Transcript Honors Physics Unit 7 Notes

Chapter 5
Section 1 Work
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.
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Chapter 5
Section 1 Work
Definition of Work
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Chapter 5
Section 1 Work
Sign Conventions for Work
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Chapter 5
Section 2 Energy
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.
1
KE  mv 2
2
1
2
kinetic energy =  mass   speed 
2
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Chapter 5
Section 2 Energy
Kinetic Energy
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Chapter 5
Section 2 Energy
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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Chapter 5
Section 2 Energy
Work-Kinetic Energy Theorem
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Chapter 5
Section 2 Energy
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 = mgh
gravitational PE = mass  free-fall acceleration  height
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Section 2 Energy
Chapter 5
Potential Energy
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Chapter 5
Section 2 Energy
Potential Energy, continued
•
Elastic potential energy is the energy available for
use when a deformed elastic object returns to its
original configuration.
1 2
PEelastic  kx
2
elastic PE =
1
 spring constant  (distance compressed or stretched)
2
•
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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2
Chapter 5
Section 2 Energy
Elastic Potential Energy
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Section 2 Energy
Chapter 5
Spring Constant
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Chapter 5
Section 3 Conservation of
Energy
Conserved Quantities
• When we say that something is conserved, we mean
that it remains constant.
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Chapter 5
Section 3 Conservation of
Energy
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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Chapter 5
Section 3 Conservation of
Energy
Conservation of Mechanical Energy
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Chapter 5
Section 3 Conservation of
Energy
Mechanical Energy, continued
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•
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.
Chapter 5
Section 4 Power
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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Chapter 5
Section 4 Power
Power
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