Pendulum (mathematics)

Simple gravity pendulum

A so-called "simple pendulum" is an idealization of a "real pendulum" but in an isolated system using the following assumptions:

The rod or cord on which the bob swings is massless, inextensible and always remains taut;

The bob is a point mass;

Motion occurs only in two dimensions, i.e. the bob does not trace an ellipse but an arc.

The motion does not lose energy to friction or air resistance.

The gravitational field is uniform.

The support does not move.

The differential equation which represents the motion of a simple pendulum is

d 2 θ d t 2 + g ℓ sin ⁡ θ = 0  Eq. 1

where g is acceleration due to gravity, l is the length of the pendulum, and θ 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

θ ≪ 1 ,

then substituting for sin θ into Eq. 1 using the small-angle approximation,

sin ⁡ θ ≈ θ ,

yields the equation for a harmonic oscillator,

d 2 θ d t 2 + g ℓ θ = 0.

The error due to the approximation is of order θ³ (from the Maclaurin series for sin θ).

Given the initial conditions θ(0) = θ₀ and dθ/dt(0) = 0, the solution becomes

The motion is simple harmonic motion where θ₀ 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 θ₀; this is the property of isochronism that Galileo discovered.

Rule of thumb for pendulum length

T 0 = 2 π ℓ g can be expressed as ℓ = g π 2 T 0 2 4 .

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 g ≈ 9.81 m/s², and g/π² ≈ 1 (0.994 is the approximation to 3 decimal places).

Therefore, a relatively reasonable approximation for the length and period are,

ℓ ≈ T 0 2 4 , T 0 ≈ 2 ℓ

where T₀ is the number of seconds between two beats (one beat for each side of the swing), and l 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 (Eq. 2),

d t d θ = ℓ 2 g 1 cos ⁡ θ − cos ⁡ θ 0

and then integrating over one complete cycle,

T = t ( θ 0 → 0 → − θ 0 → 0 → θ 0 ) ,

or twice the half-cycle

T = 2 t ( θ 0 → 0 → − θ 0 ) ,

or four times the quarter-cycle

T = 4 t ( θ 0 → 0 ) ,

which leads to

T = 4 ℓ 2 g ∫ 0 θ 0 1 cos ⁡ θ − cos ⁡ θ 0 d θ .

Note that this integral diverges as θ₀ approaches the vertical

lim θ 0 → π T = ∞ ,

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

T = 4 ℓ g F ( θ 0 2 , csc ⁡ θ 0 2 ) csc ⁡ θ 0 2

where F is the incomplete elliptic integral of the first kind defined by

F ( φ , k ) = ∫ 0 φ 1 1 − k 2 sin 2 ⁡ u d u .

Or more concisely by the substitution

sin ⁡ u = sin ⁡ θ 2 sin ⁡ θ 0 2

expressing θ in terms of u,

T = 4 ℓ g K ( k ) , k = sin ⁡ θ 0 2  Eq. 3

where K is the complete elliptic integral of the first kind defined by

K ( k ) = F ( π 2 , k ) = ∫ 0 π 2 1 1 − k 2 sin 2 ⁡ u d u .

For comparison of the approximation to the full solution, consider the period of a pendulum of length 1 m on Earth (g = {{val|9.80665|u=m/s²}) at initial angle 10 degrees is

4 1  m g   K ( sin ⁡ 10 ∘ 2 ) ≈ 2.0102  s .

The linear approximation gives

2 π 1  m g ≈ 2.0064  s .

The difference between the two values, less than 0.2%, is much less than that caused by the variation of g with geographical location.

From here there are many ways to proceed to calculate the elliptic integral.

Legendre polynomial solution for the elliptic integral

Given Eq. 3 and the Legendre polynomial solution for the elliptic integral:

K ( k ) = π 2 ( 1 + ( 1 2 ) 2 k 2 + ( 1 ⋅ 3 2 ⋅ 4 ) 2 k 4 + ⋯ + ( ( 2 n − 1 ) ! ! ( 2 n ) ! ! ) 2 k 2 n + ⋯ ) ,

where n!! denotes the double factorial, an exact solution to the period of a pendulum is:

T = 2 π ℓ g ( 1 + ( 1 2 ) 2 sin 2 ⁡ θ 0 2 + ( 1 ⋅ 3 2 ⋅ 4 ) 2 sin 4 ⁡ θ 0 2 + ( 1 ⋅ 3 ⋅ 5 2 ⋅ 4 ⋅ 6 ) 2 sin 6 ⁡ θ 0 2 + ⋯ ) = 2 π ℓ g ⋅ ∑ n = 0 ∞ ( ( ( 2 n ) ! ( 2 n ⋅ n ! ) 2 ) 2 ⋅ sin 2 n ⁡ θ 0 2 ) .

Figure 4 shows the relative errors using the power series. T₀ is the linear approximation, and T₂ to T₁₀ 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:

sin ⁡ θ 0 2 = 1 2 θ 0 − 1 48 θ 0 3 + 1 3 840 θ 0 5 − 1 645 120 θ 0 7 + ⋯ .

is used in the Legendre polynomial solution above. The resulting power series is:

T = 2 π ℓ g ( 1 + 1 16 θ 0 2 + 11 3 072 θ 0 4 + 173 737 280 θ 0 6 + 22 931 1 321 205 760 θ 0 8 + 1 319 183 951 268 147 200 θ 0 10 + 233 526 463 2 009 078 326 886 400 θ 0 12 + ⋯ ) .

Arithmetic-geometric mean solution for elliptic integral

Given Eq. 3 and the arithmetic–geometric mean solution of the elliptic integral:

K ( k ) = π 2 M ( 1 − k , 1 + k ) ,

where M(x,y) is the arithmetic-geometric mean of x and y.

This yields an alternative and faster-converging formula for the period:

T = 2 π M ( 1 , cos ⁡ θ 0 2 ) ℓ g .

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 I around the pivot point.

The equation of torque gives:

τ = I α

where:

α is the angular acceleration. τ is the torque

The torque is generated by gravity so:

τ = − m g L sin ⁡ θ

where:

m is the mass of the body L is the distance from the pivot to the center of mass of the pendulum θ is the angle from the vertical

Hence, under the small-angle approximation sin θ ≈ θ,

α = θ ″ ≈ − m g L θ I c m + m L 2

where I_cm is the moment of inertia of the body about its center of mass.

The expression for α is of the same form as the conventional simple pendulum and gives a period of

T = 2 π I c m + m L 2 m g L

And a frequency of

f = 1 T = 1 2 π m g L I c m + m L 2

If the initial angle is taken into consideration (for large amplitudes), then the expression for α becomes:

α = θ ″ = − m g L sin ⁡ θ I c m + m L 2

and gives a period of:

T = 4 K ( sin 2 ⁡ θ 0 2 ) I c m + m L 2 m g L

where θ₀ is the maximum angle of oscillation (with respect to the vertical) and K(k) 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 θ₀ is the maximum angle of one pendulum and 180° − θ₀ is the maximum angle of another, then the real period of each is the magnitude of the imaginary period of the other. This interpretation, involving dual forces in opposite directions, might be further clarified and generalized to other classical problems in mechanics with dual solutions.