Diffe Q, a forced harmonic oscillator

In summary, when solving a forced harmonic oscillator for an initial condition using the method of undetermined coefficients, you must use the general solution of both the homogeneous and non-homogeneous parts to satisfy the initial conditions. This can be seen in the example of solving y"+ 4y= x, y(0)= 0, y'(0)= 0, where the general solution is y(x)= kcos(2x)+ csin(2x)+ x/4. The initial conditions apply to the entire equation, not just the homogeneous part.
  • #1
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okay this is kind of a quick one when youre solving a forced harmonic oscillator for an initial condition by the method of undetermined coefficients, you do the part for the homogenoues equation first and come up with somethign like ke^at + ce^bt. Now when you goto solve for those constants k and c using your initial conditions do you use the general soltion form the nonhomogenous part as well? so it would either be ke^at + ce^bt = d -or- ke^at + ce^bt+re^dt = f

Thanks in advance and sorry about my rapant use of variables and poor math notation
 
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  • #2
Yes, of course. The initial conditions apply to the function that satisfies the entire equation. You must use that function, both "homogeneous" and "non-homogeneous" parts to satisfy the initial conditions.

For example, to solve y"+ 4y= x, y(0)= 0, y'(0)= 0.
The associated homogeneous equation is y"+ 4y= 0 which has characteristic equation r2+ 4= 0 and so characteristic values 2i and -2i. The general solution to that homogeneous equation is
y(x)= kcos(2x)+ c sin(2x). Now, using the method of "undetermined coefficients, we seek a particular solution to the entire equation of the form y(x)= Ax+ B. Then y'= A and y"= 0 so the equation becomes
0+ 4(Ax+ B)= 4Ax+ 4B= x. Clearly, A= 1/4 and B= 0. The general solution to the entire equation is y(x)= kcos(2x)+ csin(2x)+ x/4. Of course, then, y'(x)= -2ksin(2x)+ 2ccos(2x)+ 1/4.

Now, we have y(0)= k= 0 and y'(0)= 2c+ 1/4= 0 so c= -1/8. The solution to the problem is y(x)= -(1/8)sin(2x)+ x/4.

Since y(x)= 0 (y identically equal to 0) satisfies any homogeneous differential equation, if we just used the homogeneous equation to satisfy these initial conditions we would have just got y(x)= 0 and so, adding the particular solution, y(x)= x/4 which does NOT satisfy
y'(0)= 0.
 
  • #3
thanks man , haha yeah i guess it is pretty obvious looking back at it now. Thanks a bunch man
 

FAQ: Diffe Q, a forced harmonic oscillator

What is a forced harmonic oscillator?

A forced harmonic oscillator is a physical system that experiences oscillatory motion due to the application of an external periodic force. The motion of the oscillator is described by a differential equation known as the "differential equation of motion" or "diffe Q".

How is a forced harmonic oscillator different from a free harmonic oscillator?

A free harmonic oscillator experiences oscillatory motion without the influence of any external forces, while a forced harmonic oscillator is subjected to an external periodic force. This external force causes the amplitude and frequency of the oscillations to change from their natural values.

What factors affect the behavior of a forced harmonic oscillator?

The behavior of a forced harmonic oscillator is affected by the amplitude, frequency, and phase of the external force, as well as the mass, spring constant, and damping coefficient of the oscillator itself. These factors determine the amplitude, frequency, and phase of the oscillator's motion.

How is the motion of a forced harmonic oscillator mathematically described?

The motion of a forced harmonic oscillator is described by the differential equation of motion, which is a second-order linear differential equation. This equation can be solved to obtain the position and velocity of the oscillator as a function of time.

What are some real-world examples of forced harmonic oscillators?

Some real-world examples of forced harmonic oscillators include a swing pushed by a person, a pendulum affected by wind, a guitar string plucked by a musician, and a car suspension system reacting to bumps on the road. These systems experience oscillatory motion due to the application of an external force.

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