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# CH7 - What force on the string A pendulum of length 1.15 m...

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A pendulum of length 1.15 m swings in a vertical plane (see figure). When the pendulum is in the two horizontal positions = 90° and = 270°, its speed is 4.00 m/s. (a) Find the magnitude of the radial acceleration and tangential acceleration for these positions. (b) Draw a vector diagram to determine the direction of the total acceleration for these two positions. c) Calculate the magnitude and direction of the total acceleration. What force on the string?

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Chapter 7 Energy and Energy Transfer
Chapter 7

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Chapter 7
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

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Introduction to Energy
Energy Approach to Problems The energy approach to describing motion is particularly useful when the force is not constant An approach will involve Conservation of Energy This could be extended to biological organisms, technological systems and engineering situations

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Systems A system is a small portion of the Universe We will ignore the details of the rest of the Universe A critical skill is to identify the system
Valid System A valid system may be a single object or particle be a collection of objects or particles be a region of space vary in size and shape

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Problem Solving Does the problem require the system approach? What is the particular system and what is its nature? Can the problem be solved by the particle approach? The particle approach is what we have been using to this time
Environment 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

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Work 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
Work

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Work, cont. W = F r cos The displacement is that of the point of application of the force A force does no work on the object if the force does not move through a displacement 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
Work Example The normal force, n, and the gravitational force, m g, do no work on the object cos = cos 90° = 0 The force F does do work on the object

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More About Work The system and the environment must be determined when dealing with work The environment does work on the system Work by the environment on
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