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The pendulum is a classical physics system that we’re all conversant in. Be it a grandfather clock or a toddler on a swing, we’ve seen the common, periodic movement of the pendulum. A single pendulum is effectively outlined in classical physics, however the double pendulum (a pendulum hooked up to the top of one other pendulum) is literal chaos. On this article, we’re going to construct on our intuitive understanding of pendulums and mannequin the chaos of the double pendulum. The physics is fascinating and the numerical strategies wanted are an important device in anybody’s arsenal.

On this article we’ll:

- Study harmonic movement and mannequin the habits of a single pendulum
- Be taught the basics of chaos idea
- Mannequin the chaotic habits of a double pendulum numerically

## Easy Harmonic Movement

We describe the periodic oscillating motion of a pendulum as harmonic motion. Harmonic movement happens when there may be motion in a system that’s balanced out by a proportional restoring drive in the wrong way of stated motion. We see an instance of this in determine 2 the place a mass on a spring is being pulled down on account of gravity, however this places power into the spring which then recoils and pulls the mass again up. Subsequent to the spring system, we see the peak of the mass going round in a circle referred to as a phasor diagram which additional illustrates the common movement of the system.

Harmonic movement may be damped (lowering in amplitude on account of drag forces) or pushed (rising in amplitude on account of outdoors drive being added), however we’ll begin with the best case of indefinite harmonic movement with no outdoors forces performing on it (undamped movement). That is sort of movement is an efficient approximation for modeling a single pendulum that swings at a small angle/low amplitude. On this case we are able to mannequin the movement with equation 1 beneath.

We are able to simply put this perform into code and simulate a easy pendulum over time.

`def simple_pendulum(theta_0, omega, t, phi):`

theta = theta_0*np.cos(omega*t + phi)

return theta#parameters of our system

theta_0 = np.radians(15) #levels to radians

g = 9.8 #m/s^2

l = 1.0 #m

omega = np.sqrt(g/l)

phi = 0 #for small angle

time_span = np.linspace(0,20,300) #simulate for 20s break up into 300 time intervals

theta = []

for t in time_span:

theta.append(simple_pendulum(theta_0, omega, t, phi))

#Convert again to cartesian coordinates

x = l*np.sin(theta)

y = -l*np.cos(theta) #damaging to ensure the pendulum is dealing with down

## Full Pendulum Movement with Lagrangian Mechanics

A easy small angle pendulum is an efficient begin, however we need to transcend this and mannequin the movement of a full pendulum. Since we are able to now not use small angle approximations it’s best to mannequin the pendulum utilizing Lagrangian mechanics. That is an important device in physics that switches us from wanting on the forces in a system to wanting on the power in a system. We’re switching our body of reference from driving drive vs restoring drive to kinetic vs potential power.

The Lagrangain is the distinction between kinetic and potential power given in equation 2.

Substituting within the Kinetic and Potential of a pendulum given in equation 3 yields the Lagrangain for a pendulum seen is equation 4

With the Lagrangian for a pendulum we now describe the power of our system. There’s one final math step to undergo to rework this into one thing that we are able to construct a simulation on. We have to bridge again to the dynamic/drive oriented reference from the power reference utilizing the Euler-Lagrange equation. Utilizing this equation we are able to use the Lagrangian to get the angular acceleration of our pendulum.

After going by means of the maths, we’ve angular acceleration which we are able to use to get angular velocity and angle itself. It will require some numerical integration that shall be specified by our full pendulum simulation. Even for a single pendulum, the non-linear dynamics means there isn’t any analytical resolution for fixing for *theta, *thus the necessity for a numerical resolution. The mixing is sort of easy (however highly effective), we use angular acceleration to replace angular velocity and angular velocity to replace *theta *by including the previous amount to the latter and multiplying this by a while step. This will get us an approximation for the realm below the acceleration/velocity curve. The smaller the time step, the extra correct the approximation.

