What Steps Can Resolve This Second-Order Differential Equation?

In summary: Do you know the variation of parameters method? That might work.I didn't think that would work because I thought the exponent on the x term would have to be 1, while in this case its -2...Just integrate (1/2)x^(-2) twice. Don't forget to keep the constants of integration around.so...integrating once...x'=.0345*x^-2*tthen again...x=.0345/2*x^-2*t^2+C\?so...integrating once...x'=.0345*x^-2*tthen again...x=.0345/2
  • #1
PennyGirl
23
0

Homework Statement


Solve the differential equation...
X'' = .5*x^-2
taken with respect to t
at t=0, x'=0 and x=10

Homework Equations


N/A


The Attempt at a Solution


I tried to split the variables (ie d^2 X * X^2 = .5 dt^2) but didn't get the right answer with this (i plugged it bak in and it didn't work?)
How should I start this problem?
 
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  • #2
Show your computations.
 
  • #3
d^2 X * X^2 = .5 dt^2
integrate both sides...
dx * x^3/3=(.5*t + C) *dt
at t=0, dx/dt=0 and x=10...
0*10^3/3 = .5*0 + C
C=0

dx * x^3/3 = .5*t*dt
integrate both sides again...
x^4 / 12 = .25*t^2 + C
same refs...
10^4/12 = .25*0 +C
C = 833.3

x^4/12 = .25*t^2 + 833.3

algebra...

x= (3*t^2 + 10000)^(1/4)

but that doesn't work...
 
  • #4
Do you know the variation of parameters method? That might work.
 
  • #5
I didn't think that would work because I thought the exponent on the x term would have to be 1, while in this case its -2...
 
  • #6
Just integrate (1/2)x^(-2) twice. Don't forget to keep the constants of integration around.
 
  • #7
so...
integrating once...
x'=.0345*x^-2*t

then again...
x=.0345/2*x^-2*t^2+C\
?
 
Last edited:
  • #8
PennyGirl said:
so...
integrating once...
x'=.0345*x^-2*t

then again...
x=.0345/2*x^-2*t^2+C\
?

No, you aren't missing anything. I am. I didn't notice t was the independent variable.
 
  • #9
PennyGirl said:
d^2 X * X^2 = .5 dt^2
integrate both sides...
NO. You cannot separate a second derivative like this. A second derivative cannot be treated like a fraction.

Your original equation can be written as [itex]x^2d^2x/dt^2= 1/2[/math]. Let y= dx/dt. Then, by the chain rule, [itex]d^2x/dt= dy/dt= (dy/dx)(dx/dt)= y dy/dx. Your equation becomes y dy/dx= (1/2)x-2. Since that is now a first derivative, it can be treated like a fraction and separated: ydy= (1/2)x-2dx. Integrate that to find y, as a function of x, and then integrate dx/dt= y to find x.` Since you know that x(0)= 10 and x'(0)= 0, you know that y= 0 when x= 10 so can find the first constant of integration before the second integral.

dx * x^3/3=(.5*t + C) *dt
at t=0, dx/dt=0 and x=10...
0*10^3/3 = .5*0 + C
C=0

dx * x^3/3 = .5*t*dt
integrate both sides again...
x^4 / 12 = .25*t^2 + C
same refs...
10^4/12 = .25*0 +C
C = 833.3

x^4/12 = .25*t^2 + 833.3

algebra...

x= (3*t^2 + 10000)^(1/4)

but that doesn't work...
 

FAQ: What Steps Can Resolve This Second-Order Differential Equation?

What is a differential equation?

A differential equation is a mathematical equation that describes the relationship between a function and its derivatives. It involves the use of derivatives, or rates of change, to model and predict the behavior of a system over time.

What are the types of differential equations?

There are several types of differential equations, including ordinary differential equations, which involve a single independent variable, and partial differential equations, which involve multiple independent variables. Other types include linear and nonlinear, first-order and higher-order, and homogeneous and non-homogeneous differential equations.

What is the importance of differential equations in science and engineering?

Differential equations are used to model and analyze a wide range of phenomena in science and engineering, including physical systems, chemical reactions, biological processes, and economic systems. They are also essential in fields such as physics, engineering, and economics for making predictions and solving problems.

How are differential equations solved?

The methods for solving differential equations vary depending on the type and complexity of the equation. Some common techniques include separation of variables, variation of parameters, and using specific formulas for certain types of equations. Numerical methods, such as Euler’s method and Runge-Kutta methods, can also be used to approximate solutions.

What are some real-world applications of differential equations?

Differential equations have many practical applications, including predicting population growth, analyzing the spread of diseases, modeling weather patterns, and designing control systems for engineering and industrial processes. They are also used in fields such as finance, biology, and physics to understand and analyze complex systems.

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