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Edurev123 
6. Application of PDE 
6.1 A tightly stretched string has its end fixed at ?? =?? and ?? =?? . At time ?? =?? , the 
string is given a shape defined by ?? (?? )=???? (?? -?? ) , where ?? is a constant, and then 
released. Find the displacement of any point ?? of the string at time ?? >?? . 
(2009 : 30 Marks) 
Solution: 
The one dimensional wave equation is 
?
2
?? ??? 2
=
1
?? 2
?
2
?? ??? 2
(1)
 
Boundary condition (BC) are 
?? (0,?? )=?? (??,?? )=0 (2) 
Initial condition (IC) are: 
?? (?? ,0)=?? (?? )=h?? (?? -?? ) (3)
?? ?? (?? ,0)=0 (4)
 
Let ?? (?? ,?? )
'
=?? (?? )?? (?? ) be the trial solution where 
?? (?? ) is a function of ?? only. 
?? (?? ) is a function of ?? only. 
Then clearly by (1) 
?? '
?? =????
?? '
?? =
?? '
?? =?? (say) 
?? -???? =0
 and ??? -???? =0
 
Using (2) and (5), we get 
Now, 
?? (0,?? )?=0 and ?? (??,?? )=0
?? (?? ,?? )?=?? (?? )?? (?? )
 
As 
Page 2


Edurev123 
6. Application of PDE 
6.1 A tightly stretched string has its end fixed at ?? =?? and ?? =?? . At time ?? =?? , the 
string is given a shape defined by ?? (?? )=???? (?? -?? ) , where ?? is a constant, and then 
released. Find the displacement of any point ?? of the string at time ?? >?? . 
(2009 : 30 Marks) 
Solution: 
The one dimensional wave equation is 
?
2
?? ??? 2
=
1
?? 2
?
2
?? ??? 2
(1)
 
Boundary condition (BC) are 
?? (0,?? )=?? (??,?? )=0 (2) 
Initial condition (IC) are: 
?? (?? ,0)=?? (?? )=h?? (?? -?? ) (3)
?? ?? (?? ,0)=0 (4)
 
Let ?? (?? ,?? )
'
=?? (?? )?? (?? ) be the trial solution where 
?? (?? ) is a function of ?? only. 
?? (?? ) is a function of ?? only. 
Then clearly by (1) 
?? '
?? =????
?? '
?? =
?? '
?? =?? (say) 
?? -???? =0
 and ??? -???? =0
 
Using (2) and (5), we get 
Now, 
?? (0,?? )?=0 and ?? (??,?? )=0
?? (?? ,?? )?=?? (?? )?? (?? )
 
As 
Since ?? (?? )=0 leads to ?? =0 
So suppose that ?? (?? )?0 
Then clearly equation (8) gives us that ?? (0)=0 and ?? (??)=0 
Then ( 9 ) is modified boundary condition 
Now solving (6) and (9), three cases arise clearly: 
Case 1:?? =0 then clearly solution (6) will be ?? '
=0 
So,                          ?? ''
(?? )=?? 
?????????????????????????????????????? (?? )=???? +?? 
where ?? and ?? are arbitrary constants 
Clearly, ?? (0)=0 and ?? (?? )=0 
So, ??? =0 and ???? =0??? =0(?? ?0) 
So,                          ?? (?? )=0 
This leads to ?? (?? )=0 which does not satisfy (3) and (4). 
So, reject ?? =0 case. 
Case 2:?? is positive, i.e., ?? >0 
Clearly, ?? =?? 2
:(?? ?0)(?? >0) 
So clearly,              ?? '
-?? 2
?? =0. 
So,                            ?? 2
-?? 2
=0 
?                                        ?? =±?? 
So,                                 ?? (?? )=?? ?? ????
+?? ?? -????
 
Using (9), ?? (0)=0 and ?? (??)=0 
?                                 ?? +?? =0 and ?? ?? ????
+?? ?? -????
=0 
Now,                                  ?? =-?? as -?? (?? ????
-?? -????
)=0 
As                        ?? ????
-?? -????
=0 
?                                       ?? =0 
So                                     ?? =0 
Page 3


Edurev123 
6. Application of PDE 
6.1 A tightly stretched string has its end fixed at ?? =?? and ?? =?? . At time ?? =?? , the 
string is given a shape defined by ?? (?? )=???? (?? -?? ) , where ?? is a constant, and then 
released. Find the displacement of any point ?? of the string at time ?? >?? . 
(2009 : 30 Marks) 
Solution: 
The one dimensional wave equation is 
?
2
?? ??? 2
=
1
?? 2
?
2
?? ??? 2
(1)
 
