Intermediate engineering analysis/Problem set 1

R.1.1: Equation of Motion for a Spring-dashpot Parallel System in Series with Mass and Applied Force[1]Edit

Problem StatementEdit

Given a spring-dashpot system in parallel with an applied force, find the equation of motion.

 

Background TheoryEdit

For this problem, Newton's second law is used,

 

 

 

 

 

(1-1)

and applied to the mass at the end of a spring and damper in parallel.

SolutionEdit

Assuming no rotation of the mass,

 

 

 

 

 

(1-2)

Therefore, the spring and damper forces can be written as, respectively,

 

 

 

 

 

(1-3)

and

 

 

 

 

 

(1-4)

And the resultant force on the mass can be written as

 

 

 

 

 

Now, from equations (1-3 ) and (1-4 ) all of the forces can be substituted into Newton's second law, and since each distance is equal,

 

 

 

 

 

(1-5)

Substituting these exact force equations,

 

 

 

 

 

A little algebraic manipulation yields

 

 

 

 

 

Finally, dividing by the mass to put the equation in standard form gives the final equation of motion for the mass:

 

 

 

 

 

(1-6)

ReferencesEdit

  1. EGM4313: Section 1 Notes (.djvu)


R.1.2: Equation of Motion for a Spring-mass-dashpot System with Applied Force[1]Edit

Problem StatementEdit

For this problem, the task was to derive the equation of motion of the spring-mass-dashpot in Fig. 53, in K 2011 p.85 with an applied force r(t).

 

Background TheoryEdit

To solve this problem, note that the ODE of a damped mass-spring system is

 

 

 

 

 

(2-1)

When there is a external formal added to the model r(t) on the right. This then gives up the equation

 

 

 

 

 

(2-2)

SolutionEdit

In this problem referencing back to equation (1-2 )

 

 

 

 

 

The resultant force for the system can be described as stated in equation (1-5 )

 

 

 

 

 

Since r(t) is the external force,

 

 

 

 

 

The model of a mass-spring system ODE with external force on the right is modeled as

 

 

 

 

 

The internal force is equal to the force of the spring making this equation

 

 

 

 

 

When a dashpot is added, the force of the dashpot cy’ is added to the equation making it

 

 

 

 

 

(2-3)

For the characteristic equation m is divided throughout the whole equation making it

 

 

 

 

 

(2-4)

ReferencesEdit

  1. EGM4313: Section 1 Notes (.djvu)


R.1.3: Free-body Diagram and Equation of Motion for Spring-dashpot-mass System[1]Edit

Problem StatementEdit

For this problem we were to draw the FBDs and derive the equation of motion for the following system.
 

Background TheoryEdit

To start solving this problem, we have to first look at how the spring, dashpot, and mass interact with each other by analyzing their respective free-body diagrams.

Free-Body Diagram Derived Equation
   

and
 

 

 

 

 


Next we take a look at what we already know.

We know from kinematics that:

 

 

 

 

 

(3-1)

We also know from kinetics that:

 

 

 

 

 

(3-2)

 

 

 

 

 

(3-3)

From the constitutive relations we know:

 

 

 

 

 

(3-4)

 

 

 

 

 

(3-5)

We also know:

 

 

 

 

 

(3-6)

 

 

 

 

 

(3-7)

by solving for   we get

 

 

 

 

 

(3-8)

SolutionEdit

To derive the equation of motion we have to manipulate and combine a few formulas that we know. We first take what we found in (3-2 )

and substitute in

 

 

 

 

 

(3-9)

for  . From equations (3-3 ), (3-4 ), and (3-5

) we can substitute   for

 

 

 

 

 

After the substitutions into equation (3-2 ) we have

 

 

 

 

 

(3-10)

From the equation (3-8

) we know

 

 

 

 

 

We can then substitute this into equation (3-10 ) getting

 

 

 

 

 

To get the answer in its final form we divide both sides of the equation by m, and our final answer is

 

 

 

 

 

(3-11)

ReferencesEdit

  1. EGM4313: Section 1 Notes (.djvu)


R.1.4: Derivation of Voltage-Charge and Voltage-Current Relationships[1]Edit

Problem OverviewEdit

For this problem, the goal is to derive two different equations from the circuit equation

 

 

 

 

 

(4-1)

Each separate derivation is presented in Voltage-Charge Derivation and Voltage-Current Derivation

