//

// V0.1    2020-04-17    NPGS

//

​

const PIX = 500;

const PIX_P_M = 200;

const BOB_DIAM = 20;

​

const L = 1;

const THETA = 0.7;        

const THETA_DOT = +6.15;

const STOKES_DRAG = 0.05;

​

const G = 9.80665;

const TSTEP = 0.033;        // Reciprocal of frame rate

​

function setup() {

  createCanvas(PIX, PIX);

  fill(0, 20, 200);

  stroke(150);

  background(225);

  frameRate(30);

​

  theta = THETA;

  theta_dot = THETA_DOT;

  theta_dotdot = 0;

}

​

function draw() {

  background(225);

​

  x = PIX_P_M * L * sin(theta);

  y = PIX_P_M * L * cos(theta);

  line(PIX / 2, PIX / 2, PIX / 2 + x, PIX / 2 + y);

  circle(PIX / 2 + x, PIX / 2 + y, BOB_DIAM);

​

  theta_dotdot = -G * sin(theta) / L - STOKES_DRAG * L * theta_dot;

  theta_dot = theta_dot + theta_dotdot * TSTEP;

  theta = theta + theta_dot * TSTEP;

}