Follow 2. Based on code from Keith Peters (www.bit-101.com).

A two-segmented arm follows the cursor position. The relative angle between the segments is calculated with atan2() and the position calculated with sin() and cos().

float[] x = new float[2];
float[] y = new float[2];
float segLength = 50;

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

void draw() {
  background(226);
  dragSegment(0, mouseX, mouseY);
  dragSegment(1, x[0], y[0]);
}

void dragSegment(int i, float xin, float yin) {
  float dx = xin - x[i];
  float dy = yin - y[i];
  float angle = atan2(dy, dx);  
  x[i] = xin - cos(angle) * segLength;
  y[i] = yin - sin(angle) * segLength;
  segment(x[i], y[i], angle);
}

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