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A quantity representing the sum of the kinetic and the potential energy of a mechanical system at a certain moment in time.
 
A quantity representing the sum of the kinetic and the potential energy of a mechanical system at a certain moment in time.
  
Suppose, for example, that in a bounded domain <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e0356401.png" /> with piecewise-smooth boundary <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e0356402.png" /> for a hyperbolic partial differential equation
+
Suppose, for example, that in a bounded domain $  G $
 +
with piecewise-smooth boundary $  S $
 +
for a hyperbolic partial differential equation
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e0356403.png" /></td> <td valign="top" style="width:5%;text-align:right;">(1)</td></tr></table>
+
$$ \tag{1 }
 +
\rho
 +
\frac{\partial  ^ {2} u }{\partial  t  ^ {2} }
 +
  = \
 +
\mathop{\rm div} ( p  \mathop{\rm grad}  u ) - q u + F
 +
( x , t )  \equiv  - L u + F ( x , t ) ,
 +
$$
  
 
where
 
where
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e0356404.png" /></td> </tr></table>
+
$$
 +
p  \in  C  ^ {1} ( G) ,\  q  \in  C ( \overline{G}\; ) ,\ \
 +
p ( x)  > 0 ,\  q ( x)  \geq  0 ,\ \
 +
\rho  \in  C ( G) ,
 +
$$
  
 
one poses the mixed problem
 
one poses the mixed problem
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e0356405.png" /></td> <td valign="top" style="width:5%;text-align:right;">(2)</td></tr></table>
+
$$ \tag{2 }
 +
\left . u \right | _ {t = + 0 }  = u _ {0} ( x) ,\ \
 +
\left .  
 +
\frac{\partial  u }{\partial  t }
 +
\right | _ {t = + 0 }  = \
 +
u _ {1} ( x) ,
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e0356406.png" /></td> <td valign="top" style="width:5%;text-align:right;">(3)</td></tr></table>
+
$$ \tag{3 }
 +
\alpha u + \beta \left .
 +
\frac{\partial  u }{\partial  n }
 +
\right | _ {S}  = 0 ,\  t > 0 ,
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e0356407.png" /></td> </tr></table>
+
$$
 +
\alpha , \beta  \in  C ( S) ,\  \alpha ( x),\
 +
\beta ( x)  \geq  0 ,\  \alpha ( x) + \beta ( x)  > 0 .
 +
$$
  
A classical solution of (2)–(3) is a function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e0356408.png" /> of class <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e0356409.png" /> satisfying (1) in the cylinder <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e03564010.png" />, the initial conditions (2) on the lower base, and the boundary condition (3) on the lateral surface of the cylinder.
+
A classical solution of (2)–(3) is a function $  u ( x , t ) $
 +
of class $  C  ^ {2} ( G \times ( 0 , \infty ) ) \cap C  ^ {1} ( \overline{ {G \times ( 0 , \infty ) }}\; ) $
 +
satisfying (1) in the cylinder $  G \times ( 0 , \infty ) $,
 +
the initial conditions (2) on the lower base, and the boundary condition (3) on the lateral surface of the cylinder.
  
 
Then the relation
 
Then the relation
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e03564011.png" /></td> <td valign="top" style="width:5%;text-align:right;">(4)</td></tr></table>
+
$$ \tag{4 }
 +
J  ^ {2} ( t)  = J  ^ {2} ( 0) + \int\limits _ { 0 } ^ { t }  \int\limits _ { G }
 +
F ( x , \tau )
 +
\frac{\partial  u ( x, \tau ) }{\partial  \tau }
 +
\
 +
d \tau  dx ,\  t \geq  0 ,
 +
$$
  
 
holds, where
 
holds, where
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e03564012.png" /></td> </tr></table>
+
$$
 +
J  ^ {2} ( 0)  =
 +
\frac{1}{2}
 +
\int\limits _ { G } ( \rho u _ {1}  ^ {2} + p |
 +
\mathop{\rm grad}  u _ {0} |  ^ {2} + q u _ {0}  ^ {2} )  dx +
 +
\frac{1}{2}
 +
 
 +
\int\limits _ {S _ {0} } p
 +
\frac \alpha  \beta
 +
u _ {0}  ^ {2}  d S .
 +
$$
  
 
The energy integral is defined as the quantity
 
The energy integral is defined as the quantity
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e03564013.png" /></td> </tr></table>
+
$$
 +
J  ^ {2} ( t)  =
 +
\frac{1}{2}
 +
\int\limits _ { G } \left [
 +
\rho \left (
 +
\frac{\partial  u }{\partial  t }
 +
\right )  ^ {2} + p
 +
|  \mathop{\rm grad}  u |  ^ {2} + q u  ^ {2} \right ]  d x +
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e03564014.png" /></td> </tr></table>
+
$$
 +
+
  
For <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e03564015.png" /> equality (4) takes the form
+
\frac{1}{2}
 +
\int\limits _ {S _ {0} } p
 +
\frac \alpha  \beta
 +
u  ^ {2}  d S .
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e035/e035640/e03564016.png" /></td> </tr></table>
+
For  $  F = 0 $
 +
equality (4) takes the form
 +
 
 +
$$
 +
J  ^ {2} ( t)  = J  ^ {2} ( 0) ,\ \
 +
t \geq  0 .
 +
$$
  
 
The physical meaning of the energy integral consists in the fact that the total energy of an oscillating system in the absence of external perturbations does not change in time (the law of conservation of energy).
 
