The center of mass of an object (also called the center of gravity) is the point where all the mass can be considered to be concentrated for certain purposes.  The center of mass of a meter stick, for example would be its geometrical center (at the 0.50 m mark).  If you support the center of mass, you can balance the whole meter stick.  If you were to throw the meter stick through the air, its center of mass would move on a parabolic trajectory, just like a baseball, and the meter stick would rotate about its center of mass.

			We can find the center of mass of an object (or a group of objects) by multiplying each mass by its distance from a reference point, adding the values together and then dividing by the total mass:

			Example:

			http://www.youtube.com/watch?v=sDGbGAx97f4&fmt=18

			http://www.youtube.com/watch?v=vWde8sMxe1w&NR=1&fmt=18

			http://www.youtube.com/watch?v=GmejB075Uo0&fmt=18

			In the absence of any external forces, momentum is conserved in both the x-direction and the y-direction.

			Example:

			http://www.youtube.com/watch?v=SZVQNzah-OA&fmt=18

			Before:

			After:

			Conservation of momenum:

			Conservation of Kinetic Energy

			Solving 3 equations in 3 unknowns:  v1, v2 and θ:

			http://www.youtube.com/watch?v=E47hWdy7ciE&fmt=18

			http://www.youtube.com/watch?v=9tLi03bj5z4&feature=channel&fmt=18

			Read pp. 226-234 of the text.

			Answer the questions in HW 13 on WebAssign:  http://webassign.net/login.html