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2.4 Mechanical vibrations

Note: 2 lectures, §3.4 in [EP], §3.7 in [BD]

Let us look at some applications of linear second order constant coefficient equations.

2.4.1 Some examples

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Our first example is a mass on a spring. Suppose we have a mass m > 0 (in kilograms) connected by a spring with spring constant k > 0 (in newtons per meter) to a fixed wall. There may be some external force F(t) (in newtons) acting on the mass. Finally, there is some friction measured by c ≥ 0 (in newton-seconds per meter) as the mass slides along the floor (or perhaps there is a damper connected).

Let x be the displacement of the mass (x = 0 is the rest position), with x growing to the right (away from the wall). The force exerted by the spring is proportional to the compression of the spring by Hooke’s law. Therefore, it is kx in the negative direction. Similarly the amount of force exerted by friction is proportional to the velocity of the mass. By Newton’s second law we know that force equals mass times acceleration and hence mx ′′ = F(t) - cx′ - kx or

mx ′′ + cx′ + kx = F(t).

This is a linear second order constant coefficient ODE. We set up some terminology about this equation. We say the motion is

(i)
forced, if F ↙ 0 (if F is not identically zero),
(ii)
unforced or free, if F ≡ 0 (if F is identically zero),
(iii)
damped, if c > 0 , and
(iv)
undamped, if c = 0 .

This system appears in lots of applications even if it does not at first seem like it. Many real world scenarios can be simplified to a mass on a spring. For example, a bungee jump setup is essentially a mass and spring system (you are the mass). It would be good if someone did the math before you jump off the bridge, right? Let us give two other examples.

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Here is an example for electrical engineers. Suppose that we have the pictured RLC circuit. There is a resistor with a resistance of R ohms, an inductor with an inductance of L henries, and a capacitor with a capacitance of C farads. There is also an electric source (such as a battery) giving a voltage of E (t) volts at time t (measured in seconds). Let Q (t) be the charge in coulombs on the capacitor and I(t) be the current in the circuit. The relation between the two is Q ′ = I . By elementary principles we have that LI ′ + RI + Q∕C = E . If we differentiate we get

 1 LI ′′(t) + RI ′(t) +-I(t) = E′(t). C

This is an nonhomogeneous second order constant coefficient linear equation. Further, as L, R , and C are all positive, this system behaves just like the mass and spring system. The position of the mass is replaced by the current. Mass is replaced by the inductance, damping is replaced by resistance and the spring constant is replaced by one over the capacitance. The change in voltage becomes the forcing function. Hence for constant voltage this is an unforced motion.

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Our next example behaves like a mass and spring system only approximately. Suppose we have a mass m on a pendulum of length L . We wish to find an equation for the angle θ(t) . Let g be the force of gravity. Elementary physics mandates that the equation is of the form

 ′′ g- θ + L sin θ = 0.

Let us derive this equation using Newton’s second law; force equals mass times acceleration. The acceleration is  ′′ Lθ and mass is m . So  ′′ mL θ has to be equal to the tangential component of the force given by the gravity, that is mg sin θ in the opposite direction. So mL θ′′ = - mg sinθ . The m curiously cancels from the equation.

Now we make our approximation. For small θ we have that approximately sinθ ≈ θ . This can be seen by looking at the graph. In Figure 2.1 we can see that for approximately - 0.5 < θ < 0.5 (in radians) the graphs of sinθ and θ are almost the same.


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Figure 2.1: The graphs of sin θ and θ (in radians).


Therefore, when the swings are small, θ is always small and we can model the behavior by the simpler linear equation

 g θ′′ +--θ = 0. L

Note that the errors that we get from the approximation build up. So after a very long time, the behavior of the real system might be substantially different from our solution. Also we will see that in a mass-spring system, the amplitude is independent of the period. This is not true for a pendulum. Nevertheless, for reasonably short periods of time and small swings (for example if the pendulum is very long), the approximation is reasonably good.

