I try to create reverse gravity, and I have an issue after I click several times on the button "physics" : it is supposed to restart with the default values, but after 10 times, the balls go too fast and I don't understand why. The function works fine the first time, but then it starts to accelerate each time you click on "physics".

https://codepen.io/Arthur222/pen/QWOgMXq

let isOn = 0;
let canvas = document.getElementById("canvas");
if (canvas.getContext('2d') == null ) alert("erreur javascript");
let ctx = canvas.getContext('2d');
let velocity=1, force=1, mass = 1, radius=30;
let lastTs;

//FCN


window.addEventListener('load', ()=>{
    resize();
    window.addEventListener('resize', resize);
    createBalls();
    drawBalls();
});

// PHYSICS


let balls = [];
function createBalls(){
  for ( let i=0; i<6; i++){
    let x = radius+(i*radius*2);
    if (i>0) x = radius+(i*(radius+10)*2);
    let vel = Math.random();
    let ball = {x:x, y:250, xBase:x, yBase:250, velocity:vel, velocityBase:vel, radius:30, active:true}
    balls.push(ball);
  }
}


function startPhy1(){

  if (isOn == 1){
    console.log("canvas.width, canvas.height : "+canvas.width, canvas.height);
    ctx.clearRect(0,0,canvas.width, canvas.height);
    balls.forEach(element => {
      element.y = element.yBase;
      element.active = true;
      element.velocity = element.velocityBase;
    });
  }

  isOn = 1;
  lastTs = getTimestamp();
  runPhy();
  draw();
} 

function runPhy(){
  let t = (getTimestamp() - lastTs)/1000;

  balls.forEach(element => {
    if ( element.active ){ //still below
      element.y -= element.velocity * t;
      element.velocity += ( force / mass ) * t;
      if ( element.y <= element.radius+10 ){//already on top
        element.active = false;
        element.y = element.radius;//stay on top
      }
    }
  });
  requestAnimationFrame(runPhy);
  
}

function getTimestamp() {
  return new Date().getTime();
}
// DRAW 

function draw(){
  drawBalls();
  requestAnimationFrame(draw);
}

function drawBalls(){
  ctx.clearRect(0,0,canvas.width, canvas.height);
  for (let i=0; i< balls.length; i++){

    
    ctx.beginPath();
    ctx.arc(balls[i].x, balls[i].y, balls[i].radius, 0, 2 * Math.PI, false);
    ctx.strokeStyle = "green";
    ctx.lineWidth = 2;
    ctx.stroke();
  }
  
}


function resize(){
  ctx.canvas.width = window.innerWidth*0.7;
  ctx.canvas.height = window.innerHeight*0.9;
}