// Daniel Shiffman

// http://natureofcode.com

​

// Pendulum

​

// A Simple Pendulum Class

// Includes functionality for user can click and drag the pendulum

​

// This constructor could be improved to allow a greater variety of pendulums

class Pendulum {

​

  constructor(x, y, r) {

    // Fill all variables

    this.origin = createVector(x, y);

    this.position = createVector();

    this.r = r;

    this.angle = PI / 4;

​

    this.aVelocity = 0.0;

    this.aAcceleration = 0.0;

    this.damping = 0.995; // Arbitrary damping

    this.ballr = 48.0; // Arbitrary ball radius

​

    this.dragging = false;

  }

​

  // Function to update position

  update() {

    // As long as we aren't dragging the pendulum, let it swing!

    if (!this.dragging) {

      let gravity = 0.4; // Arbitrary constant

      this.aAcceleration = (-1 * gravity / this.r) * sin(this.angle); // Calculate acceleration (see: http://www.myphysicslab.com/pendulum1.html)

      this.aVelocity += this.aAcceleration; // Increment velocity

      this.aVelocity *= this.damping; // Arbitrary damping

      this.angle += this.aVelocity; // Increment angle

    }

  }

​

  display() {

    this.position.set(this.r * sin(this.angle), this.r * cos(this.angle), 0); // Polar to cartesian conversion

    this.position.add(this.origin); // Make sure the position is relative to the pendulum's origin

​

    stroke(0);

    strokeWeight(2);

    // Draw the arm

    line(this.origin.x, this.origin.y, this.position.x, this.position.y);

    fill(127);

    if (this.dragging) fill(200);

    // Draw the ball

    ellipse(this.position.x, this.position.y, this.ballr, this.ballr);

  }

​

​

  // The methods below are for mouse interaction

​

  // This checks to see if we clicked on the pendulum ball

  clicked(mx, my) {

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

    if (d < this.ballr) {

      this.dragging = true;

    }

  }

​

  // This tells us we are not longer clicking on the ball

  stopDragging() {

    this.aVelocity = 0; // No velocity once you let go

    this.dragging = false;

  }

​

  drag() {

    // If we are draging the ball, we calculate the angle between the

    // pendulum origin and mouse position

    // we assign that angle to the pendulum

    if (this.dragging) {

      let diff = p5.Vector.sub(this.origin, createVector(mouseX, mouseY)); // Difference between 2 points

      this.angle = atan2(-1 * diff.y, diff.x) - radians(90); // Angle relative to vertical axis

    }

  }

}