The physical meaning of the energy integral consists in the fact that the total energy of an oscillating system in the absence of external perturbations does not change in time (the law of conservation of energy).
Line 41: Line 116:
 
====References====
 
====References====
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  V.S. Vladimirov,  "Equations of mathematical physics" , MIR  (1984)  (Translated from Russian)  {{MR|0764399}} {{ZBL|0954.35001}} {{ZBL|0652.35002}} {{ZBL|0695.35001}} {{ZBL|0699.35005}} {{ZBL|0607.35001}} {{ZBL|0506.35001}} {{ZBL|0223.35002}} {{ZBL|0231.35002}} {{ZBL|0207.09101}} </TD></TR></table>
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  V.S. Vladimirov,  "Equations of mathematical physics" , MIR  (1984)  (Translated from Russian)  {{MR|0764399}} {{ZBL|0954.35001}} {{ZBL|0652.35002}} {{ZBL|0695.35001}} {{ZBL|0699.35005}} {{ZBL|0607.35001}} {{ZBL|0506.35001}} {{ZBL|0223.35002}} {{ZBL|0231.35002}} {{ZBL|0207.09101}} </TD></TR></table>
 
 
  
 
====Comments====
 
====Comments====
 
  
 
====References====
 
====References====
 
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  O.A. Ladyzhenskaya,  N.N. Ural'tseva,  "Equations aux dérivées partielles de type elliptique" , Dunod  (1969)  (Translated from Russian)  {{MR|}} {{ZBL|0164.13001}} </TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  O.A. Ladyzhenskaya,  V.A. Solonnikov,  N.N. Ural'tseva,  "Linear and quasi-linear equations of parabolic type" , Amer. Math. Soc.  (1968)  (Translated from Russian)  {{MR|}} {{ZBL|0174.15403}} </TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  L. Bers,  F. John,  M. Schechter,  "Partial differential equations" , Interscience  (1966)  {{MR|0208121}} {{ZBL|0143.32403}} </TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  F. John,  "Partial differential equations" , Springer  (1978)  {{MR|0514404}} {{MR|0502136}} {{MR|0470380}} {{ZBL|0456.65060}} {{ZBL|0426.35002}} {{ZBL|0413.93002}} {{ZBL|0397.35041}} {{ZBL|0396.35037}} {{ZBL|0381.35059}} {{ZBL|0377.65045}} {{ZBL|0368.34003}} </TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  P.R. Garabedian,  "Partial differential equations" , Wiley  (1967)  {{MR|1657375}} {{MR|0943117}} {{MR|0162045}} {{MR|0176086}} {{MR|0129167}} {{MR|0120441}} {{MR|0060698}} {{MR|0054819}} {{MR|0046440}} {{ZBL|0913.35001}} {{ZBL|0124.30501}} {{ZBL|0133.04402}} {{ZBL|0096.06503}} {{ZBL|0058.08902}} {{ZBL|0050.10002}} </TD></TR></table>
 
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  O.A. Ladyzhenskaya,  N.N. Ural'tseva,  "Equations aux dérivées partielles de type elliptique" , Dunod  (1969)  (Translated from Russian)  {{MR|}} {{ZBL|0164.13001}} </TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  O.A. Ladyzhenskaya,  V.A. Solonnikov,  N.N. Ural'tseva,  "Linear and quasi-linear equations of parabolic type" , Amer. Math. Soc.  (1968)  (Translated from Russian)  {{MR|}} {{ZBL|0174.15403}} </TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  L. Bers,  F. John,  M. Schechter,  "Partial differential equations" , Interscience  (1966)  {{MR|0208121}} {{ZBL|0143.32403}} </TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  F. John,  "Partial differential equations" , Springer  (1978)  {{MR|0514404}} {{MR|0502136}} {{MR|0470380}} {{ZBL|0456.65060}} {{ZBL|0426.35002}} {{ZBL|0413.93002}} {{ZBL|0397.35041}} {{ZBL|0396.35037}} {{ZBL|0381.35059}} {{ZBL|0377.65045}} {{ZBL|0368.34003}} </TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  P.R. Garabedian,  "Partial differential equations" , Wiley  (1967)  {{MR|1657375}} {{MR|0943117}} {{MR|0162045}} {{MR|0176086}} {{MR|0129167}} {{MR|0120441}} {{MR|0060698}} {{MR|0054819}} {{MR|0046440}} {{ZBL|0913.35001}} {{ZBL|0124.30501}} {{ZBL|0133.04402}} {{ZBL|0096.06503}} {{ZBL|0058.08902}} {{ZBL|0050.10002}} </TD></TR></table>

Revision as of 19:37, 5 June 2020


A quantity representing the sum of the kinetic and the potential energy of a mechanical system at a certain moment in time.

