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Chapter 5
Definition of Work
• Work is done on an object when a force causes a displacement of the object.
• Work is done only when components of a force are parallel to a displacement.
Section 1 Work
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Chapter 5
Definition of Work
Section 1 Work
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Visual Concept
Chapter 5 Section 1 Work
Sign Conventions for Work
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Section 2 EnergyChapter 5
Kinetic Energy
• Kinetic Energy
The energy of an object that is due to the object’s motion is called kinetic energy.
• Kinetic energy depends on speed and mass.
2
2
1
21
kinetic energy = mass speed2
KE mv
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Visual Concept
Chapter 5 Section 2 Energy
Kinetic Energy
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Section 2 EnergyChapter 5
Kinetic Energy, continued
• Work-Kinetic Energy Theorem– The net work done by all the forces acting on an
object is equal to the change in the object’s kinetic energy.
• The net work done on a body equals its change in kinetic energy.
Wnet = ∆KE
net work = change in kinetic energy
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Visual Concept
Chapter 5 Section 2 Energy
Work-Kinetic Energy Theorem
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Section 2 EnergyChapter 5
Potential Energy
• Potential Energy is the energy associated with an object because of the position, shape, or condition of the object.
• Gravitational potential energy is the potential energy stored in the gravitational fields of interacting bodies.
• Gravitational potential energy depends on height from a zero level.
PEg = mghgravitational PE = mass free-fall acceleration height
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Visual Concept
Chapter 5 Section 2 Energy
Potential Energy
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Section 2 EnergyChapter 5
Potential Energy, continued• Elastic potential energy is the energy available for
use when a deformed elastic object returns to its original configuration.
2
2
1elastic PE = spring constant (distance compressed or stretched)
2
1
2elasticPE kx
• The symbol k is called the spring constant, a parameter that measures the spring’s resistance to being compressed or stretched.
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Chapter 5
Elastic Potential Energy
Section 2 Energy
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Visual Concept
Chapter 5 Section 2 Energy
Spring Constant
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Section 3 Conservation of EnergyChapter 5
Conserved Quantities
• When we say that something is conserved, we mean that it remains constant.
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Section 3 Conservation of EnergyChapter 5
Mechanical Energy
• Mechanical energy is the sum of kinetic energy and all forms of potential energy associated with an object or group of objects.
ME = KE + ∑PE
• Mechanical energy is often conserved.
MEi = MEf
initial mechanical energy = final mechanical energy (in the absence of friction)
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Visual Concept
Chapter 5Section 3 Conservation of Energy
Conservation of Mechanical Energy
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Section 3 Conservation of EnergyChapter 5
Mechanical Energy, continued
• Mechanical Energy is not conserved in the presence of friction.
• As a sanding block slides on a piece of wood, energy (in the form of heat) is dissipated into the block and surface.
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Section 4 PowerChapter 5
Rate of Energy Transfer
• Power is a quantity that measures the rate at which work is done or energy is transformed.
P = W/∆t
power = work ÷ time interval
• An alternate equation for power in terms of force and speed is
P = Fv
power = force speed
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Visual Concept
Chapter 5 Section 4 Power
Power
