Pendulums, Simple and otherwsie

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Problem Description

An animated  simulation of three varieties of pendulum: Simple, Double,  and Forced

Background & Techniques

I recently implemented a Delphi version of the Runge-Kutta technique for solving second order ordinary differential equations.   This type of equation is good at describing the motion of things ( the second order part translates to acceleration) so sliding, rolling, bouncing, vibrating, rotating, swinging things are all candidates.  I chose swinging for this program and learned some interesting things about pendulums.  The following "User guide" is also available in the program buy clicking the Info buttons in the program.  


This program explores three different types of pendulums.    Each of the following pages allows the user to set parameters defining the pendulums and set them in motion to observe the effect.  The program was written to test a recently developed Runge-Kutta technique for solving a single ordinary differential equation (ODE) or a system of ODEs.   If you are not into higher mathematics or programming, it’s still fun to play with the parameters and observe results.  The dual double pendulums and the forced damped pendulums make interesting viewing.

Simple Pendulum

 A Simple Pendulum consists of a weight or “bob” hanging freely from the end of a weightless rod of length L.  Rotating the bob to some initial angle, q0, and releasing it imparts its movement.   Gravity, G, pulls the bob in a downward arc and past vertical to angle equal to the initial displacement, unless some damping force slows it down.    Damping factor, D, is a constant which when multiplied by the angular velocity, q', produces a force acting to decrease the velocity.     The differential equation is:

 q'' = - G/L sin (q) – Dq'

In words, the acceleration of the bob is equal to that part of the gravitational force acting  perpendicular to the shaft minus the damping force.   The minus sign on the gravity term just tells us that gravity is the enemy here, always trying to slow things down. 

 Our Runge-Kutta procedure calls a user-defined function, passing an angle (q) and angular velocity (q') for each time interval. The function in this case applies the above equation to return a new angular acceleration value (q''). 

 Here’s the Delphi code:

Function TForm1.Pendulum1Func(T, X, Xprime: Float):Float;
     Result: = - Gravity/Len*sin(x) – Damping * Xprime;

 The simple pendulum program page lets you to change gravity, length, initial angle and damping factor and watch the pendulum run.  

 The Computational Parameters dialog allows setting maximum run time, calculated points per second (pps) and returned pps.   The default values, 100 pps calculated and 25 pps returned usually provide reasonably accurate values and smooth plots.   Points returned should be some fraction of points calculated per second, normally returning around 25% of the calculated samples will work fine for the display.  

 By the way, it is often supposed that the period of a pendulum (the time to make a complete swing), is independent of length, mass, or swing angle.  This is almost true.  The pendulum does swing faster at smaller angles as  you can check for yourself by setting the damping factor to 0 and timing 10 swings for various starting angles.  

 Note: All angles are measured counterclockwise from South (down).

 Double Pendulums   

The second pendulum page has one pendulum attached to the end of another pendulum, like holding a weight in your hand and swinging it  with your shoulder and a really flexible elbow.   Do a web search on “double pendulum” and you will find a number of sites that list the differential equations.  Unlike a simple pendulum, the equations for double pendulums do include the masses of the bobs. 

Here we test our Runge-Kutta procedure’s ability to solve a system of differential equations.  There are separate ODEs for each part of the pendulum and they interact, i.e. the acceleration of each bob is influenced by the acceleration of the other.   The ODEs are quite complex so I won’t bother to reproduce them here.    They are available on-line or in the source code for this program for those interested. 

This page includes a “chaos” demonstration; a very small change resulting in very different behavior.  In this “dual double pendulum “example, we’ll clone the defined pendulum, multiply the mass of the lower bob by 1.0001 on the clone, and then release them at the same time.  After a minute or two the behaviors will diverge and have no apparent relationship to each other.   A search on the web for  “butterfly effect” will produce some interesting reading about this sensitivity to initial conditions.

Forced Pendulum

The final page let’s you play with a Damped Forced Pendulum.  In this case, a force of specified amplitude, A, and frequency, Ω (omega), is applied to a simple pendulum.   The equation is often simplified by assuming pendulum length is equal to the value for gravity since the interesting effects involve damping factor, D, and the characteristics of the forcing frequency at time t.   I’ve chosen to leave gravity, G,  and length, L,  in the calculations.

 q'' = – Dq' - G/L sin(q)  + Asin(Ωt)

 Or, in Delphi terms:

Function TForm1.Pend3Func(T, X, XPrime: Float): Float;
     Result: = - Damping* Xprime – Gravity/Length*sin (X) +  A* cos(omega*T);

The pendulum animation displays the applied force as a green bar acting on the bob at right angles to the pendulum and with length proportional to the value of the force.

A separate chart plotting pendulum angle vs. angular velocity is also displayed.   It may take the pendulum a minute or more to stabilize due to damping effects as it starts from the resting position.  Clicking the “Clear chart” button after a couple of minutes will give a clearer impression of the steady state pattern,( if there is one).     There is also a “Real time” checkbox that toggles the display between real time and the maximum display rate for your computer and graphics card.   

An appropriate selection of input values will produce patterns that repeat in single, double loops or chaotic behavior that may never repeat.   Starting with the default values, try runs with amplitudes from 100 to 120 to see a variety of interesting behaviors.

Non-programmers are welcome to read on, but may want to skip to the bottom of this page to download executable version of the program.

Programmer Notes

The length of this program puts it in the Advanced category, but there were no great programming challenges once you understand the calling protocol for Runge-Kutta described on this  Runge-Kutta page over in the Math Topics section.

 There are a couple of other new (at least for me) items worth mentioning:

  • Putting some rich text format (.rtf)  help or info files into a resource and loading them into a TRichTextEdit control using TResourceStream
  • Drawing the force bar graphic on the Forced Pendulum page needed a new  procedure, AngledLineFromLine,  to calculate the other endpoint of a line from a  point on a given line at a specified angle and length.


Running/Exploring the Program 

Suggestions for Further Explorations

Here's and  interesting article  describing Christian Huygens observation in 1655 that pendulums on two clocks in his laboratory tended to synchronize with opposing swings regardless of the initial positions of the pendulums.   A recent study at Ga. Tech  explained the phenomenon as caused by a shared suspension beam for the pendulums.  As a pendulum swings, it applies a small force, and therefore potential small movement,  to the attachment beam (by rhian hanson).  This tends to dampen the swings and in the  case of two synchronized pendulums, to dampen them both.  If the devices swing in opposition the imparted forces cancel each other and the motion us not dampened.    So over time, the opposing swing configuration wins.      I wonder if this could be modeled with a computer program.  Sure it could. 


Original: September 20.2002 

Modified: February 18, 2016



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