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 from the myphysicslab site are: open source code, documentation, and a customizable version.
For small angles, a pendulum behaves like a linear system (see Simple Pendulum). When the angles are small in the Double Pendulum, the system behaves like the linear Double Spring. In the graph, you can see similar Lissajous curves being generated. This is because the motion is determined by simple sine and cosine functions.
For large angles, the pendulum is non-linear and the phase graph becomes much more complex. You can see this by dragging one of the masses to a larger angle and letting go.
We regard the pendulum rods as being massless and rigid. We regard the pendulum masses as being point masses. The derivation of the equations of motion is shown below, using the direct Newtonian method.
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, and acceleration in terms of the variables that specify the state of the device.
We place the origin at the pivot point of the upper pendulum. We regard y as increasing upwards. We indicate the upper pendulum by subscript 1, and the lower by subscript 2. Begin by using simple trigonometry to write expressions for the positions x_{1}, y_{1}, x_{2}, y_{2} in terms of the angles θ_{1}, θ_{2} .
x_{1} = L_{1} sin θ_{1}
y_{1} = −L_{1} cos θ_{1}
x_{2} = x_{1} + L_{2} sin θ_{2}
y_{2} = y_{1} − L_{2} cos θ_{2}
The velocity is the derivative with respect to time of the position.
x_{1}' = θ_{1}' L_{1} cos θ_{1}
y_{1}' = θ_{1}' L_{1} sin θ_{1}
x_{2}' = x_{1}' + θ_{2}' L_{2} cos θ_{2}
y_{2}' = y_{1}' + θ_{2}' L_{2} sin θ_{2}
The acceleration is the second derivative.
x_{1}'' = −θ_{1}'^{2} L_{1} sin θ_{1} + θ_{1}'' L_{1} cos θ_{1} | (1) |
y_{1}'' = θ_{1}'^{2} L_{1} cos θ_{1} + θ_{1}'' L_{1} sin θ_{1} | (2) |
x_{2}'' = x_{1}'' − θ_{2}'^{2} L_{2} sin θ_{2} + θ_{2}'' L_{2} cos θ_{2} | (3) |
y_{2}'' = y_{1}'' + θ_{2}'^{2} L_{2} cos θ_{2} + θ_{2}'' L_{2} sin θ_{2} | (4) |
We treat the two pendulum masses as point particles. Begin by drawing the free body diagram for the upper mass and writing an expression for the net force acting on it. Define these variables:
The forces on the upper pendulum mass are the tension in the upper rod T_{1} , the tension in the lower rod T_{2} , and gravity −m_{1} g . We write separate equations for the horizontal and vertical forces, since they can be treated independently. The net force on the mass is the sum of these. Here we show the net force and use Newton's law F = m a .
m_{1} x_{1}'' = −T_{1} sin θ_{1} + T_{2} sin θ_{2} | (5) |
m_{1} y_{1}'' = T_{1} cos θ_{1} − T_{2} cos θ_{2} − m_{1} g | (6) |
For the lower pendulum, the forces are the tension in the lower rod T_{2} , and gravity −m_{2} g .
m_{2} x_{2}'' = −T_{2} sin θ_{2} | (7) |
m_{2} y_{2}'' = T_{2} cos θ_{2} − m_{2} g | (8) |
In relating these equations to the diagrams, keep in mind that in the example diagram θ_{1} is positive and θ_{2} is negative, because of the convention that a counter-clockwise angle is positive.
