Difference between revisions of "Pendulum (mathematics)"

From formulasearchengine
Jump to navigation Jump to search
en>Monkbot
en>Krishnavedala
 
(One intermediate revision by one other user not shown)
Line 4: Line 4:
  
 
== Simple gravity pendulum ==
 
== Simple gravity pendulum ==
[[File:Oscillating pendulum.gif|right|300px|thumb|Animation of a pendulum showing the [[Equations of motion|velocity and acceleration vectors]]{{-}}
+
[[File:Oscillating pendulum.gif|right|300px|thumb|Animation of a pendulum showing the [[Equations of motion|velocity and acceleration vectors]].]]  
'''''PLEASE NOTE:<BR>
+
 
'''The acceleration vector '''<math>\overrightarrow{a}</math>'''<BR>
 
is related to <BR>
 
the gravitational vector:<math>\overrightarrow{g} = G \frac{m M}{r^2}\ </math><br>
 
... where<br>
 
'''m''' is the [[bob (physics)|bob]] mass<br>
 
'''M''' is the [[Earth mass]] '''M<sub>⊕</sub>'''<br>
 
'''r''' is the [[Earth radius]]<br>
 
and<br>
 
'''G''' is the [[Gravitational constant]] in [[Standard gravity]].]]
 
 
A so-called "simple pendulum" is an idealization of a "real pendulum" but in an [[isolated system]] using the following assumptions:
 
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 rod or cord on which the bob swings is massless, inextensible and always remains taut;
Line 51: Line 42:
  
 
thus:
 
thus:
 +
 +
:<math>\ell{d^2\theta\over dt^2} = -g \sin\theta</math>
 
:<math>{d^2\theta\over dt^2} + {g\over\ell}\sin\theta = 0</math>
 
:<math>{d^2\theta\over dt^2} + {g\over\ell}\sin\theta = 0</math>
 
}}
 
}}
Line 77: Line 70:
 
Again just consider the magnitude of the angular momentum.
 
Again just consider the magnitude of the angular momentum.
 
:<math> \mathbf{ |L| } = mr^2 \omega = m l^2 {d\theta \over dt} </math>.
 
:<math> \mathbf{ |L| } = mr^2 \omega = m l^2 {d\theta \over dt} </math>.
and is time derivative
+
and its time derivative
  
 
:<math> {d \over dt}\mathbf{|L|} = m l^2 {d^2\theta \over dt^2} </math>,
 
:<math> {d \over dt}\mathbf{|L|} = m l^2 {d^2\theta \over dt^2} </math>,
Line 186: Line 179:
 
:<math>T_0 = 2\pi\sqrt{\frac{\ell}{g}}</math> can be expressed as <math>\ell = {\frac{g}{\pi^2}}\times{\frac{T_0^2}{4}}.</math>
 
:<math>T_0 = 2\pi\sqrt{\frac{\ell}{g}}</math> can be expressed as <math>\ell = {\frac{g}{\pi^2}}\times{\frac{T_0^2}{4}}.</math>
  
If [[International System of Units|SI units]] are used (i.e. measure in metres and seconds), and assuming the measurement is taking place on the Earth's surface, then <math>\scriptstyle g\approx9.81</math>  m/s<sup>2</sup>, and <math>\scriptstyle g/\pi^2\approx{1}</math> (0.994 is the approximation to 3 decimal places).
+
If [[International System of Units|SI units]] are used (i.e. measure in metres and seconds), and assuming the measurement is taking place on the Earth's surface, then <math>g\approx9.81</math>  m/s<sup>2</sup>, and <math>g/\pi^2\approx{1}</math> (0.994 is the approximation to 3 decimal places).
  
 
Therefore a relatively reasonable approximation for the length and period are,
 
Therefore a relatively reasonable approximation for the length and period are,
Line 217: Line 210:
  
 
:<math>T = 4\sqrt{\ell\over 2g}\int^{\theta_0}_0 {1\over\sqrt{\cos\theta-\cos\theta_0}}\,d\theta.</math>
 
:<math>T = 4\sqrt{\ell\over 2g}\int^{\theta_0}_0 {1\over\sqrt{\cos\theta-\cos\theta_0}}\,d\theta.</math>
 +
 +
Note that this integral diverges as <math> \theta_0 </math> approaches the vertical
 +
 +
:<math> \lim_{\theta_0 \rightarrow \pi} T = \infty </math>,
 +
 +
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 re-written in terms of [[elliptic integral]]s as
 
