PY345
Damped and Forced Oscillations
Practice
When a horizontal, frictionless oscillator is subject to linear drag, the resulting equation of motion is
a) Verify that the above equation is true, then simplify it using and .
b) Try the solution then solve for in terms of and .
c) Interpret the solutions for and .
Solution
a) If we suppose that the cart is to the right of the equilibrium point (i.e. in the +x-direction), then according to the free body diagram
.
b) Let's try the solution .
Therefore the equation from Newton's Second Law becomes
.
We can solve this for .
c) Since has two different values, that means that our general solution can be written as a linear sum of those two solutions.
If , then the square roots become imaginary.
This means the terms in parentheses represent oscillations with frequency . The stuff out front of the parentheses, meanwhile, represent an exponential decay. Putting it all together, when the result is an exponentially decaying oscillator.
If , then the square root is positive, which means that no oscillation occurs.
In the simulation below, try playing around with the parameters so that you can see what each of these situations looks like.
Practice
In the previous problem, we didn't look at the case .
a) Show that the general solution is not able to describe the cart if the initial conditions are such that with .
b) Show that is a solution to when .
c) Solve for the unknown coefficients in the special case and .
Solution
a)
According to the first equation, . But if that is true, then the second equation is immediately violated.
b)
Therefore the equation becomes
c)
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Damped Oscillations
in damped oscillators, the amplitude of oscillations decreases as energy is dissipated away
in the special case of linear drag, the equation of motion is where is a decay parameter and is the natural frequency (i.e. the oscillation frequency in the absence of damping)
damping | how does compare to ? | decay parameter | general solution |
---|---|---|---|
none | 0 | ||
underdamped | |||
critically damped | |||
overdamped |
Linear Differential Operators
Suppose we define
Then the linear damped oscillator equation can be written as
Now suppose that there is an additional external force that doesn't depend on at all: where . (This is called a forced oscillator.) Then the linear damped oscillator equation could be written as
We will now show that the full solution to consists of the solution to as well as the solution to .
First, if is a linear operator, then Suppose we somehow found a solution that works in the forced oscillator equation. We will call this the particular solution.
We will call the solution to the unforced oscillator the homogeneous solution.
Now because is a linear operator, then
Therefore the most general complete solution to must also include solutions to the corresponding unforced (i.e. homogeneous) oscillator equation.
Practice
An oscillator is forced with a forcing function , where . (An example similar to this would be pushing someone who is on a swing.)
Show that satisfies the equation, where and
Solution
First it may help to write .
Then the forced oscillator equation is
Every term contains , so we can cancel it out.
Now we can turn it around and solve for .
We can also solve for .
Then we can match the real terms and the imaginary terms.
To solve for we can divide the first equation by the second.
Driven Oscillators
The general solution to is where and
Practice
a) Describe the long-term behavior of a damped, sinusoidally forced oscillator.
b) For what value of the forcing frequency is the amplitude of the long-term oscillations the greatest?
Solution
a) As time goes on, the term makes the natural oscillation disappear. It is because of this that the natural oscillation terms are referred to as transients. What remains is the oscillation due to the forcing.
b) is maximized when . Recall that is the frequency of oscillation without forcing. This means that the amplitude of oscillation is biggest when the driving frequency is equal to the natural frequency. This condition is known as resonance. The idea of resonance makes sense when you think about pushing someone on a swing. You push only when they come back to you. You wouldn't give them a starting push and then push them again while they are on their way back to you mid-swing.
This, by the way, helps us answer what happened to the Millenium Bridge. By an unfortunate accident, the tension cables of the bridge happened to create a system in which the natural frequency matched the frequency of footfalls on the bridge on that day. It didn't help that the swinging made people adjust their gait in such a way that amplified the problem.
So how was the problem solved? The bridge was closed down and extra dampeners were added to the bridge.
Practice
The equation of motion of a damped, forced oscillator is with , , , , , and . Recall that the general solution is
.
a) Use the initial conditions to solve for and in terms of and .
b) Create a plot of .
Solution
a) The general solution is
.
The first derivative is
,
where (notice that in this problem, which is why I switched the order and added ).
Substituting initial conditions, we get
.
Since the right hand side of the second equation is purely real, that means that .
Then the first equation becomes
.
Putting this back into the general solution, we get
.
Since must be real, we take the real part of the function.
In the last step, we used the fact that . The solution can be simplified further.
b) see the python code below