In real world problems it is often necessary to make these types of simplifications. We must understand both the mathematics and the physics of the situation to see if the simplification is valid in the context of the questions we are trying to answer.

2.4.2 Free undamped motion

In this section we will only consider free or unforced motion, as we cannot yet solve nonhomogeneous equations. Let us start with undamped motion where c = 0 . We have the equation

mx ′′ + kx = 0.

If we divide by m and let  √ --- ω0 = k∕m , then we can write the equation as

x′′ + ω 20x = 0.

The general solution to this equation is

x(t) = A cos(ω t) + B sin(ω t). 0 0

By a trigonometric identity, we have that for two different constants C and γ , we have

A cos(ω 0t) + B sin(ω 0t) = Cc os(ω 0t - γ).

It is not hard to compute that  √ -------- C = A2 + B 2 and tan γ = B∕A . Therefore, we let C and γ be our arbitrary constants and write x(t) = C cos(ω0t - γ ) .

Exercise 2.4.1: Justify the above identity and verify the equations for C and γ . Hint: Start with cos(α - β) = cos(α)cos(β) + sin(α)sin(β) and multiply by C . Then think what should α and β be.

While it is generally easier to use the first form with A and B to solve for the initial conditions, the second form is much more natural. The constants C and γ have very nice interpretation. We look at the form of the solution

x(t) = C cos(ω 0t - γ).

We can see that the amplitude is C , ω 0 is the (angular) frequency, and γ is the so-called phase shift. The phase shift just shifts the graph left or right. We call ω 0 the natural (angular) frequency. This entire setup is usually called simple harmonic motion.

Let us pause to explain the word angular before the word frequency. The units of ω 0 are radians per unit time, not cycles per unit time as is the usual measure of frequency. Because one cycle is 2π radians, the usual frequency is given by ω0 2π . It is simply a matter of where we put the constant 2π , and that is a matter of taste.

The period of the motion is one over the frequency (in cycles per unit time) and hence 2ωπ0 . That is the amount of time it takes to complete one full cycle.

Example 2.4.1: Suppose that m = 2 kg and k = 8N∕m . The whole mass and spring setup is sitting on a truck that was traveling at 1 m/s. The truck crashes and hence stops. The mass was held in place 0.5 meters forward from the rest position. During the crash the mass gets loose. That is, the mass is now moving forward at 1 m/s, while the other end of the spring is held in place. The mass therefore starts oscillating. What is the frequency of the resulting oscillation and what is the amplitude. The units are the mks units (meters-kilograms-seconds).

The setup means that the mass was at half a meter in the positive direction during the crash and relative to the wall the spring is mounted to, the mass was moving forward (in the positive direction) at 1 m/s. This gives us the initial conditions.

So the equation with initial conditions is

2x′′ + 8x = 0, x (0 ) = 0.5, x ′(0) = 1.

We can directly compute  --- √ -- ω 0 = √ k∕m = 4 = 2 . Hence the angular frequency is 2. The usual frequency in Hertz (cycles per second) is 2 1 ∕2π = ∕π ≈ 0.3 18 .

The general solution is

x(t) = A cos(2t) + B sin(2t).

Letting x (0) = 0.5 means A = 0.5 . Then x ′(t) = - 2(0.5)sin(2t) + 2B cos(2t) . Letting x′(0) = 1 we get B = 0.5 . Therefore, the amplitude is C = √A-2 +-B2-= √ 0.25 +-0.25-= √0.5 ≈ 0.7 07 . The solution is

x(t) = 0.5 cos(2t) + 0.5 sin (2t).

A plot of x(t) is shown in Figure 2.2.


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Figure 2.2: Simple undamped oscillation.