Boundary condition (BC) are 
?? (0,?? )=?? (??,?? )=0 (2) 
Initial condition (IC) are: 
?? (?? ,0)=?? (?? )=h?? (?? -?? ) (3)
?? ?? (?? ,0)=0 (4)
 
Let ?? (?? ,?? )
'
=?? (?? )?? (?? ) be the trial solution where 
?? (?? ) is a function of ?? only. 
?? (?? ) is a function of ?? only. 
Then clearly by (1) 
?? '
?? =????
?? '
?? =
?? '
?? =?? (say) 
?? -???? =0
 and ??? -???? =0
 
Using (2) and (5), we get 
Now, 
?? (0,?? )?=0 and ?? (??,?? )=0
?? (?? ,?? )?=?? (?? )?? (?? )
 
As 
Since ?? (?? )=0 leads to ?? =0 
So suppose that ?? (?? )?0 
Then clearly equation (8) gives us that ?? (0)=0 and ?? (??)=0 
Then ( 9 ) is modified boundary condition 
Now solving (6) and (9), three cases arise clearly: 
Case 1:?? =0 then clearly solution (6) will be ?? '
=0 
So,                          ?? ''
(?? )=?? 
?????????????????????????????????????? (?? )=???? +?? 
where ?? and ?? are arbitrary constants 
Clearly, ?? (0)=0 and ?? (?? )=0 
So, ??? =0 and ???? =0??? =0(?? ?0) 
So,                          ?? (?? )=0 
This leads to ?? (?? )=0 which does not satisfy (3) and (4). 
So, reject ?? =0 case. 
Case 2:?? is positive, i.e., ?? >0 
Clearly, ?? =?? 2
:(?? ?0)(?? >0) 
So clearly,              ?? '
-?? 2
?? =0. 
So,                            ?? 2
-?? 2
=0 
?                                        ?? =±?? 
So,                                 ?? (?? )=?? ?? ????
+?? ?? -????
 
Using (9), ?? (0)=0 and ?? (??)=0 
?                                 ?? +?? =0 and ?? ?? ????
+?? ?? -????
=0 
Now,                                  ?? =-?? as -?? (?? ????
-?? -????
)=0 
As                        ?? ????
-?? -????
=0 
?                                       ?? =0 
So                                     ?? =0 
Reject this case, as again we get a trivial solution. 
Case 3:?? <0 
So, 
?? ?=-?? 2
 then 
?? (?? )?=?? cos????? +?? sin?????
?? (0)?=0 and ?? (??)=0
 
So, ?? =0 and ?? cos????? +?? sin????? =0 
As clearly ?? ?0 
? otherwise ?? =0 which does not satisfy initial condition. 
So                                  sin????? =0 
????????????????????????????????????sin????? =sin??? 
?                                         ?? =
????
?? 
So, 
?? (?? )=?? sin?
????
?? ?? ;?? =1,2,….. 
Hence, non-zero solution of (6) is clearly given by 
?? ?? (?? )=?? ?? sin??????? ?? 
Then 
?? -?? ?? 2
?? =0 
??????????????????????????????????????????????????????????????????????? +?? 2
?? 2
?? =0 
So                                                          ?? '
+
?? 2
?? 2
?? 2
?? 2
?? =0 
So, 
?? ?? (?? )=?? 0
cos?
???????? ?? +?? 0
sin????????? ?? 
???????????????????????????????????????????? ?? (?? ,?? )=?? ?? (?? )?? ?? (?? ) 
=?? ?? sin??? ?? ?? ?? [?? ?? cos????????? ?? +?? ?? sin????????? ?? ] 
General solution 
Page 4


Edurev123 
6. Application of PDE 
6.1 A tightly stretched string has its end fixed at ?? =?? and ?? =?? . At time ?? =?? , the 
string is given a shape defined by ?? (?? )=???? (?? -?? ) , where ?? is a constant, and then 
released. Find the displacement of any point ?? of the string at time ?? >?? . 
(2009 : 30 Marks) 
Solution: 
The one dimensional wave equation is 
?
2
?? ??? 2
=
1
?? 2
?
2
?? ??? 2
(1)
 
Boundary condition (BC) are 
?? (0,?? )=?? (??,?? )=0 (2) 
Initial condition (IC) are: 
?? (?? ,0)=?? (?? )=h?? (?? -?? ) (3)
?? ?? (?? ,0)=0 (4)
 
