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float stiffness = 0.1;
float damping = 0.9;
float velocity = 0.0;
float targetY;
float y;

void setup(){
size(400,400);
noStroke();
}

void draw(){
fill(0,12);
rect(0,0,width,height);
fill(255);
float force = stiffness*(targetY-y);
velocity = damping*(velocity+force);
y += velocity;
rect(10,y,width-20,10);
targetY = mouseY;
}