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PHYC 160
Conservation of Energy
Lecture #19
Elastic potential energy
• A body is elastic if it returns
to its original shape after
being deformed.
• Elastic potential energy is the
energy stored in an elastic
body, such as a spring.
• The elastic potential energy
stored in an ideal spring is Uel
= 1/2 kx2.
• Figure 7.14 at the right
shows a graph of the elastic
potential energy for an ideal
spring.
Situations with both gravitational and elastic forces
• When a situation involves both gravitational and elastic forces,
the total potential energy is the sum of the gravitational potential
energy and the elastic potential energy: U = Ugrav + Uel.
•
•
Figure 7.15 below illustrates such a situation.
Follow Problem-Solving Strategy 7.2.
Q7.5
A block is released from rest on a
frictionless incline as shown. When the
moving block is in contact with the spring
and compressing it, what is happening to
the gravitational potential energy Ugrav and
the elastic potential energy Uel?
A. Ugrav and Uel are both increasing.
B. Ugrav and Uel are both decreasing.
C. Ugrav is increasing; Uel is decreasing.
D. Ugrav is decreasing; Uel is increasing.
E. The answer depends on how the block’s speed is changing.
© 2012 Pearson Education, Inc.
Conservative and nonconservative forces
• A conservative force allows conversion between kinetic and
potential energy. Gravity and the spring force are conservative.
• The work done between two points by any conservative force
a) can be expressed in terms of a potential energy function.
b) is reversible.
c) is independent of the path between the two points.
d) is zero if the starting and ending points are the same.
• A force (such as friction) that is not conservative is called a
nonconservative force, or a dissipative force.
Conservative and Non-conservative
• The nice thing about the conservation of mechanical
energy is that the change in the potentials only are
determined by the initial and final points of the path.
That’s because potentials always describe conservative
forces – forces where the work done by them in going
from one point to another is path independent.
Non- conservative forces
• An example of a non-conservative force is
friction. The work done by friction is definitely
dependent on the path.
• Let’s take the example of moving a book on a
table with kinetic friction:
Path 2
Path 1
• Since path 2 is longer, there will be more work
done by friction.
Force and potential energy in one dimension
• In one dimension, a conservative
force can be obtained from its
potential energy function using
Fx(x) = –dU(x)/dx
• Figure 7.22 at the right illustrates
this point for spring and
gravitational forces.
• Follow Example 7.13 for an
electric force.
Energy diagrams
• An energy diagram is a
graph that shows both the
potential-energy function
U(x) and the total
mechanical energy E.
• Figure 7.23 illustrates the
energy diagram for a glider
attached to a spring on an
air track.
Force and a graph of its potential-energy function
• Figure 7.24 below helps relate a force to a graph of its
corresponding potential-energy function.
Ball launcher problem
TOTAL potential
energy of the ballspring system