This integral can be re-written in terms of [[elliptic integral]]s as
  
:<math>T = 4\sqrt{\ell\over g}F\left( {\theta_0\over 2}, \csc{\theta_0\over2}\right)\csc {\theta_0\over 2}</math>
+
:<math>T = 4\sqrt{\ell\over g}F\left( {\theta_0}, \csc{\theta_0\over2}\right)\csc {\theta_0\over 2}</math>
  
 
where <math>F</math> is the [[elliptic integral#Incomplete elliptic integral of the first kind|incomplete elliptic integral of the first kind]] defined by
 
where <math>F</math> is the [[elliptic integral#Incomplete elliptic integral of the first kind|incomplete elliptic integral of the first kind]] defined by
Line 247: Line 246:
  
 
Figure 4 shows the relative errors using the power series.  ''T''<sub>0</sub> is the linear approximation, and ''T''<sub>2</sub> to ''T''<sub>10</sub> include respectively the terms up to the 2nd to the 10th powers.
 
Figure 4 shows the relative errors using the power series.  ''T''<sub>0</sub> is the linear approximation, and ''T''<sub>2</sub> to ''T''<sub>10</sub> include respectively the terms up to the 2nd to the 10th powers.
 
+
[[File:Pendulum phase portrait.svg|thumb|312x312px|'''Figure 5.''' Potential energy and phase portrait of a simple pendulum. Note that the ''x''-axis, being angle, wraps onto itself after every 2π radians.]]
[[Image:Pendulumphase.png|right|300px|thumb| '''Figure 5.''' Potential energy and phase portrait of a simple pendulum. Note that the ''x''-axis, being angle, wraps onto itself after every 2π radians.]]
 
 
 
 
=== Power series solution for the elliptic integral ===
 
=== Power series solution for the elliptic integral ===
 
Another formulation of the above solution can be found if the following Maclaurin series:
 
Another formulation of the above solution can be found if the following Maclaurin series:
Line 258: Line 255:
 
   | first = Robert
 
   | first = Robert
 
   | authorlink =  
 
   | authorlink =  
   | coauthors = M. G. Olsson
+
   |author2=M. G. Olsson
 
   | title = The pendulum &mdash; Rich physics from a simple system
 
   | title = The pendulum &mdash; Rich physics from a simple system
 
   | journal = American Journal of Physics
 
   | journal = American Journal of Physics
Line 281: Line 278:
 
where <math>M(x,y)</math> is the arithmetic-geometric mean of <math>x</math> and <math>y</math>.
 
where <math>M(x,y)</math> is the arithmetic-geometric mean of <math>x</math> and <math>y</math>.
  
This yields an alternative and faster-converging formula for the period:<ref>{{Citation |title=Approximations for the period of the simple pendulum based on the arithmetic-geometric mean |url=http://suppes-corpus.stanford.edu/articles/physics/431.pdf |first1=Claudio G. |last1=Carvalhaes |first2=Patrick |last2=Suppes |journal=[[American Journal of Physics|Am. J. Phys.]] |issn=0002-9505 |volume=76 |issue=12͒ |date=December 2008 |pages=1150–1154 |doi=10.1119/1.2968864͔ |accessdate=2013-12-14}}</ref>
+
This yields an alternative and faster-converging formula for the period:<ref>{{Citation |title=Approximations for the period of the simple pendulum based on the arithmetic-geometric mean |url=http://suppes-corpus.stanford.edu/articles/physics/431.pdf |first1=Claudio G. |last1=Carvalhaes |first2=Patrick |last2=Suppes |journal=[[American Journal of Physics|Am. J. Phys.]] |issn=0002-9505 |volume=76 |issue=12͒ |date=December 2008 |pages=1150–1154 |doi=10.1119/1.2968864͔ |accessdate=2013-12-14}}</ref><ref>{{Citation |title=An eloquent formula for the perimeter of an ellipse | url=http://www.ams.org/notices/201208/rtx120801094p.pdf |first=Semjon | last=Adlaj |journal=[[Notices of the American Mathematial Society |Notices of the AMS]] |issn=1088-9477 |volume=76 |issue=8 |date=September 2012 |pages=1094・099 | doi=}}</ref>
 
 
 
:<math>T = \frac{2\pi}{M(1, \cos(\theta_0/2))} \sqrt\frac{\ell}{g}.</math>
 
:<math>T = \frac{2\pi}{M(1, \cos(\theta_0/2))} \sqrt\frac{\ell}{g}.</math>
Line 295: Line 292:
 
File:Pendulum_135deg.gif|Initial angle of 135°
 
File:Pendulum_135deg.gif|Initial angle of 135°
 
File:Pendulum_170deg.gif|Initial angle of 170°
 
File:Pendulum_170deg.gif|Initial angle of 170°
 +
File:Pendulum_180deg.gif|Initial angle of 180°, unstable equilibrium.
 
