Inertia with a car

ok i’ve spent lots of hours looking up the internet trying many techniques but nothing do. i just want to add inertia to my car. i know there are tutorials and i did try to adapt them but still nothing if someone could help me that would be really great thanks

var car:Car = new Car();
var power = 10;
addChild(car);
car.x = 470;
car.y = 200;
var dx = 0;
var dy = 0;
stage.addEventListener(KeyboardEvent.KEY_DOWN, keyhandler);
function keyhandler(event:KeyboardEvent):void
{
switch(event.keyCode)
{
case Keyboard.LEFT :
car.x -= power * force.x;
break;

 case Keyboard.RIGHT : 
car.x += power;

break;

case Keyboard.UP :
car.y -= power;
break;

case Keyboard.DOWN :
car.y += power;
break; }
}