processing4/java/examples/Topics/Simulate/ForcesWithVectors/Liquid.pde

/**
 * Forces (Gravity and Fluid Resistence) with Vectors 
 * by Daniel Shiffman.  
 * 
 * Demonstration of multiple force acting on bodies (Mover class)
 * Bodies experience gravity continuously
 * Bodies experience fluid resistance when in "water"
 */
 
 // Liquid class 
 class Liquid {

  
  // Liquid is a rectangle
  float x,y,w,h;
  // Coefficient of drag
  float c;

  Liquid(float x_, float y_, float w_, float h_, float c_) {
    x = x_;
    y = y_;
    w = w_;
    h = h_;
    c = c_;
  }
  
  // Is the Mover in the Liquid?
  boolean contains(Mover m) {
    PVector l = m.location;
    if (l.x > x && l.x < x + w && l.y > y && l.y < y + h) {
      return true;
    }  
    else {
      return false;
    }
  }
  
  // Calculate drag force
  PVector drag(Mover m) {
    // Magnitude is coefficient * speed squared
    float speed = m.velocity.mag();
    float dragMagnitude = c * speed * speed;

    // Direction is inverse of velocity
    PVector drag = m.velocity.get();
    drag.mult(-1);
    
    // Scale according to magnitude
    drag.setMag(dragMagnitude);
    return drag;
  }
  
  void display() {
    noStroke();
    fill(127);
    rect(x,y,w,h);
  }

}