Background TheoryEdit

To solve these derivation, it is wise to note that capacitance is

 

 

 

 

 

(4-2)

It can also be noted that

 

This means that

 

Completing the integration results in

 

 

 

 

 

(4-3)

It should also be noted that (4-2 ) can be written in the form

 

 

 

 

 

(4-4)

It should also be noted that (4-3 ) can be written in the form

 

 

 

 

 

(4-5)

Voltage-Charge DerivationEdit


Problem StatementEdit

For this problem, the Voltage-Charge equation

 

 

 

 

 

(4-6)

are derived from (4-1 )

SolutionEdit

Taking the derivative of (4-2 ) are taken with respect to time

 

 

 

 

 

(4-7)

 

 

 

 

 

(4-8)

Substituting (4-4 ), (4-7 ), and (4-8 ) into (4-1 ) results in (4-6 ) such that

 

 

 

 

 

(4-9)

Voltage-Current DerivationEdit


Problem StatementEdit

For this problem, the following equation

 

 

 

 

 

(4-10)

are derived from (4-1 )

SolutionEdit

The first step is to take the derivative of (4-1 ) resulting in

 

 

 

 

 

(4-11)

Taking the derivative of (4-3 ) are taken with respect to time

 

 

 

 

 

(4-12)

 

 

 

 

 

(4-13)

Substituting (4-3 ), (4-12 ), and (4-13 ) into (4-11 ) results in (4-10 ) such that

 

 

 

 

 

(4-16)

ReferencesEdit

  1. EGM4313: Section 2 Notes (.djvu)


R.1.5: Solutions of General 2nd Order ODEs[1]Edit

Background Theory[2]Edit


Consider a second-order homogeneous linear ODE with constant coefficients a and b.

 

 

 

 

 

(5-1)

To solve this problem, note that the solution to a first-order linear ODE of the form:

 

 

 

 

 

is an exponential function, yielding a solution of the form:

 

 

 

 

 

(5-2)

Based upon this, we will expand the solution to apply to second-linear homogeneous ODEs. To do so, we must first find equations describing   and  , and so we will take the derivatives (with respect to x) of (5-2 ).

 

 

 

 

 

(5-3)

 

 

 

 

 

(5-4)

We will now substitute (5-2 ), (5-3 ), and (5-4 ) into (5-1 ) to obtain the relationship:

 

 

 

 

 

Simplifying, we have:

 

 

 

 

 

 

 

 

 

 

(5-5)


Since (5-5 ) follows the same form as the quadratic equation, we can solve for   and   as follows:

 

 

 

 

 

 

 

 

 

 


Referring back to algebra, we know that the solution to these two equations can be one of three cases:

Case 1Edit

Two real roots if...

 

 

 

 

 

These two roots give us two solutions:

  and  

The corresponding general solution then takes the form of the following:

 

 

 

 

 

(5-6)


Case 2Edit

A real double root if...

 

 

 

 

 

This yields only one solution:

 


In order to form a basis, a set of two linearly independent solutions, we must find another solution. To do so, we will use the method of reduction of order, where:

 
 
and
 
yeilding
 
 


Since   is a solution of (5-1), the last set of parentheses is zero.
Similarly, the first parentheses is zero as well, because

 

From integration, we get the solution:

 

If we set   and  

 


Thus, the general solution is:

 

 

 

 

 

(5-7)

Case 3Edit

Complex conjugate roots if...

 

 

 

 

 

In this case, the roots of (5-5 ) are complex:

 
and
 


Thus, the corresponding general solution is of the form:

 

 

 

 

 

(5-8)


P.2.2.4Edit


Problem StatementEdit

For this problem, we were tasked with finding the general solution to the homogeneous linear ODE given below:

 

 

 

 

 

(5a-1)

SolutionEdit

Following the process that yields (5-5 ), we find the equation:

 

 

 

 

 

(5a-2)

To find the form of the general solution, we must solve for the values of   using the quadratic formula:

File:224 solution.png

Since the solutions to   are complex numbers, our solution takes the form of (5-8 ):

 

 

 

 

 

(5a-3)

A and B are constant coefficients of unknown value. Had we been given initial values of   and  , we would solve for those values as well.

Confirmation of SolutionEdit

To confirm that the solution is true, we will substitute  ,  , and   into (5a-1 ). If the final result is a tautology[3] then the solution is confirmed. If the result is a contradiction, a statement that is never true, then our solution is disproved.