`def full_pendulum(g,l,theta,theta_velocity, time_step):`

#Numerical Integration

theta_acceleration = -(g/l)*np.sin(theta) #Get acceleration

theta_velocity += time_step*theta_acceleration #Replace velocity with acceleration

theta += time_step*theta_velocity #Replace angle with angular velocity

return theta, theta_velocityg = 9.8 #m/s^2

l = 1.0 #m

theta = [np.radians(90)] #theta_0

theta_velocity = 0 #Begin with 0 velocity

time_step = 20/300 #Outline a time step

time_span = np.linspace(0,20,300) #simulate for 20s break up into 300 time intervals

for t in time_span:

theta_new, theta_velocity = full_pendulum(g,l,theta[-1], theta_velocity, time_step)

theta.append(theta_new)

#Convert again to cartesian coordinates

x = l*np.sin(theta)

y = -l*np.cos(theta)

We have now simulated a full pendulum, however that is nonetheless a effectively outlined system. It’s now time to step into the chaos of the double pendulum.

Chaos, within the mathematical sense, refers to programs which are extremely delicate to their preliminary circumstances. Even slight adjustments within the system’s begin will result in vastly totally different behaviors because the system evolves. This completely describes the movement of the double pendulum. In contrast to the only pendulum, it’s not a effectively behaved system and can evolve in a vastly totally different manner with even slight adjustments in beginning angle.

To mannequin the movement of the double pendulum, we’ll use the identical Lagrangian method as earlier than (see full derivation).

We can even be utilizing the identical numerical integration scheme as earlier than when implementing this equation into code and discovering theta.

`#Get theta1 acceleration `

def theta1_acceleration(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g):

mass1 = -g*(2*m1 + m2)*np.sin(theta1)

mass2 = -m2*g*np.sin(theta1 - 2*theta2)

interplay = -2*np.sin(theta1 - theta2)*m2*np.cos(theta2_velocity**2*l2 + theta1_velocity**2*l1*np.cos(theta1 - theta2))

normalization = l1*(2*m1 + m2 - m2*np.cos(2*theta1 - 2*theta2))theta1_ddot = (mass1 + mass2 + interplay)/normalization

return theta1_ddot

#Get theta2 acceleration

def theta2_acceleration(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g):

system = 2*np.sin(theta1 - theta2)*(theta1_velocity**2*l1*(m1 + m2) + g*(m1 + m2)*np.cos(theta1) + theta2_velocity**2*l2*m2*np.cos(theta1 - theta2))

normalization = l1*(2*m1 + m2 - m2*np.cos(2*theta1 - 2*theta2))

theta2_ddot = system/normalization

return theta2_ddot

#Replace theta1

def theta1_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step):

#Numerical Integration

theta1_velocity += time_step*theta1_acceleration(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g)

theta1 += time_step*theta1_velocity

return theta1, theta1_velocity

#Replace theta2

def theta2_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step):

#Numerical Integration

theta2_velocity += time_step*theta2_acceleration(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g)

theta2 += time_step*theta2_velocity

return theta2, theta2_velocity

#Run full double pendulum

def double_pendulum(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step,time_span):

theta1_list = [theta1]

theta2_list = [theta2]

for t in time_span:

theta1, theta1_velocity = theta1_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step)

theta2, theta2_velocity = theta2_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step)

theta1_list.append(theta1)

theta2_list.append(theta2)

x1 = l1*np.sin(theta1_list) #Pendulum 1 x

y1 = -l1*np.cos(theta1_list) #Pendulum 1 y

x2 = l1*np.sin(theta1_list) + l2*np.sin(theta2_list) #Pendulum 2 x

y2 = -l1*np.cos(theta1_list) - l2*np.cos(theta2_list) #Pendulum 2 y

return x1,y1,x2,y2

`#Outline system parameters`

g = 9.8 #m/s^2m1 = 1 #kg

m2 = 1 #kg

l1 = 1 #m

l2 = 1 #m

theta1 = np.radians(90)

theta2 = np.radians(45)

theta1_velocity = 0 #m/s

theta2_velocity = 0 #m/s

theta1_list = [theta1]

theta2_list = [theta2]

time_step = 20/300

time_span = np.linspace(0,20,300)

x1,y1,x2,y2 = double_pendulum(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step,time_span)

