We study undamped harmonic motion as an application of second order linear differential equations.

### Spring Problems I

We consider the motion of an object of mass , suspended from a spring of
negligible mass. We say that the spring–mass system is in *equilibrium* when
the object is at rest and the forces acting on it sum to zero. The position
of the object in this case is the *equilibrium position*. We define to be the
displacement of the object from its equilibrium position measured positive
upward.

Our model accounts for the following kinds of forces acting on the object:

- The force , due to gravity.
- A force exerted by the spring resisting change in its length. The
*natural length*of the spring is its length with no mass attached. We assume that the spring obeys Hooke’s law: If the length of the spring is changed by an amount from its natural length, then the spring exerts a force , where is a positive number called the*spring constant*. If the spring is stretched then and , so the spring force is upward, while if the spring is compressed then and , so the spring force is downward. - A
*damping force*that resists the motion with a force proportional to the velocity of the object. It may be due to air resistance or friction in the spring. However, a convenient way to visualize a damping force is to assume that the object is rigidly attached to a piston with negligible mass immersed in a cylinder (called a*dashpot*) filled with a viscous liquidAs the piston moves, the liquid exerts a damping force. We say that the motion is

*undamped*if , or*damped*if . - An external force , other than the force due to gravity, that may vary with , but
is independent of displacement and velocity. We say that the motion is
*free*if , or*forced*if .

From Newton’s second law of motion,

We must now relate to . In the absence of external forces the object stretches the spring by an amount to assume its equilibrium position, as shown in the figure below.Since the sum of the forces acting on the object is then zero, Hooke’s Law implies that . If the object is displaced units from its equilibrium position, the total change in the length of the spring is , so Hooke’s law implies that Substituting this into (eq:6.1.1) yields Since this can be written as

We call this*the equation of motion*.

#### Simple Harmonic Motion

Throughout the rest of this section we’ll consider spring–mass systems without damping; that is, . We’ll consider systems with damping in the next section.

We first consider the case where the motion is also free; that is, =0. We begin with an example.

- (a)
- Set up the equation of motion and find its general solution.
- (b)
- Find the displacement of the object for if it’s initially displaced 18 inches above equilibrium and given a downward velocity of 3 ft/s.

item:6.1.1b The initial upward displacement of 18 inches is positive and must be expressed in feet. The initial downward velocity is negative; thus, Differentiating (eq:6.1.5) yields

Setting in (eq:6.1.5) and (eq:6.1.6) and imposing the initial conditions shows that and . Therefore where is in feet.We’ll now consider the equation where and are arbitrary positive numbers. Dividing through by and defining yields The general solution of this equation is

We can rewrite this in a more useful form by defining and Substituting from (eq:6.1.9) into (eq:6.1.7) and applying the identity yieldsFrom (eq:6.1.8) and (eq:6.1.9) we see that the and can be interpreted as polar coordinates of the point with rectangular coordinates .

Given and , we can compute from (eq:6.1.8). From (eq:6.1.8) and (eq:6.1.9), we see that is related to and by There are infinitely many angles , differing by integer multiples of , that satisfy these equations. We will always choose so that .

The motion described by (eq:6.1.7) or (eq:6.1.10) is *simple harmonic motion*. We see from either of
these equations that the motion is periodic, with period
This is the time required for the object to complete one full cycle of oscillation (for
example, to move from its highest position to its lowest position and back to its
highest position). Since the highest and lowest positions of the object are and , we
say that is the *amplitude* of the oscillation. The angle in (eq:6.1.10) is the *phase angle*. It’s
measured in radians. Equation (eq:6.1.10) is the *amplitude–phase form* of the displacement. If
is in seconds then is in radians per second (rad/s); it’s the *frequency* of the motion.
It is also called the *natural frequency* of the spring–mass system without
damping.

#### Undamped Forced Oscillation

In many mechanical problems a device is subjected to periodic external forces. For example, soldiers marching in cadence on a bridge cause periodic disturbances in the bridge, and the engines of a propeller driven aircraft cause periodic disturbances in its wings. In the absence of sufficient damping forces, such disturbances – even if small in magnitude – can cause structural breakdown if they are at certain critical frequencies. To illustrate, this we’ll consider the motion of an object in a spring–mass system without damping, subject to an external force where is a constant. In this case the equation of motion (eq:6.1.2) is which we rewrite as

with . We’ll see from the next two examples that the solutions of (eq:6.1.13) with behave very differently from the solutions with .It is revealing to write this in a different form. We start with the trigonometric identities

From (eq:6.1.20) we can regard as a sinusoidal variation with frequency and variable
amplitude . In the figure below, the dashed curve above the axis is , the dashed
curve below the axis is , and the displacement appears as an oscillation bounded by
them. The oscillation of for on an interval between successive zeros of is called a
*beat*.

You can see from (eq:6.1.20) and (eq:6.1.21) that moreover, if is sufficiently large compared with , then assumes values close to (perhaps equal to) this upper bound during each beat. However, the oscillation remains bounded for all . (This assumes that the spring can withstand deflections of this size and continue to obey Hooke’s law.) The next example shows that this isn’t so if .

This phenomenon of unbounded displacements of a spring–mass system in response
to a periodic forcing function at its natural frequency is called *resonance*. More
complicated mechanical structures can also exhibit resonance–like phenomena. For
example, rhythmic oscillations of a suspension bridge by wind forces or of
an airplane wing by periodic vibrations of reciprocating engines can cause
damage or even failure if the frequencies of the disturbances are close to
critical frequencies determined by the parameters of the mechanical system in
question.

### Text Source

Trench, William F., ”Elementary Differential Equations” (2013). Faculty Authored and Edited Books & CDs. 8. (CC-BY-NC-SA)