ch5+1410+FA10 - 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
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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
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Work, cont. ! ! F is the magnitude of the force ! Δ x is the magnitude of the object’s displacement ! θ is the angle between
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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
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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
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Concepts Decide for each case whether the work being done by the force shown is positive, negative, or zero. Δ x ~ F Δ x ~ F Δ x ~ F Δ x ~ F
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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
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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
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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
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Concepts 1. Thinking in terms of force, describe the motion of the car. 2. What is true in terms of work (energy)? Δ x ~ F ~ F drag w F n
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Concepts 1. Thinking graphically, is the work done by each force shown positive, negative, or zero in the figure below? 2. Find the net force graphically. Is the work done by the net force positive, negative, or zero? 3. If we added up all the work done by the individual forces would it be the same as the net work? Δ x ~ F ~ F drag w F n
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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
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SV-Ch05-P05 A horizontal force of 150 N is used to push a 40.0Lkg packing crate a distance of 6.00 m on a rough horizontal surface. If the crate moves at constant speed, find (a) the work done by the 150LN force and (b) the coefficient of kinetic friction between the crate and surface.
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Kinetic Energy !
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