We’ve lastly carried out it! We have now efficiently modeled a double pendulum, however now it’s time to look at some chaos. Our last simulation shall be of two double pendulums with barely totally different beginning situation. We are going to set one pendulum to have a *theta 1 *of 90 levels and the opposite to have a *theta 1* of 91 levels. Let’s see what occurs.

We are able to see that each pendulums begin off with comparable trajectories however rapidly diverge. That is what we imply after we say chaos, even a 1 diploma distinction in angle cascades into vastly totally different finish habits.

On this article we realized about pendulum movement and how you can mannequin it. We began from the best harmonic movement mannequin and constructed as much as the complicated and chaotic double pendulum. Alongside the best way we realized concerning the Lagrangian, chaos, and numerical integration.

The double pendulum is the best instance of a chaotic system. These programs exist all over the place in our world from population dynamics, climate, and even billiards. We are able to take the teachings we’ve realized from the double pendulum and apply them every time we encounter a chaotic programs.

## Key Take Aways

- Chaotic programs are very delicate to preliminary circumstances and can evolve in vastly other ways with even slight adjustments to their begin.
- When coping with a system, particularly a chaotic system, is there one other body of reference to take a look at it that makes it simpler to work with? (Just like the drive reference body to the power reference body)
- When programs get too difficult we have to implement numerical options to resolve them. These options are easy however highly effective and supply good approximations to the precise habits.

All figures used on this article have been both created by the creator or are from Math Images and full below the GNU Free Documentation License 1.2

Classical Mechanics, John Taylor https://neuroself.files.wordpress.com/2020/09/taylor-2005-classical-mechanics.pdf

## Easy Pendulum

`def makeGif(x,y,title):`

!mkdir framescounter=0

photographs = []

for i in vary(0,len(x)):

plt.determine(figsize = (6,6))

plt.plot([0,x[i]],[0,y[i]], "o-", shade = "b", markersize = 7, linewidth=.7 )

plt.title("Pendulum")

plt.xlim(-1.1,1.1)

plt.ylim(-1.1,1.1)

plt.savefig("frames/" + str(counter)+ ".png")

photographs.append(imageio.imread("frames/" + str(counter)+ ".png"))

counter += 1

plt.shut()

imageio.mimsave(title, photographs)

!rm -r frames

def simple_pendulum(theta_0, omega, t, phi):

theta = theta_0*np.cos(omega*t + phi)

return theta

#parameters of our system

theta_0 = np.radians(15) #levels to radians

g = 9.8 #m/s^2

l = 1.0 #m

omega = np.sqrt(g/l)

phi = 0 #for small angle

time_span = np.linspace(0,20,300) #simulate for 20s break up into 300 time intervals

theta = []

for t in time_span:

theta.append(simple_pendulum(theta_0, omega, t, phi))

x = l*np.sin(theta)

y = -l*np.cos(theta) #damaging to ensure the pendulum is dealing with down

## Pendulum

`def full_pendulum(g,l,theta,theta_velocity, time_step):`

theta_acceleration = -(g/l)*np.sin(theta)

theta_velocity += time_step*theta_acceleration

theta += time_step*theta_velocity

return theta, theta_velocityg = 9.8 #m/s^2

l = 1.0 #m

theta = [np.radians(90)] #theta_0

theta_velocity = 0

time_step = 20/300

time_span = np.linspace(0,20,300) #simulate for 20s break up into 300 time intervals

for t in time_span:

theta_new, theta_velocity = full_pendulum(g,l,theta[-1], theta_velocity, time_step)

theta.append(theta_new)

