Download 05_Work and Energy

Lecture Outline
Chapter 5
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
Chapter 5
Work and Energy
© 2010 Pearson Education, Inc.
Units of Chapter 5
Work Done by a Constant Force
Work Done by a Variable Force
The Work–Energy Theorem: Kinetic Energy
Potential Energy
Conservation of Energy
Power
© 2010 Pearson Education, Inc.
5.1 Work Done by a Constant Force
Definition of work:
The work done by a constant force acting on an
object is equal to the product of the magnitudes of the
displacement and the component of the force parallel
to that displacement.
© 2010 Pearson Education, Inc.
5.1 Work Done by a Constant Force
In (a), there is a force but no displacement: no
work is done. In (b), the force is parallel to the
displacement, and in (c) the force is at an angle
to the displacement.
© 2010 Pearson Education, Inc.
5.1 Work Done by a Constant Force
If the force is at an angle to the displacement,
as in (c), a more general form for the work
must be used:
Unit of work: newton • meter (N • m)
1 N • m is called 1 joule.
© 2010 Pearson Education, Inc.
5.1 Work Done by a
Constant Force
If the force (or a component) is
in the direction of motion, the
work done is positive.
If the force (or a component) is
opposite to the direction of
motion, the work done is
negative.
© 2010 Pearson Education, Inc.
5.1 Work Done by a Constant Force
If there is more than one force acting on an
object, it is useful to define the net work:
The total, or net, work is defined as the work done
by all the forces acting on the object, or the scalar
sum of all those quantities of work.
© 2010 Pearson Education, Inc.
5.2 Work Done by a Variable Force
The force exerted by a
spring varies linearly
with the displacement:
© 2010 Pearson Education, Inc.
5.2 Work Done by a Variable Force
A plot of force versus displacement allows
us to calculate the work done:
© 2010 Pearson Education, Inc.
5.3 The Work–Energy Theorem:
Kinetic Energy
The net force acting on an object causes the
object to accelerate, changing its velocity:
© 2010 Pearson Education, Inc.
5.3 The Work–Energy Theorem:
Kinetic Energy
We can use this relation to calculate the
work done:
© 2010 Pearson Education, Inc.
5.3 The Work–Energy Theorem:
Kinetic Energy
Kinetic energy is therefore defined:
The net work on an object changes its
kinetic energy.
© 2010 Pearson Education, Inc.
5.3 The Work–Energy Theorem:
Kinetic Energy
This relationship is called the work–energy theorem.
© 2010 Pearson Education, Inc.
5.4 Potential Energy
Potential energy may be thought of as stored
work, such as in a compressed spring or an
object at some height above the ground.
Work done also changes the potential energy
(U) of an object.
© 2010 Pearson Education, Inc.
5.4 Potential Energy
We can, therefore, define the potential
energy of a spring; note that, as the
displacement is squared, this expression is
applicable for both compressed and
stretched springs.
© 2010 Pearson Education, Inc.
5.4 Potential Energy
Gravitational
potential energy:
© 2010 Pearson Education, Inc.
5.4 Potential Energy
Only changes in potential energy are
physically significant; therefore, the point
where U = 0 may be chosen for convenience.
© 2010 Pearson Education, Inc.
5.5 Conservation of Energy
We observe that, once all forms of energy are
accounted for, the total energy of an isolated
system does not change. This is the law of
conservation of energy:
The total energy of an isolated system is always
conserved.
We define a conservative force:
A force is said to be conservative if the work done by
it in moving an object is independent of the object’s
path.
© 2010 Pearson Education, Inc.
5.5 Conservation of Energy
So, what types of forces are conservative?
Gravity is one; the work done by gravity
depends only on the difference between the
initial and final height, and not on the path
between them.
Similarly, a nonconservative force:
A force is said to be nonconservative if the work
done by it in moving an object does depend on the
object’s path.
The quintessential nonconservative force is
friction.
© 2010 Pearson Education, Inc.
5.5 Conservation of Energy
Another way of describing a conservative
force:
A force is conservative if the work done by it in
moving an object through a round trip is zero.
We define the total mechanical energy:
© 2010 Pearson Education, Inc.
5.5 Conservation of Energy
For a conservative force:
Many kinematics problems are much
easier to solve using energy conservation.
© 2010 Pearson Education, Inc.
5.5 Conservation of Energy
All three of these balls have the same initial
kinetic energy; as the change in potential
energy is also the
same for all three, their
speeds just before they
hit the bottom are the
same as well.
© 2010 Pearson Education, Inc.
5.5 Conservation of Energy
In a conservative system, the total mechanical
energy does not change, but the split between
kinetic and potential energy does.
© 2010 Pearson Education, Inc.
5.5 Conservation of Energy
If a nonconservative force or forces are
present, the work done by the net
nonconservative force is equal to the change
in the total mechanical energy.
© 2010 Pearson Education, Inc.
5.6 Power
The average power is the total amount of
work done divided by the time taken to do the
work.
If the force is constant and parallel to the
displacement,
© 2010 Pearson Education, Inc.
5.6 Power
Mechanical efficiency:
The efficiency of any real system is always
less than 100%.
© 2010 Pearson Education, Inc.
5.6 Power
© 2010 Pearson Education, Inc.
Review of Chapter 5
Work done by a constant force is the
displacement times the component of force in
the direction of the displacement.
Kinetic energy is the energy of motion.
Work–energy theorem: the net work done on
an object is equal to the change in its kinetic
energy.
Potential energy is the energy of position or
configuration.
© 2010 Pearson Education, Inc.
Review of Chapter 5
The total energy of the universe, or of an
isolated system, is conserved.
Total mechanical energy is the sum of kinetic
and potential energy. It is conserved in a
conservative system.
The net work done by nonconservative forces
is equal to the change in the total mechanical
energy.
Power is the rate at which work is done.
© 2010 Pearson Education, Inc.