Let ?? (?? ,?? )
'
=?? (?? )?? (?? ) be the trial solution where 
?? (?? ) is a function of ?? only. 
?? (?? ) is a function of ?? only. 
Then clearly by (1) 
?? '
?? =????
?? '
?? =
?? '
?? =?? (say) 
?? -???? =0
 and ??? -???? =0
 
Using (2) and (5), we get 
Now, 
?? (0,?? )?=0 and ?? (??,?? )=0
?? (?? ,?? )?=?? (?? )?? (?? )
 
As 
Since ?? (?? )=0 leads to ?? =0 
So suppose that ?? (?? )?0 
Then clearly equation (8) gives us that ?? (0)=0 and ?? (??)=0 
Then ( 9 ) is modified boundary condition 
Now solving (6) and (9), three cases arise clearly: 
Case 1:?? =0 then clearly solution (6) will be ?? '
=0 
So,                          ?? ''
(?? )=?? 
?????????????????????????????????????? (?? )=???? +?? 
where ?? and ?? are arbitrary constants 
Clearly, ?? (0)=0 and ?? (?? )=0 
So, ??? =0 and ???? =0??? =0(?? ?0) 
So,                          ?? (?? )=0 
This leads to ?? (?? )=0 which does not satisfy (3) and (4). 
So, reject ?? =0 case. 
Case 2:?? is positive, i.e., ?? >0 
Clearly, ?? =?? 2
:(?? ?0)(?? >0) 
So clearly,              ?? '
-?? 2
?? =0. 
So,                            ?? 2
-?? 2
=0 
?                                        ?? =±?? 
So,                                 ?? (?? )=?? ?? ????
+?? ?? -????
 
Using (9), ?? (0)=0 and ?? (??)=0 
?                                 ?? +?? =0 and ?? ?? ????
+?? ?? -????
=0 
Now,                                  ?? =-?? as -?? (?? ????
-?? -????
)=0 
As                        ?? ????
-?? -????
=0 
?                                       ?? =0 
So                                     ?? =0 
Reject this case, as again we get a trivial solution. 
Case 3:?? <0 
So, 
?? ?=-?? 2
 then 
?? (?? )?=?? cos????? +?? sin?????
?? (0)?=0 and ?? (??)=0
 
So, ?? =0 and ?? cos????? +?? sin????? =0 
As clearly ?? ?0 
? otherwise ?? =0 which does not satisfy initial condition. 
So                                  sin????? =0 
????????????????????????????????????sin????? =sin??? 
?                                         ?? =
????
?? 
So, 
?? (?? )=?? sin?
????
?? ?? ;?? =1,2,….. 
Hence, non-zero solution of (6) is clearly given by 
?? ?? (?? )=?? ?? sin??????? ?? 
Then 
?? -?? ?? 2
?? =0 
??????????????????????????????????????????????????????????????????????? +?? 2
?? 2
?? =0 
So                                                          ?? '
+
?? 2
?? 2
?? 2
?? 2
?? =0 
So, 
?? ?? (?? )=?? 0
cos?
???????? ?? +?? 0
sin????????? ?? 
???????????????????????????????????????????? ?? (?? ,?? )=?? ?? (?? )?? ?? (?? ) 
=?? ?? sin??? ?? ?? ?? [?? ?? cos????????? ?? +?? ?? sin????????? ?? ] 
General solution 
?? ?? (?? ,?? )=??
8
?? =1
?
sin??????? ?? [?? ?? cos????????? ?? +?? ?? sin????????? ?? ] (6) 
Now differentiating (6) w.r.t. ?? , we get 
?? ?? (?? ,?? )=??
8
?? =1
?[-?? ?? sin?
???????? ?? (
?????? ?? )+?? ?? cos????????? ?? (
?????? ?? )]
sin??????? ?? Now ??? ?? (?? ,0)=0
???0=??
8
?? =1
??? ?? ?????? ?? sin??????? ?? ????? ?? =0
?? (?? ,?? )=??
8
?? =1
??? ?? cos?2?????? ?? sin??????? ?? ?? (?? ,0)=???? (?? -?? )
?? ?? =
2
?? ? ?
1
0
????? (?? -?? )
sin??????? ?? ????
?=
2?? ?? {(???? -?? 2
)
-
cos??????? ?? ????
?? |
0
1
+
?? ????
? ?
1
0
?(?? -2?? )
cos??????? ?? ????
?=
2?? ????
[(?? -2?? )
sin??????? ?? ????
?? |?+
4?? ?? 2
?? 2
?? 2
·
?? ????
-
cos??????? ?? ]
0
?? ????
?=
-4?? ?? 3
?? 2
?? 3
[cos????? -1]
?={
0 if ?? =2?? 8?? ?? 3
(2?? -1)
3
?? 3
 if ?? =2?? -1
 