#Convert again to cartesian coordinates

x = l*np.sin(theta)

y = -l*np.cos(theta)

#Use identical perform from easy pendulum

makeGif(x,y,"pendulum.gif")

## Double Pendulum

`def theta1_acceleration(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g):`

mass1 = -g*(2*m1 + m2)*np.sin(theta1)

mass2 = -m2*g*np.sin(theta1 - 2*theta2)

interplay = -2*np.sin(theta1 - theta2)*m2*np.cos(theta2_velocity**2*l2 + theta1_velocity**2*l1*np.cos(theta1 - theta2))

normalization = l1*(2*m1 + m2 - m2*np.cos(2*theta1 - 2*theta2))theta1_ddot = (mass1 + mass2 + interplay)/normalization

return theta1_ddot

def theta2_acceleration(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g):

system = 2*np.sin(theta1 - theta2)*(theta1_velocity**2*l1*(m1 + m2) + g*(m1 + m2)*np.cos(theta1) + theta2_velocity**2*l2*m2*np.cos(theta1 - theta2))

normalization = l1*(2*m1 + m2 - m2*np.cos(2*theta1 - 2*theta2))

theta2_ddot = system/normalization

return theta2_ddot

def theta1_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step):

theta1_velocity += time_step*theta1_acceleration(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g)

theta1 += time_step*theta1_velocity

return theta1, theta1_velocity

def theta2_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step):

theta2_velocity += time_step*theta2_acceleration(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g)

theta2 += time_step*theta2_velocity

return theta2, theta2_velocity

def double_pendulum(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step,time_span):

theta1_list = [theta1]

theta2_list = [theta2]

for t in time_span:

theta1, theta1_velocity = theta1_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step)

theta2, theta2_velocity = theta2_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step)

theta1_list.append(theta1)

theta2_list.append(theta2)

x1 = l1*np.sin(theta1_list)

y1 = -l1*np.cos(theta1_list)

x2 = l1*np.sin(theta1_list) + l2*np.sin(theta2_list)

y2 = -l1*np.cos(theta1_list) - l2*np.cos(theta2_list)

return x1,y1,x2,y2

`#Outline system parameters, run double pendulum`

g = 9.8 #m/s^2m1 = 1 #kg

m2 = 1 #kg

l1 = 1 #m

l2 = 1 #m

theta1 = np.radians(90)

theta2 = np.radians(45)

theta1_velocity = 0 #m/s

theta2_velocity = 0 #m/s

theta1_list = [theta1]

theta2_list = [theta2]

time_step = 20/300

time_span = np.linspace(0,20,300)

for t in time_span:

theta1, theta1_velocity = theta1_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step)

theta2, theta2_velocity = theta2_update(m1,m2,l1,l2,theta1,theta2,theta1_velocity,theta2_velocity,g,time_step)

theta1_list.append(theta1)

theta2_list.append(theta2)

x1 = l1*np.sin(theta1_list)

y1 = -l1*np.cos(theta1_list)

x2 = l1*np.sin(theta1_list) + l2*np.sin(theta2_list)

y2 = -l1*np.cos(theta1_list) - l2*np.cos(theta2_list)

`#Make Gif`

!mkdir framescounter=0

photographs = []

for i in vary(0,len(x1)):

plt.determine(figsize = (6,6))

plt.determine(figsize = (6,6))

plt.plot([0,x1[i]],[0,y1[i]], "o-", shade = "b", markersize = 7, linewidth=.7 )

plt.plot([x1[i],x2[i]],[y1[i],y2[i]], "o-", shade = "b", markersize = 7, linewidth=.7 )

plt.title("Double Pendulum")

plt.xlim(-2.1,2.1)

plt.ylim(-2.1,2.1)

plt.savefig("frames/" + str(counter)+ ".png")

photographs.append(imageio.imread("frames/" + str(counter)+ ".png"))

counter += 1

plt.shut()

imageio.mimsave("double_pendulum.gif", photographs)

!rm -r frames

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