 

 

 

 

 

 

 

 

 

 

(5a-4)

 

 

 

 

 

 

 

 

 

 

(5a-5)


Substituting (5a-3 ), (5a-4 ), and (5a-5 ) into (5a-1 ), we are left with:

 

 

 

 

 

 

 

 

 

 


Combining similar terms allows us to clean up this solution and check our answer:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Final SolutionEdit

Since the outcome of our check is a tautology, the solution is confirmed for all values of A and B and we affirm that the final solution is:

 


P.2.2.5Edit


Problem StatementEdit

For this problem, we were tasked with finding the general solution to the homogeneous linear ODE given below:

 

 

 

 

 

(5b-1)


SolutionEdit

Following the process that yields (5-5 ), we find the equation:

 

 

 

 

 

(5b-2)


To find the form of the general solution, we must solve for the values of  . Instead of going through the quadratic formula, we can analyze the discriminant.

 

 

 

 

 

Thus we see that the discriminant follows Case 2 and the general solution must follow the form of (5-7 ), and we have the following:

 

 

 

 

 

(5b-3)

where   and   are unknown constant coefficients. If this were an initial value problem with values of   and  , we would solve for those values as well.

Confirmation of SolutionEdit

To confirm that the solution is true, we will substitute  ,  , and   into (5a-1 ) and check to see if the final result is a tautology or a contradiction.

 

 

 

 

 

(5b-4)

 

 

 

 

 

(5b-5)


Substituting (5b-3 ), (5b-4 ), and (5b-5 ) into (5b-1 ), we are left with:

 

 

 

 

 

After combining similar terms, we have the form:

 

 

 

 

 

 

 

 

 

 

Final SolutionEdit

Since the above result is a tautology for all values of   and  , (5-3 ) is confirmed and the general solution to (5-1 ) is:

 

 

 

 

 


ReferencesEdit

  1. EGM4313: Section 2 Notes (.djvu)
  2. Kreyszig, Erwin (2011). Advanced Engineering Mathematics. John Wily & Sons, Inc. ISBN 978-0-470-45836-5. Unknown parameter |coauthors= ignored (|author= suggested) (help) Pg 52-58
  3. Introduction to Propositional Logic


R.1.6: Determination of ODE Order, Linearity, and Application of the Superposition Principle[1]Edit

Problem StatementEdit

Given the eight Ordinary Differential Equations shown below:
(1) Determine the order
(2) Determine the linearity (or lack of)
(3) Show whether the principle of superposition can be applied for each equation

Background Theory[2][1]Edit

Consider a function  , defined as the sum of the homogeneous solution,  , and the particular solution,  :

 

 

 

 

 

(6-1)

Example:

Consider the Differential Equation in standard form:

 

 

 

 

 

(6-2)

The homogeneous solution is then:

 

 

 

 

 

(6-3)

and the particular solution is:

 

 

 

 

 

(6-4)

Now we sum up equations (6-3 ) and (6-4 ) to get:

 

 

 

 

 

(6-5)

If the Equation is Linear then some reductions can take place:

 

and


 

Now we can make some substitutions into (6-5 )using (6-3 )and (6-4 ):

 

 

 

 

 

(6-6)

Which is the same as equation (6-2 ):

 

SolutionEdit

Part AEdit

 

 

 

 

 

(6a-1)

Order:   Order ODE
Linearity: Linear
Superposition: Yes

This can be shown using the method from the background theory.

Recall:

 

The homogeneous solution is:

 

 

 

 

 

(6a-2)

The particular solution is:

 

 

 

 

 

(6a-3)

Now we add Equations (6a-2 ) and (6a-3 ):

 

 

 

 

 

(6a-4)

We can use (6-1 ) to simplify (6a-4 ):

 

 

 

 

 

(6a-5)

 

 

 

 

 

(6a-6)

Since (6a-1 ) and (6a-6 ) are the same then we can apply Superposition.

Part BEdit

 

 

 

 

 

(6b-1)

Order:   Order ODE
Linearity: Non-Linear
Superposition: No

This can be shown using the method from the background theory.
Before we start it is easier to do the problem if the equation is rearranged as such:

 

 

 

 

 

(6b-2)

Recall:

 

The homogeneous solution is:

 

 

 

 

 

(6b-3)

The particular solution is:

 

 

 

 

 

(6b-4)

Now we add Equations (6b-3 ) and (6b-4 ):

 

 

 

 

 

(6b-5)

We can use (6-1 ) to simplify (6b-5 ):

 

 

 

 

 

(6b-6)

Note that:

 

So we cannot apply Super position.

