chapter05

chapter05 - Chapter 5 Energy Forms of Energy Mechanical...

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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 The total amount of energy in the Universe never changes 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 W = F x This equation applies when the force is in the same direction as the displacement are in the same direction and Fx

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Work General W = (F cos ) x F is the magnitude of the force Δ x is the magnitude of the object’s displacement is the angle between and Fx
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

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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
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

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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
When Work is Zero Displacement is horizontal Force is vertical cos 90° = 0

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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
Work, Final Work doesn’t happen by itself Work is done by something in the environment, on the object of interest The forces are constant Varying force will be discussed later

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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 in the object’s kinetic energy Speed will increase if work is positive Speed will decrease if work is negative net f i W KE
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chapter05 - Chapter 5 Energy Forms of Energy Mechanical...

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