In general, for free undamped motion, a solution of the form

x (t) = Aco s(ω 0t) + Bsin(ω0t),

corresponds to the initial conditions x (0 ) = A and x′(0) = ω 0B . Therefore, it is easy to figure out A and B from the initial conditions. The amplitude and the phase shift can then be computed from A and B . In the example, we have already found the amplitude C . Let us compute the phase shift. We know that  B tan γ = ∕A = 1 . We take the arctangent of 1 and get approximately 0.785. We still need to check if this γ is in the correct quadrant (and add π to γ if it is not). Since both A and B are positive, then γ should be in the first quadrant, and 0.785 radians really is in the first quadrant.

Note: Many calculators and computer software do not only have the atan function for arctangent, but also what is sometimes called atan2. This function takes two arguments, B and A , and returns a γ in the correct quadrant for you.

2.4.3 Free damped motion

Let us now focus on damped motion. Let us rewrite the equation

mx ′′ + cx′ + kx = 0,

as

 ′′ ′ 2 x + 2px + ω 0x = 0,

where

 ∘ --- ω = k-, p = c--. 0 m 2m

The characteristic equation is

r2 + 2pr + ω 20 = 0.

Using the quadratic formula we get that the roots are

 ∘ -------- r = -p ± p 2 - ω 20.

The form of the solution depends on whether we get complex or real roots. We get real roots if and only if the following number is nonnegative:

 ( ) 2 2 -c- 2 k- c2 --4km- p - ω 0 = 2m - m = 4m 2 .

The sign of p2 - ω2 0 is the same as the sign of c2 - 4km . Thus we get real roots if and only if c2 - 4km is nonnegative, or in other words if  2 c ≥ 4km .

Overdamping

When  2 c - 4km > 0 , we say the system is overdamped. In this case, there are two distinct real roots r1 and r 2 . Both roots are negative: As ∘ -------- p2 - ω 2 0 is always less than p , then  ∘-------- - p ± p 2 - ω 2 0 is negative in either case.


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Figure 2.3: Overdamped motion for several different initial conditions.


The solution is

 r1t r2t x(t) = C1e + C2e .

Since r1,r2 are negative, x (t) → 0 as t → ∞ . Thus the mass will tend towards the rest position as time goes to infinity. For a few sample plots for different initial conditions, see Figure 2.3.

Do note that no oscillation happens. In fact, the graph will cross the x axis at most once. To see why, we try to solve  r1t r2t 0 = C 1e + C2e . Therefore,  r1t r2t C1e = - C2e and using laws of exponents we obtain

-C-1 (r2-r1)t C = e . 2

This equation has at most one solution t ≥ 0 . For some initial conditions the graph will never cross the x axis, as is evident from the sample graphs.

Example 2.4.2: Suppose the mass is released from rest. That is x(0) = x0 and x′(0) = 0 . Then

x(t) =--x0-- (r1er2t - r2er1t). r1 - r2

It is not hard to see that this satisfies the initial conditions.

Critical damping

When c2 - 4km = 0 , we say the system is critically damped. In this case, there is one root of multiplicity 2 and this root is - p . Our solution is

x(t) = C e-pt + C te-pt. 1 2

The behavior of a critically damped system is very similar to an overdamped system. After all a critically damped system is in some sense a limit of overdamped systems. Since these equations are really only an approximation to the real world, in reality we are never critically damped, it is a place we can only reach in theory. We are always a little bit underdamped or a little bit overdamped. It is better not to dwell on critical damping.

Underdamping


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Figure 2.4: Underdamped motion with the envelope curves shown.


When c2 - 4km < 0 , we say the system is underdamped. In this case, the roots are complex.

 ∘ -------- r = -p ± p 2 - ω 2 ∘ --0----- √ --- 2 2 = -p ± -1 ω0 - p = -p ± iω1,

where  ∘ -------- ω 1 = ω2 - p2 0 . Our solution is

 -pt( ) x(t) = e A cos(ω1t) + B sin(ω1t),

or

 -pt x(t) = Ce cos(ω 1t - γ).

An example plot is given in Figure 2.4. Note that we still have that x(t) → 0 as t → ∞ .

