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This is a simulation of a double pendulum. For large motions it is a chaotic system,but for small motions it is a simple linear system.
You can change parameters in the simulation such as mass, gravity, and length of rods.You can drag the pendulum with your mouse to change the starting position.
The math behind the simulation is shown below. Also available:source code,documentation andhow to customize.
For small angles, a pendulum behaves like alinear system (seeSimple Pendulum).When the angles are small in the Double Pendulum, the system behaves like the linearDouble Spring.In the graph, you can see similarLissajous curvesbeing generated. This is because the motion is determined by simple sine and cosinefunctions.
For large angles, the pendulum is non-linear and the phase graph becomes much morecomplex. You can see this by dragging one of the masses to a larger angle and lettinggo.
We regard the pendulum rods as being massless and rigid. We regard the pendulummasses as being point masses. The derivation of the equations of motion is shown below,using the direct Newtonian method.
Kinematics of the Double Pendulum
Kinematics means the relations of the parts of the device, without regard to forces.In kinematics we are only trying to find expressions for the position, velocity, andacceleration in terms of the variables that specify the state of the device.
- x = horizontal position of pendulum mass
- y = vertical position of pendulum mass
- θ = angle of pendulum (0 = vertical downwards, counter-clockwise is positive)
- L = length of rod (constant)
We place the origin at the pivot point of the upper pendulum. We regardy as increasing upwards. We indicate the upper pendulum by subscript 1,and the lower by subscript 2. Begin by using simple trigonometry to write expressionsfor the positions x1, y1, x2, y2 in terms of the angles θ1, θ2.
x1 = L1 sin θ1
y1 = −L1 cos θ1
x2 = x1 + L2 sin θ2
y2 = y1 − L2 cos θ2
The velocity is the derivative with respect to time of the position.
x1' = θ1' L1 cos θ1
y1' = θ1' L1 sin θ1
x2' = x1' + θ2' L2 cos θ2
y2' = y1' + θ2' L2 sin θ2
The acceleration is the second derivative.
| x1'' = −θ1'2 L1 sin θ1 + θ1'' L1 cos θ1 | (1) |
| y1'' = θ1'2 L1 cos θ1 + θ1'' L1 sin θ1 | (2) |
| x2'' = x1'' − θ2'2 L2 sin θ2 + θ2'' L2 cos θ2 | (3) |
| y2'' = y1'' + θ2'2 L2 cos θ2 + θ2'' L2 sin θ2 | (4) |
Forces in the Double Pendulum
upper mass
lower mass
We treat the two pendulum masses as point particles. Begin by drawing the free bodydiagram for the upper mass and writing an expression for the net force acting on it.Define these variables:
- T = tension in the rod
- m = mass of pendulum
- g = gravitational constant
The forces on the upper pendulum mass are the tension in the upper rod T1,the tension in the lower rod T2, and gravity −m1 g. We writeseparate equations for the horizontal and vertical forces, since they can be treatedindependently. The net force on the mass is the sum of these. Here we show the netforce and use Newton's law F = m a.
| m1 x1'' = −T1 sin θ1 + T2 sin θ2 | (5) |
| m1 y1'' = T1 cos θ1 − T2 cos θ2 −m1 g | (6) |
For the lower pendulum, the forces are the tension in the lower rod T2, andgravity −m2 g.
| m2 x2'' = −T2 sin θ2 | (7) |
| m2 y2'' = T2 cos θ2 − m2 g | (8) |
In relating these equations to the diagrams, keep in mind that in the examplediagram θ1 is positive and θ2 is negative, because of the conventionthat a counter-clockwise angle is positive.
Direct Method for Finding Equations of Motion
Now we do some algebraic manipulations with the goal of finding expressions forθ1'', θ2'' in terms of θ1, θ1', θ2, θ2'. Begin by solving equations(7), (8) for T2 sin θ2 and T2 cos θ2 and then substituting intoequations (5) and (6).
| m1 x1'' = −T1 sin θ1 − m2 x2'' | (9) |
| m1 y1'' = T1 cos θ1 − m2 y2'' − m2 g −m1 g | (10) |
Multiply equation (9) by cos θ1 and equation (10) by sin θ1 andrearrange to get
| T1 sin θ1 cos θ1 = −cos θ1 (m1 x1'' +m2 x2'') | (11) |
| T1 sin θ1 cos θ1 = sin θ1 (m1 y1'' + m2 y2'' +m2 g +m1 g) | (12) |
This leads to the equation
| sin θ1 (m1 y1'' + m2 y2'' + m2 g +m1 g) =−cos θ1 (m1 x1'' + m2 x2'') | (13) |
Next, multiply equation (7) by cos θ2 and equation (8) by sin θ2 andrearrange to get
| T2 sin θ2 cos θ2 = −cos θ2 (m2 x2'') | (14) |
| T2 sin θ2 cos θ2 = sin θ2 (m2 y2'' + m2 g) | (15) |
which leads to
| sin θ2 (m2 y2'' + m2 g) = −cos θ2 (m2 x2'') | (16) |
Next we need to use a computer algebra program to solve equations (13) and (16) forθ1'', θ2'' in terms of θ1, θ1', θ2, θ2'. Note that we also include thedefinitions given by equations (1-4), so that we have 2 equations (13, 16) and 2unknowns (θ1'', θ2''). The result is somewhat complicated, but is easy enoughto program into the computer.
| θ1''= | −g (2 m1 + m2) sin θ1 − m2 g sin(θ1 − 2 θ2) − 2 sin(θ1 − θ2) m2 (θ2'2 L2 + θ1'2 L1 cos(θ1 − θ2)) |
| L1 (2 m1 + m2 − m2 cos(2 θ1 − 2 θ2)) |
| θ2''= | 2 sin(θ1 − θ2) (θ1'2 L1 (m1 + m2) + g(m1 + m2) cos θ1 + θ2'2 L2 m2 cos(θ1 − θ2)) |
| L2 (2 m1 + m2 − m2 cos(2 θ1 − 2 θ2)) |
These are the equations of motion for the double pendulum.
Numerical Solution
The above equations are now close to the form needed for theRunge Kutta method. The finalstep is convert these two 2nd order equations into four 1st order equations. Define thefirst derivatives as separate variables:
- ω1 = angular velocity of top rod
- ω2 = angular velocity of bottom rod
Then we can write the four 1st order equations:
θ1' = ω1
θ2' = ω2
| ω1'= | −g (2 m1 + m2) sin θ1 − m2 g sin(θ1 − 2 θ2) − 2 sin(θ1 − θ2) m2 (ω22 L2 + ω12 L1 cos(θ1 − θ2)) |
| L1 (2 m1 + m2 − m2 cos(2 θ1 − 2 θ2)) |
| ω2'= | 2 sin(θ1−θ2) (ω12 L1 (m1 + m2) + g(m1 + m2) cos θ1 + ω22 L2 m2 cos(θ1 − θ2)) |
| L2 (2 m1 + m2 − m2 cos(2 θ1 − 2 θ2)) |
This is now exactly the form needed to plug in to the Runge-Kutta method fornumerical solution of the system.
This web page was first published February 2002.
© Erik Neumann, 2004-2021
revised Dec 19 2023 9:56
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