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chapter5_151

# Essentials of College Physics (with CengageNOW 2-Semester and Personal Tutor Printed Access Card)

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Chapter 5 Energy

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Forms of Energy Mechanical Focus for now May be kinetic (associated with motion) or  potential (associated with position) Chemical Electromagnetic Nuclear
Some Energy Considerations Energy can be transformed from one  form to another Essential to the study of physics,  chemistry, biology, geology, astronomy Can be used in place of Newton’s laws  to solve certain problems more simply

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Work Provides a link between force and  energy The work,  W , done by a constant force  on an object is defined as the product of  the component of the force along the  direction of displacement and the  magnitude of the displacement
Work, cont.   F is the magnitude of  the force Δ x is the magnitude  of the object’s  displacement θ  is the angle  between  x ) cos F ( W θ and F x r r

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Work, cont. This gives no information about the time it took for the displacement to  occur the velocity or acceleration of the object Work is a scalar quantity
Units of Work SI Newton • meter = Joule N • m = J J = kg • m 2  / s 2 US Customary foot  • pound ft • lb no special name

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More About Work The work done by a force is zero when  the force is perpendicular to the  displacement cos 90° = 0 If there are multiple forces acting on an  object, the total work done is the  algebraic sum of the amount of work  done by each force
More About Work, cont. Work can be positive or negative Positive if the force and the displacement  are in the same direction Negative if the force and the displacement  are in the opposite direction

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When Work is Zero Displacement is  horizontal Force is vertical cos 90° = 0
Work Can Be Positive or  Negative Work is positive  when lifting the box Work would be  negative if lowering  the box The force would still  be upward, but the  displacement would  be downward

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Work and Dissipative Forces Work can be done by friction The energy lost to friction by an object goes  into heating both the object and its  environment Some energy may be converted into sound For now, the phrase “Work done by friction”  will denote the effect of the friction processes  on mechanical energy alone
Kinetic Energy Energy associated with the motion of an  object   Scalar quantity with the same units as  work Work is related to kinetic energy 2 mv 2 1 KE =

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Work-Kinetic Energy Theorem When work is done by a net force on an  object and the only change in the object is its  speed, the work done is equal to the change
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chapter5_151 - Chapter5 Energy FormsofEnergy Mechanical...

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