In the figure we also show the envelope curves Ce-pt and - Ce-pt . The solution is the oscillating line between the two envelope curves. The envelope curves give the maximum amplitude of the oscillation at any given point in time. For example if you are bungee jumping, you are really interested in computing the envelope curve so that you do not hit the concrete with your head.

The phase shift γ just shifts the graph left or right but within the envelope curves (the envelope curves do not change if γ changes).

Finally note that the angular pseudo-frequency (we do not call it a frequency since the solution is not really a periodic function) ω 1 becomes smaller when the damping c (and hence p ) becomes larger. This makes sense. When we change the damping just a little bit, we do not expect the behavior of the solution to change dramatically. If we keep making c larger, then at some point the solution should start looking like the solution for critical damping or overdamping, where no oscillation happens. So if c2 approaches 4km , we want ω1 to approach 0.

On the other hand when c becomes smaller, ω 1 approaches ω 0 (ω 1 is always smaller than ω 0 ), and the solution looks more and more like the steady periodic motion of the undamped case. The envelope curves become flatter and flatter as c (and hence p ) goes to 0.

2.4.4 Exercises

Exercise 2.4.2: Consider a mass and spring system with a mass m = 2 , spring constant k = 3 , and damping constant c = 1 . a) Set up and find the general solution of the system. b) Is the system underdamped, overdamped or critically damped? c) If the system is not critically damped, find a c that makes the system critically damped.

Exercise 2.4.3: Do Exercise 2.4.2 for m = 3 , k = 12 , and c = 12 .

Exercise 2.4.4: Using the mks units (meters-kilograms-seconds), suppose you have a spring with spring constant 4 N/m. You want to use it to weigh items. Assume no friction. You place the mass on the spring and put it in motion. a) You count and find that the frequency is 0.8 Hz (cycles per second). What is the mass? b) Find a formula for the mass m given the frequency ω in Hz.

Exercise 2.4.5: Suppose we add possible friction to Exercise 2.4.4. Further, suppose you do not know the spring constant, but you have two reference weights 1 kg and 2 kg to calibrate your setup. You put each in motion on your spring and measure the frequency. For the 1 kg weight you measured 1.1 Hz, for the 2 kg weight you measured 0.8 Hz. a) Find k (spring constant) and c (damping constant). b) Find a formula for the mass in terms of the frequency in Hz. Note that there may be more than one possible mass for a given frequency. c) For an unknown object you measured 0.2 Hz, what is the mass of the object? Suppose that you know that the mass of the unknown object is more than a kilogram.

Exercise 2.4.6: Suppose you wish to measure the friction a mass of 0.1 kg experiences as it slides along a floor (you wish to find c ). You have a spring with spring constant k = 5N∕m . You take the spring, you attach it to the mass and fix it to a wall. Then you pull on the spring and let the mass go. You find that the mass oscillates with frequency 1 Hz. What is the friction?

Exercise 2.4.101: A mass of 2 kilograms is on a spring with spring constant k newtons per meter with no damping. Suppose the system is at rest and at time t = 0 the mass is kicked and starts traveling at 2 meters per second. How large does k have to be to so that the mass does not go further than 3 meters from the rest position?

Exercise 2.4.102: Suppose we have an RLC circuit with a resistor of 100 miliohms (0.1 ohms), inductor of inductance of 50 millihenries (0.05 henries), and a capacitor of 5 farads, with constant voltage. a) Set up the ODE equation for the current I . b) Find the general solution. c) Solve for I(0) = 10 and I′(0) = 0 .

Exercise 2.4.103: A 5000 kg railcar hits a bumper (a spring) at 1 m/s, and the spring compresses by 0.1 m. Assume no damping. a) Find k . b) Find out how far does the spring compress when a 10000 kg railcar hits the spring at the same speed. c) If the spring would break if it compresses further than 0.3 m, what is the maximum mass of a railcar that can hit it at 1 m/s? d) What is the maximum mass of a railcar that can hit the spring without breaking at 2 m/s?