​

class Bob {

​

  constructor(x, y) {

    this.position = createVector(x, y);

    this.velocity = createVector();

    this.acceleration = createVector();

    this.mass = 24;

    // Arbitrary damping to simulate friction / drag

    this.damping = 0.98;

    // For user interaction

    this.dragOffset = createVector();

    this.dragging = false;

  }

​

  // Standard Euler integration

  update() {

    this.velocity.add(this.acceleration);

    this.velocity.mult(this.damping);

    this.position.add(this.velocity);

    this.acceleration.mult(0);

  }

​

  // Newton's law: F = M * A

  applyForce(force) {

    let f = force.copy();

    f.div(this.mass);

    this.acceleration.add(f);

  }

​

  // Draw the bob

  display() {

    stroke(255);

    strokeWeight(2);

    fill(127);

    if (this.dragging) {

      fill(200);

    }

    ellipse(this.position.x, this.position.y, this.mass * 2, this.mass * 2);

  }

​

  handleClick(mx, my) {

    let d = dist(mx, my, this.position.x, this.position.y);

    if (d < this.mass) {

      this.dragging = true;

      this.dragOffset.x = this.position.x - mx;

      this.dragOffset.y = this.position.y - my;

    }

  }

​

  stopDragging() {

    this.dragging = false;

  }

​

  handleDrag(mx, my) {

    if (this.dragging) {

      this.position.x = mx + this.dragOffset.x;

      this.position.y = my + this.dragOffset.y;

    }

  }

}