Reach 1. Based on code from Keith Peters (www.bit-101.com)

The arm follows the position of the mouse by calculating the angles with atan2().

 
float x = 100;
float y = 100;
float x2 = 100;
float y2 = 100;
float segLength = 30;

void setup() {
  size(200, 200);
  smooth(); 
  strokeWeight(20.0);
  stroke(0, 100);
}

void draw() {
  background(226);
  
  float dx = mouseX - x;
  float dy = mouseY - y;
  float angle1 = atan2(dy, dx);  
  
  float tx = mouseX - cos(angle1) * segLength;
  float ty = mouseY - sin(angle1) * segLength;
  dx = tx - x2;
  dy = ty - y2;
  float angle2 = atan2(dy, dx);  
  x = x2 + cos(angle2) * segLength;
  y = y2 + sin(angle2) * segLength;
  
  segment(x, y, angle1); 
  segment(x2, y2, angle2); 
}

void segment(float x, float y, float a) {
  pushMatrix();
  translate(x, y);
  rotate(a);
  line(0, 0, segLength, 0);
  popMatrix();
}