Pendulum (mathematics)

A pendulum is a body suspended from a fixed support so that it swings freely back and forth under the influence of gravity. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position. When released, the restoring force acting on the pendulum's mass causes it to oscillate about the equilibrium position, swinging it back and forth. The mathematics of pendulums are in general quite complicated. Simplifying assumptions can be made, which in the case of a simple pendulum allow the equations of motion to be solved analytically for small-angle oscillations.
Simple gravity pendulum.
A "simple gravity pendulum" is an idealized mathematical model of a real pendulum. This is a weight (or bob) on the end of a massless cord suspended from a pivot, without friction. Since in this model there is no frictional energy loss, when given an initial displacement it will swing back and forth at a constant amplitude. The model is based on these assumptions:
The differential equation which represents the motion of a simple pendulum is
where is acceleration due to gravity, is the length of the pendulum, and is the angular displacement.
Small-angle approximation.
The differential equation given above is not easily solved, and there is no solution that can be written in terms of elementary functions. However, adding a restriction to the size of the oscillation's amplitude gives a form whose solution can be easily obtained. If it is assumed that the angle is much less than 1 radian (often cited as less than 0.1 radians, about 6°), or
then substituting for into using the small-angle approximation,
yields the equation for a harmonic oscillator,
The error due to the approximation is of order (from Taylor expansion for ).
Given the initial conditions and , the solution becomes
The motion is simple harmonic motion where is the amplitude of the oscillation (that is, the maximum angle between the rod of the pendulum and the vertical). The period of the motion, the time for a complete oscillation (outward and return) is
which is known as Christiaan Huygens's law for the period. Note that under the small-angle approximation, the period is independent of the amplitude ; this is the property of isochronism that Galileo discovered.
Rule of thumb for pendulum length.
If SI units are used (i.e. measure in metres and seconds), and assuming the measurement is taking place on the Earth's surface, then , and (0.994 is the approximation to 3 decimal places).
Therefore, a relatively reasonable approximation for the length and period are,
where is the number of seconds between "two" beats (one beat for each side of the swing), and is measured in metres.
Arbitrary-amplitude period.
For amplitudes beyond the small angle approximation, one can compute the exact period by first inverting the equation for the angular velocity obtained from the energy method (),
and then integrating over one complete cycle,
or twice the half-cycle
or four times the quarter-cycle
which leads to
Note that this integral diverges as approaches the vertical
so that a pendulum with just the right energy to go vertical will never actually get there. (Conversely, a pendulum close to its maximum can take an arbitrarily long time to fall down.)
This integral can be rewritten in terms of elliptic integrals as
where is the incomplete elliptic integral of the first kind defined by
Or more concisely by the substitution
expressing in terms of ,
Here is the complete elliptic integral of the first kind defined by
For comparison of the approximation to the full solution, consider the period of a pendulum of length 1 m on Earth ( = ) at initial angle 10 degrees is
The linear approximation gives
The difference between the two values, less than 0.2%, is much less than that caused by the variation of with geographical location.
From here there are many ways to proceed to calculate the elliptic integral.
Legendre polynomial solution for the elliptic integral.
Given and the Legendre polynomial solution for the elliptic integral:
where denotes the double factorial, an exact solution to the period of a pendulum is:
Figure 4 shows the relative errors using the power series. is the linear approximation, and to include respectively the terms up to the 2nd to the 10th powers.
Power series solution for the elliptic integral.
Another formulation of the above solution can be found if the following Maclaurin series:
is used in the Legendre polynomial solution above.
The resulting power series is:
more fractions available in .
Arithmetic-geometric mean solution for elliptic integral.
Given and the arithmetic–geometric mean solution of the elliptic integral:
where is the arithmetic-geometric mean of and .
This yields an alternative and faster-converging formula for the period:
The first iteration of this algorithm gives
This approximation has the relative error of less than 1% for angles up to 96.11 degrees. Since formula_27 the expression can be written more concisely as
The second order expansion of formula_29 reduces to formula_30
A second iteration of this algorithm gives
This second approximation has a relative error of less than 1% for angles up to 163.10 degrees.
Approximate formulae for the nonlinear pendulum period.
