A form of energy associated with the presence of conservative interactions such as gravity or a spring.
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Motivation for Concept
Conservative interactions like [gravity] have the ability to "store" kinetic energy. Consider an object thrown up to a high roof. If the object is thrown perfectly, the force of gravity will slow it to a stop just as it reaches the roof. The object will then remain at rest on the roof until disturbed. But, as it falls, gravity will restore all the speed that was removed on the way up. Because we can perfectly "recover" the kinetic energy "removed" by gravity, we can consider the total energy to be constant in such a situation if we can associate some energy with the object's height. That energy is the [gravitational potential energy].
Definition
Finding Potential Energy From Force
The work done by a general force is given by:
\begin
[ W = \int_
\vec
\cdot d\vec
]\end
The [work energy theorem] tells us that when work is done on a system, the system's kinetic energy will change:
\begin
[ K_
+ W = K_
]\end
Suppose that we consider a system acted upon by a single, conservative force. If we want to define a potential energy U to represent this interaction in such a way that the mechanical energy of the system is conserved, we must take:
\begin
[ U_
- U_
= W^
]\end
With this definition, the work-energy theorem takes the form:
\begin
[ K_
+ (U_
-U_
) = K_
]\end
which is equivalent to the conservation of mechanical energy:
\begin
[ K_
+ U_
= K_
+ U_
]\end
The definition we have arrived at expresses potential energy in terms of force through the application of a path integral:
\begin
[ U_
- U_
= - \int_
\vec
^
\cdot d\vec
]\end
it is important to note, however, that the work done by conservative forces is, by definition, path independent. Thus, the integrals can be done using the most advantageous path, and the value will depend only upon the initial and final positions of the system. We can therefore write:
\begin
[ U_
- U_
= - \int_{\vec
_{i}}^{\vec
_{f}} \vec
^
\cdot d\vec
] \end
Note also that the expression we have found is only useful for computing potential energy differences. The formula's validity does not depend upon the precise value of Uf or Ui, but instead upon the difference. That means that an arbitrary constant can be added to the potential energy without affecting its usefulness. In problems involving potential energy, then, it is customary to specify a zero point for the potential energy (r0) such that:
\begin
[ U(\vec
_
) = 0 ]\end
Finding Force From Potential Energy
Taking the componentwise derivative of the above definition of potential energy with respect to position yields the three expressions:
\begin
[ -\frac
= F^
_
]
[ -\frac
= F^
_
]
[ -\frac
= F^
_
]\end
Thus, given information about the dependence of the potential energy on position, the force acting on the system subject to that potential energy can be determined.
Potential Energy Curves
A [potential energy curve] is a graphical representation of a system's potential energy as a function of posittion. This can be done for any system, but it is most often drawn for a system confined to move in one dimension (since multidimensional graphs are difficult to draw and interpret). The graph can be useful in furthering both qualitative and quantitative understanding of the system's behavior.
Common Types
Gravitational Potential Energy Near Earth
Near the earth's surface, if we assume coordinates with the +y direction pointing upward, the force of gravity can be written:
\begin
[ \vec
= -mg \hat
]\end
Since the "natural" ground level varies depending upon the specific situation, it is customary to specify the coordinate system such that:
\begin
[ U(0) \equiv 0]\end
The [gravitational potential energy] at any other height y can then be found by choosing a path for the work integral that is perfectly vertical, such that:
\begin
[ U
= U(0) - \int_
^
(-mg)\;dy = mgy]\end
For an object in vertical freefall (no horizontal motion) the associated potential energy curve would then be:
POTENTIAL ENERGY CURVE
For movement under pure near-earth gravity, then, there is no equilibrium point. At least one other force, such as a normal force, tension, etc., must be present to produce equilibrium.
Elastic Potential Energy
Assuming an object attached to a spring that obeys Hooke's Law with the motion confined to the x direction, it is customary to choose the coordinates such that x = 0 when the object is in a position such that the spring is at its natural length. The force on the object from the spring is then:
\begin
[ \vec
= - kx \hat
]\end
It is also customary to make the assignment:
\begin
[ U(0) \equiv 0]\end
Thus, the potential can be defined:
\begin
[ U
= U(0) - \int_
^
(-kx)\:dx = \frac
kx^
]\end
For an object moving under the influence of a spring only, the associated potential energy curve would then be:
POTENTIAL ENERGY CURVE
For such a motion, then, there is one stable equilibrium point at x = 0.
Universal Gravitational Potential Energy
For two spherically symmetric objects (objects 1 and 2), it is customary to analyze the [gravitational] interaction by placing one of the objects at the origin.