File:Pendulum_190deg.gif|Pendulum with just barely enough energy for a full swing
 
File:Pendulum_190deg.gif|Pendulum with just barely enough energy for a full swing
 
File:Pendulum_220deg.gif|Pendulum with enough energy for a full swing
 
File:Pendulum_220deg.gif|Pendulum with enough energy for a full swing
File:Pendulum_180deg.gif|Initial angle of 180°, unstable equilibrium.
 
 
</gallery>
 
</gallery>
  
Line 344: Line 341:
 
*[[Double pendulum]]
 
*[[Double pendulum]]
 
*[[Inverted pendulum]]
 
*[[Inverted pendulum]]
 +
*[[Kapitza's pendulum]]
 
*[[Spring pendulum]]
 
*[[Spring pendulum]]
 
*[[Mathieu function]]
 
*[[Mathieu function]]

Latest revision as of 20:37, 29 November 2014

Template:Dynamics Template:Seeintro The mathematics of pendulums are in general quite complicated. Simplifying assumptions can be made, which in the case of a simple pendulum allows the equations of motion to be solved analytically for small-angle oscillations.

Simple gravity pendulum

Animation of a pendulum showing the velocity and acceleration vectors.

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 differential equation which represents the motion of a simple pendulum is

Template:NumBlk where is acceleration due to gravity, is the length of the pendulum, and is the angular displacement.

Template:Show

Template:Show

Template:Show

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, or

,

then substituting for sin θ into Template:EquationNote using the small-angle approximation,

,

yields the equation for a harmonic oscillator,

The error due to the approximation is of order θ 3 (from the Maclaurin series for sin θ).

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

The motion is simple harmonic motion where θ0 is the semi-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

can be expressed as

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 m/s2, and (0.994 is the approximation to 3 decimal places).

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

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 (Template:EquationNote),

Figure 3. Deviation of the "true" period of a pendulum from the small-angle approximation of the period. "True" value was obtained using Matlab to numerically evaluate the elliptic integral.

and then integrating over one complete cycle,

or twice the half-cycle

or 4 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 re-written 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 ,

Template:NumBlk where 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 (g = 9.80665 m/s2) 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 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 Template:EquationNote and the Legendre polynomial solution for the elliptic integral:

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

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.

Figure 5. Potential energy and phase portrait of a simple pendulum. Note that the x-axis, being angle, wraps onto itself after every 2π radians.

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:[1]

Arithmetic-geometric mean solution for elliptic integral

Given Template:EquationNote 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:[2][3]

Examples

The animations below depict several different modes of oscillation given different initial conditions. The small graph above the pendulums are their phase portraits.

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:

where:

is the angular acceleration.
is the torque

The torque is generated by gravity so:

where:

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 ,

This is of the same form as the conventional simple pendulum and this gives a period of:

[4]

And a frequency of:

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:[5] 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.[6]

See also

References

  1. {{#invoke:Citation/CS1|citation |CitationClass=journal }}
  2. {{#invoke:citation/CS1|citation |CitationClass=citation }}
  3. {{#invoke:citation/CS1|citation |CitationClass=citation }}
  4. Physical Pendulum
  5. Paul Appell, "Sur une interprétation des valeurs imaginaires du temps en Mécanique", Comptes Rendus Hebdomadaires des Scéances de l'Académie des Sciences, volume 87, number 1, July, 1878
  6. Adlaj, S. Mechanical interpretation of negative and imaginary tension of a tether in a linear parallel force field , Selected papers of the International Scientific Conference on Mechanics "SIXTH POLYAKHOV READINGS", January 31 - February 3, 2012, Saint-Petersburg, Russia, pp. 13-18.

Further reading

  • {{#invoke:citation/CS1|citation

|CitationClass=book }}

  • {{#invoke:Citation/CS1|citation

|CitationClass=journal }}

  • {{#invoke:Citation/CS1|citation

|CitationClass=journal }}

External links