Chapter 8 Rotational Motion, Part 2 Circular Motion Rotational Inertia Torque Center of Mass and Center of Gravity Centripetal/Centrifugal Force Angular Momentum Conservation of Angular Momentum Last time Skip pgs 147-149

Center of Mass/Center of Gravity A baseball Bat doesn’t’! It follows a wobbly trajectory. A baseball follows a smooth parabolic trajectory. But, the bat’s Center of Mass follows a smooth parabolic trajectory. Center of Mass: The average of the positions of all the mass in an object. The point of a body at which the force of gravity can be considered to act A crescent wrench sliding across a frictionless surface follows a complicated motion, but its center of mass follows a straight line path. Center of Mass Applet 1 Center of Mass Applet 2 Physics Place Videos

Question 1

Centripetal Force Centripetal Force is the force that makes an object move in a curved path. Newton’s First Law: An object in motion will stay in motion with constant speed and direction unless acted on by an external force. If a force doesn’t act, an object in motion will stay in motion in a straight line. Examples: The force exerted radially inward by friction on the tires makes the car move in a circle. The centripetal force that makes the airplane move in a circle is exerted on the wings by air moving over them. The can moves in a circle because of the force exerted on it by the rope

Question 2 Does a centripetal force do work on an object? A. Yes B. No C. Only on Tuesdays

Angular Momentum We have learned that if an object has inertia and it’s in motion, we can describe it’s “momentum” as its mass times its velocity (momentum = mv). Similarly, a rotating object has rotational inertia and also has a rotational momentum, called “Angular Momentum”. Angular momentum depends on an object’s rotational inertia and rotational velocity: Angular momentum = Rotational Inertia x Rotational Velocity - Or - Angular momentum = I If an object is in revolution around an external point, its angular momentum can be expressed as: v Angular momentum = mvr r m

Conservation of Angular Momentum In linear motion, to gain or lose momentum, an impulse must be delivered to an object. If no impulse acts, the momentum of the object (or system) doesn’t change … momentum is conserved. In rotational motion, Angular Momentum is conserved. If no external net torque acts on a rotating system, the angular momentum of that system remains constant. Large Rotational Inertia Small Rotational Velocity Small Rotational Inertia Large Rotational Velocity