var avel = 0;

var aacc = 0.001;

var slider;

function setup() { 

  createCanvas(400, 400);

  slider = createSlider(-0.01,0.01,0,0.001);

} 

​

function draw() { 

  background(220);

  translate(width/2,height/2);

  rectMode(CENTER);

  rotate(angle);

  rect(0,0,100,20);

  aacc = slider.value();

  angle += avel;

  avel += aacc;

  avel = constrain(avel,-0.1,0.1);

}