# Chain.

Chain.

One mass is attached to the mouse position and the other is attached the position of the other mass. The gravity in the environment pulls down on both.

```

Spring2D s1, s2;

float gravity = 9.0;
float mass = 2.0;

void setup() {
size(640, 360);
fill(255, 126);
// Inputs: x, y, mass, gravity
s1 = new Spring2D(0.0, width/2, mass, gravity);
s2 = new Spring2D(0.0, width/2, mass, gravity);
}

void draw() {
background(0);
s1.update(mouseX, mouseY);
s1.display(mouseX, mouseY);
s2.update(s1.x, s1.y);
s2.display(s1.x, s1.y);
}

class Spring2D {
float vx, vy; // The x- and y-axis velocities
float x, y; // The x- and y-coordinates
float gravity;
float mass;
float radius = 30;
float stiffness = 0.2;
float damping = 0.7;

Spring2D(float xpos, float ypos, float m, float g) {
x = xpos;
y = ypos;
mass = m;
gravity = g;
}

void update(float targetX, float targetY) {
float forceX = (targetX - x) * stiffness;
float ax = forceX / mass;
vx = damping * (vx + ax);
x += vx;
float forceY = (targetY - y) * stiffness;
forceY += gravity;
float ay = forceY / mass;
vy = damping * (vy + ay);
y += vy;
}

void display(float nx, float ny) {
noStroke();
ellipse(x, y, radius*2, radius*2);
stroke(255);
line(x, y, nx, ny);
}
}
```

## Functions Used

fill()

Sets the color used to fill shapes

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stroke()

Sets the color used to draw lines and borders around shapes

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size()

Defines the dimension of the display window width and height in units of pixels

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ellipse()

Draws an ellipse (oval) to the screen

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setup()

The setup() function is run once, when the program starts

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draw()

Called directly after setup(), the draw() function continuously executes the lines of code contained inside its block until the program is stopped or noLoop() is called

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noStroke()

Disables drawing the stroke (outline)

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line()

Draws a line (a direct path between two points) to the screen

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background()

The background() function sets the color used for the background of the Processing window

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