Today's plan: Wednesday, January 23, 2013 Seating: sit in your assigned seat unless you are new to the class period, then just sit in an empty seat until further notice. What is your new "Advisory Period"? Begin Chapter 10 - Center of Gravity Notes Concept Practice Exercises If you are completely new to Physics class, see Mr. Kipp before you leave.

Warm up - Independent reading: Open your textbooks to page 136, Ch. 10. Read the first two (2) pages of the chapter silently to yourself. We spent Chapters 1-8 dealing with linear motion, forces, and energy. We will continue to study circular motion/forces before we get into the concepts of gravity, relativity, sound, and light (the majority of 2nd semester physics).

Why learn about Center of Gravity? Learning the nature behind The Center of Gravity (CG) helps you further understand equilibrium and the balance or imbalance of opposing forces. Ch 10 - CG relates to Ch 9 - circular motion (rotation about an axis) and Ch 11 - rotational mechanics (torques, etc.) For our purposes, the center of gravity is the same as the center of mass. Video: https://www.youtube.com/watch?v=hqDhW8HkOQ8

Try this at home! Amaze your friends and family! You can also balance a soda can on the edge of a glass by applying the physics behind CG! Here is another trick you can do: https://www.youtube.com/watch?v=IJFCNBwsslo

Center of Gravity/Mass The CG of an object is the point located at the object's average position of mass; where all the mass of an object can be considered to be concentrated. For a symmetrical object, the center of mass is at the geometric center of the object. For irregularly shaped objects, the location of the center of mass varies. The CG of an object is, loosely speaking, kind of like its axis that it would rotate around. Think of a rotating planet, a vinyl record, a wheel, etc.

The basic concepts of CG are explained in the following video: Center of Gravity by Tutorvista https://www.youtube.com/watch?v=R8wKV0UQtlo

Examples: (draw diagrams) The CG of your textbook, for example, is at the geometrical center. It can be easily found by the intersection of diagonal lines from opposite corners. (See page 136-137 in text.) A baseball bat is an irregularly shaped object, it has an axis of symmetry that runs along the bat's length. A baseball, on the other hand, has axes of symmetry in all spatial directions.

Spin can be applied to an object by applying a force that does not pass through the object's center of mass. Kicking a football in the middle, for example, will make it travel without rotating. Kicking the football above or below its center will make it rotate.

Examples of CG: If you throw a wrench so that it rotates as it moves through the air, you'll see it wobble about its CG. see page 137 The center of gravity itself would follow a parabolic path. An object's CG is its balance point; supporting the CG supports the entire object. A meter stick can be balanced by applying a force at its geometric midpoint -- the location of its CG. Any object suspended at a single point will hang with its CG directly below the point of suspension.

Weeble Wobble toy - an example of CG

Humans experiment with CG at very young ages: Babies learn quickly how to knock things over based on the location of an object's CG. They also learn how easily they can topple over if they don't manage their own CG properly. People that play sports often have a very good concept of other objects' CG, whether it be the CG of their opponents, teammates, or the sports equipment.

Extra Credit Opportunity: If you are interested in making a simple and fun video or multimedia project demonstrating these concepts of CG for extra credit, see Mr. Kipp for the assignment sheet. Example: https://www.youtube.com/watch?v=EE5SVATQzjc

Today's assignment: Page 147: Review Questions # 1-10. You MUST write out questions and answers to receive full credit. Draw diagrams/sketches where appropriate. Due before you leave (turn in to basket).