ppt_chapter06.ppt - Raymond A Serway John W Jewett Chapter 6 Energy of a System Introduction to Energy • • The concept of energy is one of the most

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Raymond A. Serway John W. Jewett Chapter 6 Energy of a System
Introduction to Energy The concept of energy is one of the most important topics in science Every physical process that occurs in the Universe involves energy and energy transfers or transformations Energy is not easily defined
Energy Approach to Problems The energy approach to describing motion is particularly useful when the force is not constant A global approach to problems involving energy and energy transfers will be developed This could be extended to biological organisms, technological systems and engineering situations
6.1 Systems and Environments A system is a small portion of the Universe We will ignore the details of the rest of the Universe This is a simplification model A critical skill is to identify the system
6.1 Systems and Environments A valid system may: be a single object or particle be a collection of objects or particles be a region of space vary with time in size and shape
6.1 Systems and Environments There is a system boundary around the system The boundary is an imaginary surface It does not necessarily correspond to a physical boundary The boundary divides the system from the environment The environment is the rest of the Universe
6.2 Work Done by a Constant Force The work , W , done on a system by an agent exerting a constant force on the system is the product of the magnitude, F , of the force, the magnitude r of the displacement of the point of application of the force, and cos  where is the angle between the force and the displacement vectors:
6.2 Work Done by a Constant Force The displacement is at the point of application of the force. A force does no work on the object if the force does not move through a displacement
6.2 Work Done by a Constant Force The work done by a force on a moving object is zero when the force applied is perpendicular to the displacement of its point of application. The normal force, n , and the gravitational force, mg , do no work on the object
6.2 Work Done by a Constant Force Work is a scalar quantity It can be either negative or positive When an object is lifted, the work done by the applied force on the object is positive because the direction of that force is upward, in the same direction as the displacement As an object is lifted, the work done by the gravitational force on the object is negative. The unit of work is a joule (J) 1 joule = 1 newton × 1 meter J = N · m
Example 6.1 Mr. Clean A man cleaning a floor pulls a vacuum cleaner with a force of magnitude F = 50.0 N at an angle of 30.0 with the horizontal. Calculate the work done by the force on the vacuum cleaner as the vacuum cleaner is displaced 3.00 m to the right.

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