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Solving Differential Equations using the D operator
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Contents
- Theory Of Differential Operator (differential Module)
- The D Operator And The Fundamental Laws Of Algebra
- The Use Of The D Operator To Find The Complementary Function For Linear Equations
- Three Useful Formulae Based On The Operator D
- Linear First Order D Equations With Constant Coefficients
- Linear Second Order D Equations With Constant Coefficients
- Physical Examples
- Page Comments
Theory Of Differential Operator (differential Module)
Definition
- A differential operator is an operator defined as a function of the differentiation operator. It is helpful, as a matter of notation first, to consider differentiation as an abstract operation, accepting a function and returning another (in the style of a higher-order function in computer science).The most commonly used differential operator is the action of taking the derivative itself. Common notations for this operator include: and if generalize Note
is an operator and must therefore always be followed by some expression on which it operates.
Simple Equivalents
- means but
- Similarly and
The D Operator And The Fundamental Laws Of Algebra
The following differential equation:
may be expressed as: or
This can be factorised to give:
Examples
But is it justifiable to treat D in this way?
Algebraic procedures depend upon three laws.
- The Distributive Law:
- The Commutative Law:
- The Index Law:
If D satisfies these Laws, then it can be used as an Algebraic operator(or a linear operator). However:
- only when u is a constant.
Thus we can see that D does satisfy the Laws of Algebra very nearly except that it is not interchangeable with variables.
In the following analysis we will write
are constants and is a positive integer. As has been seen, we can factorise this or perform any operation depending upon the fundamental laws of Algebra.
We can now apply this principle to a number of applications.
The Use Of The D Operator To Find The Complementary Function For Linear Equations
It is required to solve the following equations:
Example:
Example - Simple example
Problem
Solve the following equation:-
Workings
Using the D operator this can be written as:-
Solution
Three Useful Formulae Based On The Operator D
Equation A
- Let represent a polynomial functionSinceand
From which it can be seen that:
Example:
Example - Equation A example
Problem
Workings
This can be re-written as:
Solution
We can put D = 4
Equation B
- Where is any function of xApplying Leibniz's theorem for the differential coefficient of a product.Similarly and so on
therefore
Example:
Example - Equation B example
Problem
Find the Particular Integral of:
Workings
We have used D as if it were an algebraic constant but it is in fact an operator where
Solution
Equation C - Trigonometrical Functions
- And so onsimilarly
Therefore
Example:
Example - Trigonometric example
Problem
Find the Particular Integral of:-
Workings
This can be re-written as:-
Using equation 1 we can put
If we multiply the top and bottom of this equation by
But
Solution
But since
Linear First Order D Equations With Constant Coefficients
These equations have on the right hand side
This equation is
Using an Integrating Factor of the equation becomes:-
Which is the General Solution.
Linear Second Order D Equations With Constant Coefficients
Where are the roots of the quadratic equation. i.e. the auxiliary equation.
Where is an arbitrary Constant
This equation can be re-written as:-
Integrating
- Thus when we can write the General Solution as:-
Where A and B are arbitrary Constants.
Example:
Example - Linear second order example
Problem
Workings
The roots of this equation are:-
Therefore the General Solution is
- The Special Case where
From Equation (41)
or
- The roots of the Auxiliary Equation are complex.
If the roots of the are complex then the General Solution will be of the form , and the solution will be given by:-
Solution
The roots of this equation are :-
Physical Examples
Example:
Example - Small oscilations
Problem
Show that if satisfies the differential equation with k < n and if when
The complete period of small oscillations of a simple pendulum is 2 secs. and the angularretardation due to air resistance is 0.04 X the angular velocity of the pendulum. The bob is heldat rest so the the string makes a small angle with the downwards vertical andthen let go. Show that after 10 complete oscillations the string will make an angle of about 40'with the vertical.(LU)
Workings
Using the "D" operator we can write
When t = 0 = 0 and = 0
and
Solution
At t = 0
We have been given that k = 0.02 and the time for ten oscillations is 20 secs.
Last Modified: 30 Jul 12 @ 13:02 Page Rendered: 2023-01-22 22:34:10