter 7 Slide 15 To compute your grade (This information is on the syllabus.) Homework Average _____ 40 = _______ Exam 1 _____ 150 = _______ Exam 2 _____ 150 = _______ Lab Exam 1 _____ 50 = _______ Exam 3 _____ 150 = _______ Final Exam _____ 150 = _______ Lab Exam 2 _____ 50 = _______ Lab Grades _____ 100 = _______ Total = _________ 8 Your Average = _________ Slide 16 Notice n The Physics 101 lab grades are posted outside of your lab room. n You can pick up your old labs there as well. n Use your old labs and the notes on the study guide to prepare for the lab exam. n You can pick up you homework and in-class assignments outside of Dr. Brutons office (room 330). Slide 17 Circular Motion n Linear speed - the distance moved per unit time. Also called simply speed. n Rotational speed - the number of rotations or revolutions per unit time. n Rotational speed is often measured in revolutions per minute (RPM). Slide 18 n The linear speed is directly proportional to both rotational speed and radial distance. v = r v = r n What are two ways that you can increase your linear speed on a rotating platform? Answers: Move away from the rotation axis. Have the platform spin faster. Slide 19 Example Question n Two ladybugs are sitting on a phonograph record that rotates at 33 1/3 RPM. (a) Which ladybug has a great linear speed? Answer: The one on the outside edge. (b) Which ladybug has a great rotational speed? (b) Which ladybug has a great rotational speed? Answer: Both have the same rotational speed. Slide 20 You sit on a rotating platform halfway between the rotating axis and the outer edge. You have a rotational speed of 20 RPM and a tangential speed of 2 m/s. What will be the linear speed of your friend who sit at the outer edge? Answer: 4 m/s What will be his rotational speed? Answer: 20 RPM Example Question