Part CEdit

 

 

 

 

 

(6c-1)

Order:   Order ODE
Linearity: Non-Linear
Superposition: No

This can be shown using the method from the background theory.
Before we start it is easier to do the problem if the equation is rearranged like this:

 

 

 

 

 

(6c-2)

Recall:

 

The homogeneous solution is:

 

 

 

 

 

(6c-3)

The particular solution is:

 

 

 

 

 

(6c-4)

Now we add Equations (6c-3 ) and (6c-4 ):

 

 

 

 

 

(6c-5)

We can use (6-1 ) to simplify (6c-5 ):

 

 

 

 

 

(6c-6)

Note that:

 

So we cannot apply Super position.

Part DEdit

 

 

 

 

 

(6d-1)

Order:   Order ODE
Linearity: Linear
Superposition: Yes

This can be shown using the method from the background theory.

Recall:

 

The homogeneous solution is:

 

 

 

 

 

(6d-2)

The particular solution is:

 

 

 

 

 

(6d-3)

Now we add Equations (6d-2 ) and (6d-3 ):

 

 

 

 

 

(6d-4)

We can use (6-1 ) to simplify (6d-4 ):

 

 

 

 

 

(6d-5)

 

 

 

 

 

(6d-6)

Since (6d-1 ) and (6d-6 ) are the same then we can apply Superposition.

Part EEdit

 

 

 

 

 

(6e-1)

Order:   Order ODE
Linearity: Linear
Superposition: Yes

This can be shown using the method from the background theory.

Recall:

 

The homogeneous solution is:

 

 

 

 

 

(6e-2)

The particular solution is:

 

 

 

 

 

(6e-3)

Now we add Equations (6e-2 ) and (6e-3 ):

 

 

 

 

 

(6e-4)

We can use (6-1 ) to simplify (6e-4 ):

 

 

 

 

 

(6e-5)

 

 

 

 

 

(6e-6)

Since (6e-1 ) and (6e-6 ) are the same then we can apply Superposition.

Part FEdit

 

 

 

 

 

(6f-1)

Order:   Order ODE
Linearity: Linear
Superposition: Yes

This can be shown using the method from the background theory.

Recall:

 

The homogeneous solution is:

 

 

 

 

 

(6f-2)

The particular solution is:

 

 

 

 

 

(6f-3)

Now we add Equations (6f-2 ) and (6f-3 ):

 

 

 

 

 

(6f-4)

We can use (6-1 ) to simplify (6f-4 ):

 

 

 

 

 

(6f-5)

 

 

 

 

 

(6f-6)

Since (6f-1 ) and (6f-6 ) are the same then we can apply Superposition.

Part GEdit

 

 

 

 

 

(6g-1)

Order:   Order ODE
Linearity: Linear
Superposition: Yes

This can be shown using the method from the background theory.

Recall:

 

The homogeneous solution is:

 

 

 

 

 

(6g-2)

The particular solution is:

 

 

 

 

 

(6g-3)

Now we add Equations (6g-2 ) and (6g-3 ):

 

 

 

 

 

(6g-4)

We can use (6-1 ) to simplify (6g-4 ):

 

 

 

 

 

(6g-5)

 

 

 

 

 

(6g-6)

Since (6g-1 ) and (6g-6 ) are the same then we can apply Superposition.

Part HEdit

 

 

 

 

 

(6h-1)

Order:   Order ODE
Linearity: Non-Linear
Superposition: No

This can be shown using the method from the background theory.

Recall:

 

The homogeneous solution is:

 

 

 

 

 

(6h-2)

The particular solution is:

 

 

 

 

 

(6h-2)

Now we add Equations (6h-2 ) and (6h-3 ):

 

 

 

 

 

(6h-4)

We can use (6-1 ) to simplify (6h-4 ):

 

 

 

 

 

(6h-5)

Note that:

 

So we cannot apply Super position.

ReferencesEdit

  1. 1.0 1.1 EGM4313: Section 2 Notes (.djvu)
  2. Kreyszig, Erwin (2011). Advanced Engineering Mathematics. John Wily & Sons, Inc. ISBN 978-0-470-45836-5. Unknown parameter |coauthors= ignored (|author= suggested) (help) Pg 46-48