Suppose, for example, that in a bounded domain $ G $ with piecewise-smooth boundary $ S $ for a hyperbolic partial differential equation

$$ \tag{1 } \rho \frac{\partial ^ {2} u }{\partial t ^ {2} } = \ \mathop{\rm div} ( p \mathop{\rm grad} u ) - q u + F ( x , t ) \equiv - L u + F ( x , t ) , $$

where

$$ p \in C ^ {1} ( G) ,\ q \in C ( \overline{G}\; ) ,\ \ p ( x) > 0 ,\ q ( x) \geq 0 ,\ \ \rho \in C ( G) , $$

one poses the mixed problem

$$ \tag{2 } \left . u \right | _ {t = + 0 } = u _ {0} ( x) ,\ \ \left . \frac{\partial u }{\partial t } \right | _ {t = + 0 } = \ u _ {1} ( x) , $$

$$ \tag{3 } \alpha u + \beta \left . \frac{\partial u }{\partial n } \right | _ {S} = 0 ,\ t > 0 , $$

$$ \alpha , \beta \in C ( S) ,\ \alpha ( x),\ \beta ( x) \geq 0 ,\ \alpha ( x) + \beta ( x) > 0 . $$

A classical solution of (2)–(3) is a function $ u ( x , t ) $ of class $ C ^ {2} ( G \times ( 0 , \infty ) ) \cap C ^ {1} ( \overline{ {G \times ( 0 , \infty ) }}\; ) $ satisfying (1) in the cylinder $ G \times ( 0 , \infty ) $, the initial conditions (2) on the lower base, and the boundary condition (3) on the lateral surface of the cylinder.

Then the relation

$$ \tag{4 } J ^ {2} ( t) = J ^ {2} ( 0) + \int\limits _ { 0 } ^ { t } \int\limits _ { G } F ( x , \tau ) \frac{\partial u ( x, \tau ) }{\partial \tau } \ d \tau dx ,\ t \geq 0 , $$

holds, where

$$ J ^ {2} ( 0) = \frac{1}{2} \int\limits _ { G } ( \rho u _ {1} ^ {2} + p | \mathop{\rm grad} u _ {0} | ^ {2} + q u _ {0} ^ {2} ) dx + \frac{1}{2} \int\limits _ {S _ {0} } p \frac \alpha \beta u _ {0} ^ {2} d S . $$

The energy integral is defined as the quantity

$$ J ^ {2} ( t) = \frac{1}{2} \int\limits _ { G } \left [ \rho \left ( \frac{\partial u }{\partial t } \right ) ^ {2} + p | \mathop{\rm grad} u | ^ {2} + q u ^ {2} \right ] d x + $$

$$ + \frac{1}{2} \int\limits _ {S _ {0} } p \frac \alpha \beta u ^ {2} d S . $$

For $ F = 0 $ equality (4) takes the form

$$ J ^ {2} ( t) = J ^ {2} ( 0) ,\ \ t \geq 0 . $$

The physical meaning of the energy integral consists in the fact that the total energy of an oscillating system in the absence of external perturbations does not change in time (the law of conservation of energy).

References

[1] V.S. Vladimirov, "Equations of mathematical physics" , MIR (1984) (Translated from Russian) MR0764399 Zbl 0954.35001 Zbl 0652.35002 Zbl 0695.35001 Zbl 0699.35005 Zbl 0607.35001 Zbl 0506.35001 Zbl 0223.35002 Zbl 0231.35002 Zbl 0207.09101

Comments

References

[a1] O.A. Ladyzhenskaya, N.N. Ural'tseva, "Equations aux dérivées partielles de type elliptique" , Dunod (1969) (Translated from Russian) Zbl 0164.13001
[a2] O.A. Ladyzhenskaya, V.A. Solonnikov, N.N. Ural'tseva, "Linear and quasi-linear equations of parabolic type" , Amer. Math. Soc. (1968) (Translated from Russian) Zbl 0174.15403
[a3] L. Bers, F. John, M. Schechter, "Partial differential equations" , Interscience (1966) MR0208121 Zbl 0143.32403
[a4] F. John, "Partial differential equations" , Springer (1978) MR0514404 MR0502136 MR0470380 Zbl 0456.65060 Zbl 0426.35002 Zbl 0413.93002 Zbl 0397.35041 Zbl 0396.35037 Zbl 0381.35059 Zbl 0377.65045 Zbl 0368.34003
[a5] P.R. Garabedian, "Partial differential equations" , Wiley (1967) MR1657375 MR0943117 MR0162045 MR0176086 MR0129167 MR0120441 MR0060698 MR0054819 MR0046440 Zbl 0913.35001 Zbl 0124.30501 Zbl 0133.04402 Zbl 0096.06503 Zbl 0058.08902 Zbl 0050.10002
How to Cite This Entry:
Energy integral. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Energy_integral&oldid=28187
This article was adapted from an original article by A.B. Ivanov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article