# Elementary Differential Equations and Boundary Value Problems - Boyce W.E.

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T i ó 4km J ó 8km J

where again the last approximation is valid when ó 2/4km is small. Thus, small damping increases the quasi period.

Equations (27) and (28) reinforce the significance of the dimensionless ratio ó 2/4km. It is not the magnitude of ó alone that determines whether damping is large or small, but the magnitude of ó 2 compared to 4km. When ó 2/4km is small, then we can neglect the effect of damping in calculating the quasi frequency and quasi period of the motion. On the other hand, if we want to study the detailed motion of the mass for all time, then we can never neglect the damping force, no matter how small.

As ó2/4km increases, the quasi frequency fi decreases and the quasi period Td increases. In fact, i ^ 0 and Td as y ^ 2 ë/km. As indicated by Eqs. (23),

(24), and (25), the nature of the solution changes as y passes through the value 2 ë/km. This value is known as critical damping, while for larger values of y the motion is said to be overdamped. In these cases, given by Eqs. (24) and (23), respectively, the mass creeps back to its equilibrium position, but does not oscillate about it, as for small y . Two typical examples of critically damped motion are shown in Figure 3.8.6, and the situation is discussed further in Problems 21 and 22.

FIGURE 3.8.6 Critically damped motions: u" + u' + 0.25u = 0; u = (A + Bt)e t/2.

The motion of a certain spring-mass system is governed by the differential equation

u" + 0.125 u'+ u = 0, (29)

where u is measured in feet and t in seconds. If u(0) = 2 and u'(0) = 0, determine the position of the mass at any time. Find the quasi frequency and the quasi period, as well

194

Chapter 3. Second Order Linear Equations

as the time at which the mass first passes through its equilibrium position. Also find the time t such that |u(t)| < 0.1 for all t > ò.

The solution of Eq. (29) is

u = e-t/16

' . Ó255 D . 7255

A cos , , t + B sin , , t

16

16

To satisfy the initial conditions we must choose A = 2 and B = 2/Ó255; hence the solution of the initial value problem is

u = e-t/16 (^cos^2551 + 16

32 ;e-t- S

V255

16

(30)

where tan S = 1/Ó255, so S = 0.06254. The displacement of the mass as afunctionof time is shown in Figure 3.8.7. For the purpose of comparison we also show the motion if the damping term is neglected.

FIGURE 3.8.7 Vibration with small damping (solid curve) and with no damping (dashed curve).

The quasi frequency is /ë = V255/16 = 0.998 and the quasi period is Td = 2ë/ë = 6.295 sec. These values differ only slightly from the corresponding values (1 and 2n, respectively) for the undamped oscillation. This is evident also from the graphs in Figure 3.8.7, which rise and fall almost together. The damping coefficient is small in this example, only one-sixteenth of the critical value, in fact. Nevertheless, the amplitude of the oscillation is reduced rather rapidly. Figure 3.8.8 shows the graph of the solution for 40 < t < 60, together with the graphs of u = ±0.1. From the

3.8 Mechanical and Electrical Vibrations

195

graph it appears that ò is about 47.5 and by a more precise calculation we find that t = 47.5149 sec.

To find the time at which the mass first passes through its equilibrium position, we refer to Eq. (30) and set V255t/16 — S equal to n/2, the smallest positive zero of the cosine function. Then, by solving for t, we obtain

16 / Ï \

t = ( —+ S ) = 1.637 sec.

V255 V2 /

u. >

0.1 u

0.05 II

0.

u = C3^ e-t/16cos ((t- 0.06254)

V255 \ 16 )

- 0.05 40 45 1 50 55 60 1

_ '

FIGURE 3.8.8 Solution of Example 3; determination of ò.

Electric Circuits. A second example of the occurrence of second order linear differential equations with constant coefficients is as a model of the flow of electric current in the simple series circuit shown in Figure 3.8.9. The current I, measured in amperes, is a function of time t. The resistance R (ohms), the capacitance C (farads), and the inductance L (henrys) are all positive and are assumed to be known constants. The impressed voltage E (volts) is a given function of time. Another physical quantity that enters the discussion is the total charge Q (coulombs) on the capacitor at time t. The relation between charge Q and current I is

I = dQ/dt. (31)

Resistance R Capacitance C

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