So, clearly we have 
?? (?? ,?? )= ??
8
?? -1
8?? ?? 3
(2?? -1)
3
?? 3
·
cos?(2?? -1)?? ct
?? ·
sin?(2?? -1)????
?? 
6.2 Solve the following heat equation 
?? ?? -?? ????
?=?? , ???????? ?? <?? <?? ,?? >?? ?? (?? ,?? )?=?? (?? ,?? )=?? ???????? ?? >?? ?? (?? ,?? )?=?? (?? -?? ), ???????? ?? =?? =?? 
Solution: 
Given, the equation is ?? ?? -?? ????
=0 popularly known as "Heat Equation". 
Page 5


Edurev123 
6. Application of PDE 
6.1 A tightly stretched string has its end fixed at ?? =?? and ?? =?? . At time ?? =?? , the 
string is given a shape defined by ?? (?? )=???? (?? -?? ) , where ?? is a constant, and then 
released. Find the displacement of any point ?? of the string at time ?? >?? . 
(2009 : 30 Marks) 
Solution: 
The one dimensional wave equation is 
?
2
?? ??? 2
=
1
?? 2
?
2
?? ??? 2
(1)
 
Boundary condition (BC) are 
?? (0,?? )=?? (??,?? )=0 (2) 
Initial condition (IC) are: 
?? (?? ,0)=?? (?? )=h?? (?? -?? ) (3)
?? ?? (?? ,0)=0 (4)
 
Let ?? (?? ,?? )
'
=?? (?? )?? (?? ) be the trial solution where 
?? (?? ) is a function of ?? only. 
?? (?? ) is a function of ?? only. 
Then clearly by (1) 
?? '
?? =????
?? '
?? =
?? '
?? =?? (say) 
?? -???? =0
 and ??? -???? =0
 
Using (2) and (5), we get 
Now, 
?? (0,?? )?=0 and ?? (??,?? )=0
?? (?? ,?? )?=?? (?? )?? (?? )
 
As 
Since ?? (?? )=0 leads to ?? =0 
So suppose that ?? (?? )?0 
Then clearly equation (8) gives us that ?? (0)=0 and ?? (??)=0 
Then ( 9 ) is modified boundary condition 
Now solving (6) and (9), three cases arise clearly: 
Case 1:?? =0 then clearly solution (6) will be ?? '
=0 
So,                          ?? ''
(?? )=?? 
?????????????????????????????????????? (?? )=???? +?? 
where ?? and ?? are arbitrary constants 
Clearly, ?? (0)=0 and ?? (?? )=0 
So, ??? =0 and ???? =0??? =0(?? ?0) 
So,                          ?? (?? )=0 
This leads to ?? (?? )=0 which does not satisfy (3) and (4). 
So, reject ?? =0 case. 
Case 2:?? is positive, i.e., ?? >0 
Clearly, ?? =?? 2
:(?? ?0)(?? >0) 
So clearly,              ?? '
-?? 2
?? =0. 
So,                            ?? 2
-?? 2
=0 
?                                        ?? =±?? 
So,                                 ?? (?? )=?? ?? ????
+?? ?? -????
 
Using (9), ?? (0)=0 and ?? (??)=0 
?                                 ?? +?? =0 and ?? ?? ????
+?? ?? -????
=0 
Now,                                  ?? =-?? as -?? (?? ????
-?? -????
)=0 
As                        ?? ????
-?? -????
=0 
?                                       ?? =0 
So                                     ?? =0 
Reject this case, as again we get a trivial solution. 
Case 3:?? <0 
So, 
?? ?=-?? 2
 then 
?? (?? )?=?? cos????? +?? sin?????
?? (0)?=0 and ?? (??)=0
 