Now we do some algebraic manipulations with the goal of finding expressions for θ_{1}'', θ_{2}'' in terms of θ_{1}, θ_{1}', θ_{2}, θ_{2}' . Begin by solving equations (7), (8) for T_{2} sin θ_{2} and T_{2} cos θ_{2} and then substituting into equations (5) and (6).
m_{1} x_{1}'' = −T_{1} sin θ_{1} − m_{2} x_{2}'' | (9) |
m_{1} y_{1}'' = T_{1} cos θ_{1} − m_{2} y_{2}'' − m_{2} g − m_{1} g | (10) |
Multiply equation (9) by cos θ_{1} and equation (10) by sin θ_{1} and rearrange to get
T_{1} sin θ_{1} cos θ_{1} = −cos θ_{1} (m_{1} x_{1}'' + m_{2} x_{2}'') | (11) |
T_{1} sin θ_{1} cos θ_{1} = sin θ_{1} (m_{1} y_{1}'' + m_{2} y_{2}'' + m_{2} g + m_{1} g) | (12) |
This leads to the equation
sin θ_{1} (m_{1} y_{1}'' + m_{2} y_{2}'' + m_{2} g + m_{1} g) = −cos θ_{1} (m_{1} x_{1}'' + m_{2} x_{2}'') | (13) |
Next, multiply equation (7) by cos θ_{2} and equation (8) by sin θ_{2} and rearrange to get
T_{2} sin θ_{2} cos θ_{2} = −cos θ_{2} (m_{2} x_{2}'') | (14) |
T_{2} sin θ_{2} cos θ_{2} = sin θ_{2} (m_{2} y_{2}'' + m_{2} g) | (15) |
which leads to
sin θ_{2} (m_{2} y_{2}'' + m_{2} g) = −cos θ_{2} (m_{2} x_{2}'') | (16) |
Next we need to use a computer algebra program to solve equations (13) and (16) for θ_{1}'', θ_{2}'' in terms of θ_{1}, θ_{1}', θ_{2}, θ_{2}' . Note that we also include the definitions given by equations (1-4), so that we have 2 equations (13, 16) and 2 unknowns ( θ_{1}'', θ_{2}'' ). The result is somewhat complicated, but is easy enough to program into the computer.
θ_{1}'' = | −g (2 m_{1} + m_{2}) sin θ_{1} − m_{2} g sin(θ_{1} − 2 θ_{2}) − 2 sin(θ_{1} − θ_{2}) m_{2} (θ_{2}'^{2} L_{2} + θ_{1}'^{2} L_{1} cos(θ_{1} − θ_{2})) |
L_{1} (2 m_{1} + m_{2} − m_{2} cos(2 θ_{1} − 2 θ_{2})) |
θ_{2}'' = | 2 sin(θ_{1} − θ_{2}) (θ_{1}'^{2} L_{1} (m_{1} + m_{2}) + g(m_{1} + m_{2}) cos θ_{1} + θ_{2}'^{2} L_{2} m_{2} cos(θ_{1} − θ_{2})) |
L_{2} (2 m_{1} + m_{2} − m_{2} cos(2 θ_{1} − 2 θ_{2})) |
These are the equations of motion for the double pendulum.
The above equations are now close to the form needed for the Runge Kutta method. The final step is convert these two 2nd order equations into four 1st order equations. Define the first derivatives as separate variables:
Then we can write the four 1st order equations:
θ_{1}' = ω_{1}
θ_{2}' = ω_{2}
ω_{1}' = | −g (2 m_{1} + m_{2}) sin θ_{1} − m_{2} g sin(θ_{1} − 2 θ_{2}) − 2 sin(θ_{1} − θ_{2}) m_{2} (ω_{2}^{2} L_{2} + ω_{1}^{2} L_{1} cos(θ_{1} − θ_{2})) |
L_{1} (2 m_{1} + m_{2} − m_{2} cos(2 θ_{1} − 2 θ_{2})) |
ω_{2}' = | 2 sin(θ_{1}−θ_{2}) (ω_{1}^{2} L_{1} (m_{1} + m_{2}) + g(m_{1} + m_{2}) cos θ_{1} + ω_{2}^{2} L_{2} m_{2} cos(θ_{1} − θ_{2})) |
L_{2} (2 m_{1} + m_{2} − m_{2} cos(2 θ_{1} − 2 θ_{2})) |
This is now exactly the form needed to plug in to the Runge-Kutta method for numerical solution of the system.
This web page was first published February 2002.