Transcript Chapter 5 - Physics

Chapter 5 - Physics
Work and Energy
Section 1 objectives
 Recognize
the difference between the
scientific and ordinary definition of work.
 Define work, relating it to force and
displacement.
 Identify where work in being performed
in a variety of ways.
 Calculate work done when many forces
are applied to an object.
Work
 Work
– The product of the magnitudes of
the component of a force along the
direction of displacement and the
displacement.
 Work
is not done unless the object is moved.
 Work is only done when components of a
force are parallel to a displacement
 Components of the force perpendicular to
a displacement do no work.
Work
W
= Fd(cosθ)
 Do
 Sign
sample problems 5A on page 169.
of work
 Page
170; figure 5-3
 Work is + when the force is in the same
direction of the displacement
 Work is – when the force is in the opposite
direction of the displacement
objectives

Identify several forms of mechanical energy.
 Calculate kinetic energy for objects.
 Distinguish between kinetic and potential energy.
 Classify different types of potential energy.
 Calculate an object’s potential energy.
 Relate kinetic and all forms of potential energy to the
idea of mechanical energy.
Kinetic Energy
 Kinetic
Energy-The energy of an
object due to its motion.
Depends on both mass and
velocity.
KE = ½mv2
Do practice problems 5B, page 173
Potential Energy
 The
energy associated with an object due
to its position.
 Different types of potential energy:
 Gravitational Potential Energy: The
energy assoc. w/ an object due to its
position relative to the Earth or some
other gravitational source.
 PEg=mgh
Potential Energy
 Elastic
Potential Energy: The energy in a
stretched or compressed spring
 Peelastic=½kx2
 k= spring constant
 x=distance compressed or stretched
 Spring constant= A parameter that
expresses how resistant a spring is to
being compressed or stretched.
 Do practice problems 5C; page 177
Mechanical Energy
 The
sum of the kinetic energy and all
forms of potential energy
Energy
Mechanical
Nonmechanical
Kinetic
Potential
Gravitational Elastic
Nonmechanical Energy- other forms
besides kinetic and potential
objectives
 Identify
situations in which conservation
of mechanical energy is valid.
 Recognize the forms that conserved
energy can take.
 Solve problems using conservation of
mechanical energy.
Conservation of Energy
 Energy
 See
is conserved
example pg 180; figure 5-1
 In
the absence of friction, mechanical
energy is conserved, but can change forms
 MEi=Mef
 ½mv2i + mghi
= ½mv2f + mghf
 Do practice problems 5D; pg. 182
 When
friction is present, mech. E is not
conserved – it changes to other forms of
nonmech. energy.
objectives
 Objectives
 Apply
the work-kinetic energy theorem
to solve problems.
 Relate the concepts of energy, power,
and time
 Calculate power in two different ways
 Explain the effect of machines on work
and power.
Work, Power, and Energy
 Work-Kinetic
 The
Energy Theorem
net work done on an object is equal to
the change in the kinetic energy of the
object.
 Wnet=ΔKE
 Work is a method of energy transfer
 Do practice problems 5E, pg. 186
Work, Power, and Energy

Power- the rate at which energy is transferred.





P=W/ΔT (Power = work/time)
Remember W=Fd, so P =Fd/t, but d/t = v, so this can
be simplified to say that P=Fv.
You can use any of these equations depending on the
given information.
SI unit of power = Watt (W)
 1 W = 1 J/s
 1 hp = 746W (hp-horsepower is the English unit)
Do practice problems 5F, PG. 188
Chapter 5 problem set
 Pg.
193-199
 #2, 3, 5, 6, 7, 10, 12, 13, 14, 16, 19, 23, 27,
31, 32, 35, 39, 40, 41, 48, 52.