University:
Boston University
Course:
CAS PY 252 | Principles of Physics 2
Academic year:

2020
Views:

409

Pages:

16
Author:

Giselle A.

11.3 Center of Mass The center of mass of an object is the point located at the object’s average position of mass. 11.3 Center of Mass A baseball thrown into the air follows a smooth parabolic path. A baseball bat thrown into the air does not follow a smooth path. The bat wobbles about a special point. This point stays on a parabolic path, even though the rest of the bat does not. The motion of the bat is the sum of two motions: • a spin around this point, and • a movement through the air as if all the mass were concentrated at this point. This point, called the center of mass, is where all the mass of an object can be considered to be concentrated. 11.3 Center of Mass The centers of mass of the baseball and of the spinning baseball bat each follow parabolic paths. 11.3 Center of Mass Location of the Center of Mass For a symmetrical object, such as a baseball, the center of mass is at the geometric center of the object. For an irregularly shaped object, such as a baseball bat, the center of mass is toward the heavier end. 11.3 Center of Mass The center of mass for each object is shown by the red dot. 11.3 Center of Mass Objects not made of the same material throughout may have the center of mass quite far from the geometric center. Consider a hollow ball half filled with lead. The center of mass would be located somewhere within the lead part. The ball will always roll to a stop with its center of mass as low as possible. 11.3 Center of Mass The center of mass of the toy is below its geometric center. 11.3 Center of Mass Motion About the Center of Mass As an object slides across a surface, its center of mass follows a straight-line path. 11.3 Center of Mass The center of mass of the rotating wrench follows a straight-line path as it slides across a smooth surface. 11.3 Center of Mass The motion of the wrench is a combination of straight-line motion of its center of mass and rotation around its center of mass. If the wrench were tossed into the air, its center of mass would follow a smooth parabola. 11.3 Center of Mass Internal forces during the explosion of a projectile do not change the projectile’s center of mass. If air resistance is negligible, the center of mass of the dispersed fragments as they fly through the air will be at any time where the center of mass would have been if the explosion had never occurred. 11.3 Center of Mass The center of mass of the fireworks rocket and its fragments move along the same path before and after the explosion. 11.3 Center of Mass Applying Spin to an Object When you throw a ball and apply spin to it, or when you launch a plastic flying disk, a force must be applied to the edge of the object. This produces a torque that adds rotation to the projectile. A skilled pool player strikes the cue ball below its center to put backspin on the ball. 11.3 Center of Mass A force must be applied to the edge of an object for it to spin. a. If the football is kicked in line with its center, it will move without rotating. 11.3 Center of Mass A force must be applied to the edge of an object for it to spin. a. If the football is kicked in line with its center, it will move without rotating. b. If it is kicked above or below its center, it will rotate. 11.3 Center of Mass Where is an object’s center of mass located?