## 02ps-02 Symplectic Euler Solution

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Let's go over the solution to this problem starting with the definition of the acceleration function.
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We know the acceleration due to gravity points downward.
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So let's put this factor into two components.
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We have one component right here that is parallel to the string of the pendulum
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and another component that is perpendicular to this green one.
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Now we know that the acceleration in this direction is going to be exactly cancelled out
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by the acceleration due to the tension in the rope.
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So that means that the acceleration we're looking for is really just this pink component,
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which any point along the path is going to be tangent to the trajectory.
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Now if we call this angle θ right here, we can figure out the length of this pink component
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by just saying that it is equal to length of the resultant vector times the sine of θ.
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The θ down here is actually exactly equal to θ in the diagram of the pendulum itself.
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So that means to figure out the length of this component,
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Since position is just the arc link right here of this imaginary circle,
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then the measure of that angle in radiant is going to be equal to the length of the arc
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that it corresponds to divided by the radius of that circle.
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So that means that in our case, θ is equal to arc length over radius or position over length.
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So since θ equals position over length and we want the sine of θ,
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we fill in our definition for acceleration as -g or negative
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magnitude of the acceleration due to gravity times sine of the position over length.
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Okay, moving on towards symplectic Euler function.
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We have to fill in this for loop with the input num_initial conditions.
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As I said in the InterVideo of the problem, we wanted the initial x to vary from 1.75 to 2.25
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and the initial v to vary from -2 to 2
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corresponding to the coordinates of every point along that green circle that I'd showed you.
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Now a convenient way to make a variable cycle through values that are symmetric
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about an equilibrium value is to use sine or cosine.
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So we're going to keep that in mind.
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Now as you learn from the circular orbit problem of Unit 1, if we consider any point
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along the circumference of the circle, then we can define an angle that corresponds to that point.
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These are coming from right here as a zero radian mark.
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You can write the coordinates of this point then as the radius of the circle times the cosine of the angle.
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That's for the horizontal component and for the vertical component,
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we get the radius times the sine of the angle.
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In the phase based plot that I showed you in the InterVideo, we saw that position lying along
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the horizontal axis and velocity lying along the vertical axis.
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So we wanted to plot the coordinates of the points on that green circle--the initial condition circle
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where the position is going to correspond to cosine and the velocity is going to use sine.
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Now I created a variable called phi. You could pick any name you want I guess.
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And phi effectively split the circle into 49 segments
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by marking out 50 different points along the circumference.
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So every time I increases by one, we're going to step to the next point along the circumference.
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Since as we saw in the phase base plot, we have a complete circle of green points.
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The x values of those green points vary like this with 2 as the middle value
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and the v coordinates vary like that.
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You noticed that the amplitude in either case corresponds to the
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half link of the green shape in that direction.
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So actually we have in a phase base plot is an ellipse for that set of initial conditions.
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Now that we have our starting additions figured out, we can finally use the symplectic Euler method
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to proximate the values with x and v at later sets.
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This code right here is just a direct transition pretty much of the equations that I showed you earlier.
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Now let's go back to looking at the plot that we get things plugged in
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but first let's look at our top two plots.
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The horizontal axis in both of them represents time measured in seconds.
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The vertical axis in the top one is x measured in meters
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and here it is v measured in meters per second.
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So you can see that our initial values of x go from 1.5 to 2.25 and v from -2 to 2.
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So that corresponds to this green ellipse right here.
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phase base is that if we look closely at each one of these ellipses they all have the same area.
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Now let's look at the shapes that we have down here in this bottom graph.
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If you look closely and do a bit of calculating, you'll notice that all these different color shapes
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have the same exact area even though they are well shaped very differently.
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This is a great example of how phase base is conserve in the system where energy is conserve.
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Now the fact that its conservation principle holds in this diagram
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shows how the symplectic Euler method improve upon the accuracy of the forward Euler method.
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When the forward Euler method is used, it often result in the energy suddenly increasing.
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So it means that the area of each of these shapes down here will get progressively bigger.
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Symplectic Euler method, however, confirms much better the equations of motion in physics,
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It never reflects exactly radical predictions more accurately. Great job with the first problem in Unit 2.
Title:
02ps-02 Symplectic Euler Solution
Team:
Udacity
Project:
CS222 - Differential Equations
Duration:
04:48
 Amara Bot edited English subtitles for 02ps-02 Symplectic Euler Solution Gundega added a translation

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