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

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3.8 Mechanical and Electrical Vibrations

189

only time unit mentioned is the second, so we will measure t in seconds. On the other hand, both the foot and inch appear in the statement as units of length. It is immaterial

which one we use, but having made a choice, it is important to be consistent. To be

definite, let us measure the displacement u in feet.

Since nothing is said in the statement of the problem about an external force, we assume that F( t) = 0. To determine m note that

w 4 lb 1 lb-sec2

g 32 ft/ sec2 8 ft

The damping coefficient y is determined from the statement that y u' is equal to 6 lb when u' is 3 ft/sec. Therefore

6 lb

Y =

2

lb-sec

3 ft/sec ft

The spring constant k is found from the statement that the mass stretches the spring by

2 in., or by 1/6 ft. Thus

k =

4 lb Ø

-------= 24—.

1/6 ft ft

Consequently, Eq. (7) becomes

8 u" + 2d + 24u = 0,

or

The initial conditions are

u" + 16uf + 192u = 0.

u (0) = 2, u'(0) = 0.

(9)

(10)

The second initial condition is implied by the word “released” in the statement of the problem, which we interpret to mean that the mass is set in motion with no initial velocity.

Undamped Free Vibrations. If there is no external force, then F (t) = 0 in Eq. (7). Let us also suppose that there is no damping, so that y = 0; this is an idealized configuration of the system, seldom (if ever) completely attainable in practice. However, if the actual damping is very small, the assumption of no damping may yield satisfactory results over short to moderate time intervals. Then the equation of motion (7) reduces to

mu” + ku = 0. (11)

The general solution of Eq. (11) is

u = A cos m0t + Bsin m0t, (12)

where

= k/ m. (13)

The arbitrary constants A and B can be determined if initial conditions of the form (8) are given.

190

Chapter 3. Second Order Linear Equations

In discussing the solution of Eq. (11) it is convenient to rewrite Eq. (12) in the form

u = Rcos(w0t - S), (14)

or

u = Rcos S cos m0t + Rsin S sinm0t. (15)

By comparing Eq. (15) with Eq. (12), we find that A, B, R, and S are related by the equations

A = Rcos S, B = Rsin S. (16)

Thus

R = Ó A2 + B2, tan S = B/ A. (17)

In calculating S care must be taken to choose the correct quadrant; this can be done by checking the signs of cos S and sin S in Eqs. (16).

The graph of Eq. (14), or the equivalent Eq. (12), for a typical set of initial conditions is shown in Figure 3.8.3. The graph is a displaced cosine wave that describes a periodic, or simple harmonic, motion of the mass. The period of the motion is

^ 2n / m \1/2

T = — = 2n (-) . (18)

m0 V k /

The circular frequency þ0 = Vk/ m, measured in radians per unit time, is called the natural frequency of the vibration. The maximum displacement R of the mass from equilibrium is the amplitude of the motion. The dimensionless parameter S is called the phase, or phase angle, and measures the displacement of the wave from its normal position corresponding to S = 0.

Note that the motion described by Eq. (14) has a constant amplitude that does not diminish with time. This reflects the fact that in the absence of damping there is no way for the system to dissipate the energy imparted to it by the initial displacement and velocity. Further, for a given mass m and spring constant k, the system always vibrates at the same frequency m0, regardless of the initial conditions. However, the initial conditions do help to determine the amplitude of the motion. Finally, observe from Eq. (18) that T increases as m increases, so larger masses vibrate more slowly. On the other hand, T decreases as k increases, which means that stiffer springs cause the system to vibrate more rapidly.

u

1 R

R cos 8

1 8 8+ n 8 + 2 n m0t

-R

FIGURE 3.8.3 Simple harmonic motion; u = Rcos(«0t - S).

3.8 Mechanical and Electrical Vibrations

191

EXAMPLE

2

Suppose that a mass weighing 10 lb stretches a spring 2 in. If the mass is displaced an additional 2 in. and is then set in motion with an initial upward velocity of1 ft/sec, determine the position of the mass at any later time. Also determine the period, amplitude, and phase of the motion.

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