So, ?? =0 and ?? cos????? +?? sin????? =0 
As clearly ?? ?0 
? otherwise ?? =0 which does not satisfy initial condition. 
So                                  sin????? =0 
????????????????????????????????????sin????? =sin??? 
?                                         ?? =
????
?? 
So, 
?? (?? )=?? sin?
????
?? ?? ;?? =1,2,….. 
Hence, non-zero solution of (6) is clearly given by 
?? ?? (?? )=?? ?? sin??????? ?? 
Then 
?? -?? ?? 2
?? =0 
??????????????????????????????????????????????????????????????????????? +?? 2
?? 2
?? =0 
So                                                          ?? '
+
?? 2
?? 2
?? 2
?? 2
?? =0 
So, 
?? ?? (?? )=?? 0
cos?
???????? ?? +?? 0
sin????????? ?? 
???????????????????????????????????????????? ?? (?? ,?? )=?? ?? (?? )?? ?? (?? ) 
=?? ?? sin??? ?? ?? ?? [?? ?? cos????????? ?? +?? ?? sin????????? ?? ] 
General solution 
?? ?? (?? ,?? )=??
8
?? =1
?
sin??????? ?? [?? ?? cos????????? ?? +?? ?? sin????????? ?? ] (6) 
Now differentiating (6) w.r.t. ?? , we get 
?? ?? (?? ,?? )=??
8
?? =1
?[-?? ?? sin?
???????? ?? (
?????? ?? )+?? ?? cos????????? ?? (
?????? ?? )]
sin??????? ?? Now ??? ?? (?? ,0)=0
???0=??
8
?? =1
??? ?? ?????? ?? sin??????? ?? ????? ?? =0
?? (?? ,?? )=??
8
?? =1
??? ?? cos?2?????? ?? sin??????? ?? ?? (?? ,0)=???? (?? -?? )
?? ?? =
2
?? ? ?
1
0
????? (?? -?? )
sin??????? ?? ????
?=
2?? ?? {(???? -?? 2
)
-
cos??????? ?? ????
?? |
0
1
+
?? ????
? ?
1
0
?(?? -2?? )
cos??????? ?? ????
?=
2?? ????
[(?? -2?? )
sin??????? ?? ????
?? |?+
4?? ?? 2
?? 2
?? 2
·
?? ????
-
cos??????? ?? ]
0
?? ????
?=
-4?? ?? 3
?? 2
?? 3
[cos????? -1]
?={
0 if ?? =2?? 8?? ?? 3
(2?? -1)
3
?? 3
 if ?? =2?? -1
 
So, clearly we have 
?? (?? ,?? )= ??
8
?? -1
8?? ?? 3
(2?? -1)
3
?? 3
·
cos?(2?? -1)?? ct
?? ·
sin?(2?? -1)????
?? 
6.2 Solve the following heat equation 
?? ?? -?? ????
?=?? , ???????? ?? <?? <?? ,?? >?? ?? (?? ,?? )?=?? (?? ,?? )=?? ???????? ?? >?? ?? (?? ,?? )?=?? (?? -?? ), ???????? ?? =?? =?? 
Solution: 
Given, the equation is ?? ?? -?? ????
=0 popularly known as "Heat Equation". 
???? ?? ?????????????????????????????????????????????????? ?=?? (?? )?? (?? )
?Equation?is???????????? ?? '
-?? ''
?? ?=0
?? '
?? ?=?? ''
?? ?? '
?? ?=
?? ''
?? =?? 
Case-1: 
?? ?=0
?? ''
?? ?=0
?? ?=???? +?? ?? (0,?? ) ?=?? (2,?? )=0
?? ?=0,??? =0???? =0??? =0 which is not possible. ??? ?0.
 
Case-2: 
?? =?? 2
,??? ??? ?
?? ''
?? =?? 2
? ?? ''
-?? 2
?? =0
?
?
2
?? ??? 2
-?? 2
?? =0
? ?? =?? 1
?? ????
+?? 2
 Given, ?? (0,?? )=?? (2,?? )=0 ?? 1
+?? 2
=0 and ?? ? ?? 1
=?? 2
=0
? ?? =0??? =0
 
?? =-?? 2
,??? ??? ?
?? ''
?? =-?? 2
???? ''
+?? 2
?? =0
?
?
2
?? ??? 2
+?? 2
?? =0
???? =?? 3
?? ?????? +?? 4
?? -?????? =?? 5
cos????? +?? 6
sin?????
 
( ?? 3
,?? 4
 are constant) 
( ?? 5
,?? 6
 are constant) 
Given, ?? (0,?? )=?? (2,?? )=0 
? ?? 5
=0
 and ?? 5
cos?2?? +?? 6
sin?2?? =0
? sin?2?? =0
? 2?? =????
 
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