Chapter 8 Rotational Motion Rotational Inertia An object
Chapter 8 Rotational Motion
Rotational Inertia An object rotating about an axis tends to remain rotating unless interfered with by some external influence. This influence is called torque. Rotation adds stability to linear motion. – Examples: spinning football bicycle tires Frisbee
The greater the distance between the bulk of an object's mass and its axis of rotation, the greater the rotational inertia. Examples: – Tightrope walker – Inertia Bars – Ring and Disk on an Incline – Metronome
Torque is the product of the force and lever-arm distance, which tends to produce rotation. Torque = force ´ lever arm – Examples: wrenches see-saws
Center of Mass The center of mass of an object is the average position of mass. Objects tend to rotate about their center of mass. Examples: Meter stick Map of Texas Rotating Hammer
Stability For stability center of gravity must be over area of support. Examples: Tower of Pisa Touching toes with back to wall Meter stick over the edge Rolling Double-Cone
Direction of Motion Centripetal Force Centrifugal Force
Centripetal Force …is applied by some object. Centripetal means "center seeking". Centrifugal Force …results from a natural tendency. Centrifugal means "center fleeing".
What is that force that throws you to the right if you turn to the left in your car? centrifugal force. What is that force that keeps you in your seat when you turn left in your car? centripetal force.
Examples Centripetal Centrifugal Force water in bucket Bucket Nature moon and earth Earth’s gravity Nature car on circular path coin on a hanger jogging in a space station Nature Road Friction Nature Hanger Nature Space Station Floor
Chapter 8 Rotational Motion
Conservation of Angular Momentum angular momentum = rotational inertia ´ rotational velocity L=Iw Newton's first law for rotating systems: – “A body will maintain its state of angular momentum unless acted upon by an unbalanced external torque. ”
Examples: – 1. – 2. – 3. – 4. ice skater spin cat dropped on back Diving into water Collapsing Stars (neutron stars)
Circular Motion Linear speed - the distance moved per unit time. Also called simply speed. Rotational speed - the number of rotations or revolutions per unit time. Rotational speed is often measured in revolutions per minute (RPM).
The linear speed is directly proportional to both rotational speed and radial distance. v=wr 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.
Example Question Two ladybugs are sitting on a phonograph record that rotates at 33 1/3 RPM. 1. Which ladybug has a great linear speed? A. The one closer to the center. B. The on the outside edge. C. The both have the same linear speed
Example Question Two ladybugs are sitting on a phonograph record that rotates at 33 1/3 RPM. 1. Which ladybug has a great linear speed? A. The one closer to the center. B. The on the outside edge. C. The both have the same linear speed
Example Question Two ladybugs are sitting on a phonograph record that rotates at 33 1/3 RPM. 2. Which ladybug has a great rotational speed? A. The one closer to the center. B. The on the outside edge. C. The both have the same rotational speed
Example Question 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?
Example Question 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? A. 4 m/s B. 2 m/s C. 20 RPM D. 40 RPM E. None of these
Example Question 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 rotational speed of your friend who sit at the outer edge? A. 4 m/s B. 2 m/s C. 20 RPM D. 40 RPM E. None of these
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