SECTION 8.4 • Changes in Mechanical Energy for Nonconservative Forces
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This equation should look familiar to you. It is the general form of the equation for
work done by the gravitational force (Eq. 8.4) as an object moves relative to the Earth
and that for the work done by the spring force (Eq. 7.11) as the extension of the
spring changes.
Nonconservative Forces
A force is
nonconservative if it does not satisfy properties 1 and 2 for conservative
forces. Nonconservative forces acting within a system cause a change in the mechanical
energy E
mech
of the system. We have defined mechanical energy as the sum of the ki-
netic and all potential energies. For example, if a book is sent sliding on a horizontal
surface that is not frictionless, the force of kinetic friction reduces the book’s kinetic
energy. As the book slows down, its kinetic energy decreases. As a result of the friction
force, the temperatures of the book and surface increase. The type of energy associ-
ated with temperature is internal energy, which we introduced in Chapter 7. Only part
of the book’s kinetic energy is transformed to internal energy in the book. The rest ap-
pears as internal energy in the surface. (When you trip and fall while running across a
gymnasium floor, not only does the skin on your knees warm up, so does the floor!)
Because the force of kinetic friction transforms the mechanical energy of a system into
internal energy, it is a nonconservative force.
As an example of the path dependence of the work, consider Figure 8.10. Suppose
you displace a book between two points on a table. If the book is displaced in a straight
line along the blue path between points ! and " in Figure 8.10, you do a certain
amount of work against the kinetic friction force to keep the book moving at a con-
stant speed. Now, imagine that you push the book along the brown semicircular path
in Figure 8.10. You perform more work against friction along this longer path than
along the straight path. The work done depends on the path, so the friction force can-
not be conservative.
8.4 Changes in Mechanical Energy
for Nonconservative Forces
As we have seen, if the forces acting on objects within a system are conservative, then
the mechanical energy of the system is conserved. However, if some of the forces acting
on objects within the system are not conservative, then the mechanical energy of the
system changes.
Consider the book sliding across the surface in the preceding section. As the book
moves through a distance d, the only force that does work on it is the force of kinetic
friction. This force causes a decrease in the kinetic energy of the book. This decrease
was calculated in Chapter 7, leading to Equation 7.20, which we repeat here:
(8.13)
Suppose, however, that the book is part of a system that also exhibits a change in po-
tential energy. In this case, # f
k
d is the amount by which the mechanical energy of the
system changes because of the force of kinetic friction. For example, if the book moves
on an incline that is not frictionless, there is a change in both the kinetic energy and
the gravitational potential energy of the book–Earth system. Consequently,
In general, if a friction force acts within a system,
(8.14)
where !U is the change in all forms of potential energy. Notice that Equation 8.14 re-
duces to Equation 8.9 if the friction force is zero.
!
E
mech
" !
K % !U " #
f
k
d
!
E
mech
" !
K % !U
g
" #
f
k
d
!
K " #
f
k
d
!
"
Figure 8.10 The work done
against the force of kinetic friction
depends on the path taken as the
book is moved from ! to ". The
work is greater along the red path
than along the blue path.
Change in mechanical energy of
a system due to friction within
the system