Though the exact period formula_32 can be determined, for any finite amplitude formula_33 rad, by evaluating the corresponding complete elliptic integral formula_34, where formula_35, this is often avoided in applications because it is not possible to express this integral in a closed form in terms of elementary functions. This has made way for research on simple approximate formulae for the increase of the pendulum period with amplitude (useful in introductory physics labs, classical mechanics, electromagnetism, acoustics, electronics, superconductivity, etc. The approximate formulae found by different authors can be classified as follows:
with respect to the exact period increases monotonically with amplitude, being unsuitable for amplitudes near to formula_37rad. One of the simplest formulae found in literature is the following one by Lima (2006): formula_38, where formula_39.
amplitudes (i.e., unsuitable for small amplitudes). One of the better such formulae is that by Cromer, namely: formula_41.
Of course, the increase of formula_32 with amplitude is more apparent when formula_43, as has been observed in many experiments using either a rigid rod or a disc. As accurate timers and sensors are currently available even in introductory physics labs, the experimental errors found in ‘very large-angle’ experiments are already small enough for a comparison with the exact period and a very good agreement between theory and experiments in which friction is negligible has been found. Since this activity has been encouraged by many instructors, a simple approximate formula for the pendulum period valid for all possible amplitudes, to which experimental data could be compared, was sought. In 2008, Lima derived a weighted-average formula with this characteristic:
formula_44,
where formula_45, which presents a maximum error of only 0.6% (at formula_46).
Arbitrary-amplitude angular displacement Fourier series.
The Fourier series expansion of formula_47 is given by
where formula_49 is the elliptic nome, formula_50, and formula_51 the angular frequency.
If one defines
formula_49 can be approximated using the expansion
(see ). Note that for formula_55 we have formula_56, thus the approximation is applicable even for large amplitudes.
Examples.
The animations below depict the motion of a simple (frictionless) pendulum with increasing amounts of initial displacement of the bob, or equivalently increasing initial velocity. The small graph above each pendulum is the corresponding phase plane diagram; the horizontal axis is displacement and the vertical axis is velocity. With a large enough initial velocity the pendulum does not oscillate back and forth but rotates completely around the pivot.
Compound pendulum.
A compound pendulum (or physical pendulum) is one where the rod is not massless, and may have extended size; that is, an arbitrarily shaped rigid body swinging by a pivot. In this case the pendulum's period depends on its moment of inertia around the pivot point.
The equation of torque gives:
where:
The torque is generated by gravity so:
where:
Hence, under the small-angle approximation ,
where is the moment of inertia of the body about the pivot point.
The expression for is of the same form as the conventional simple pendulum and gives a period of
And a frequency of
If the initial angle is taken into consideration (for large amplitudes), then the expression for formula_62 becomes: 
and gives a period of: 
where is the maximum angle of oscillation (with respect to the vertical) and is the complete elliptic integral of the first kind.
Physical interpretation of the imaginary period.
The Jacobian elliptic function that expresses the position of a pendulum as a function of time is a doubly periodic function with a real period and an imaginary period. The real period is, of course, the time it takes the pendulum to go through one full cycle. Paul Appell pointed out a physical interpretation of the imaginary period: if is the maximum angle of one pendulum and is the maximum angle of another, then the real period of each is the magnitude of the imaginary period of the other.
Coupled pendula.
Coupled pendulums can affect each other's motion, either through a direction connection (such as a spring connecting the bobs) or through motions in a supporting structure (such as a tabletop). The equations of motion for two identical simple pendulums coupled by a spring connecting the bobs can be obtained using Lagrangian Mechanics.
The kinetic energy of the system is:
where formula_66 is the mass of the bobs, formula_67 is the length of the strings, and formula_68, formula_69 are the angular displacements of the two bobs from equilibrium.
The potential energy of the system is:
where formula_71 is the gravitational acceleration, and formula_72 is the spring constant. The displacement formula_73 of the spring from its equilibrium position assumes the small angle approximation.
The Lagrangian is then
which leads to the following set of coupled differential equations:
Adding and subtracting these two equations in turn, and applying the small angle approximation, gives two harmonic oscillator equations in the variables formula_76 and formula_77:
with the corresponding solutions
where
and formula_81, formula_82, formula_62, formula_84 are constants of integration.
Expressing the solutions in terms of formula_68 and formula_69 alone:
If the bobs are not given an initial push, then the condition formula_88 requires formula_89, which gives (after some rearranging):