//SOLVE: Using basic physics

## Apply basic kinematics to make your games feel more natural

Kinematics is the study of motion, and is a branch of physics. We can use some of the equations from physics to make movement seem more natural, as if gravity were affecting it.

In our example below we are going to use two equations:

• v = at + vi
This describes how velocity is affected by acceleration, in this case, due to gravity.
• d = vt + di
This describes how distance is affected by speed.

In the examples above:

• a = acceleration
• v = velocity
• vi = initial velocity
• d = distance
• di = initial distance
• t = time

One things that we have to remember in that the Y axis (top) increases as it goes down, so we have to switch the math around.

We have to abandon jQuery-based animation for physics-based animation. We will set up a frame rate of about 30 frames per second. This means that we will have to create a repeating function with an interval of 1000/30 = 33.33 millseconds.

Inside this repeating function we will recalculate the current speed and position, and then shift the coordinates to the new place.

Look at the code to see where these calculations take place inside `flappy.calculatePosition()`.

## Source code

``````<!DOCTYPE html>
<html>
<meta charset="utf-8">
<title>Basic game physics: flappy tree</title>
<script>
// flappy OBJECT
// flappy properties:
//      flappy.element
//      flappy.fps
//
//      flappy.currentSpeedTop
//      flappy.currentSpeedLeft
//      flappy.gravity
//      flappy.speedBoostPerJump
//
//      flappy.top
//      flappy.left
//      flappy.scrollOffset
//
// flappy methods
//      flappy.initialize()
//      flappy.jumpUp()
//      flappy.calculatePosition()
//      flappy.move()
//
var flappy = {
element:            null,       // the element to animate
fps:                30,         // the frames per second to animate

currentSpeedTop:    null,       // the current vertical speed
currentSpeedLeft:   120,            // the current horizontal speed
gravity:            60,         // the downward acceleration due to gravity per second
speedBoostPerJump:  80,         // the amount of speed added per jump

top:                null,       // current top coordinate
left:               null,       // current left coordinate
scrollOffset:       -200,       // the position of the flappy from the side of the screen while scrolling

// INPUT: get the element containing the flappy image and get the current top and left coordinates
initialize: function ( id ) {
console.log( 'initializing...' );

this.element        = document.getElementById( id );
this.top            = this.element.offsetTop;
this.left           = this.element.offsetLeft;

// OUTPUT: scroll to flappy's initial position
window.scrollTo( this.left + this.scrollOffset, this.top );
},

// OUTPUT: give a boost to the vertical speed
jumpUp:   function () {
this.currentSpeedTop    = this.currentSpeedTop - this.speedBoostPerJump;
},

// PROCESS: use physics math to calculate the current position...
calculatePosition:  function () {
// calculate the new vertical speed using: v = at + vi
this.currentSpeedTop    = this.gravity * ( 1 / this.fps ) + this.currentSpeedTop;

// calculate the new positions using: d = vt + di
this.top                = this.currentSpeedTop * ( 1 / this.fps ) + this.top;
this.left               = this.currentSpeedLeft * ( 1 / this.fps ) + this.left;
},

//OUTPUT: display flappy in his current position
move: function () {
this.element.style.left = this.left + "px";
this.element.style.top  = this.top + "px";

window.scrollTo( this.left + this.scrollOffset, this.top );
}
}

// triggered by start button
var mainProcedure = function () {
// turn off the button
// otherwise each space bar hit will trigger the button again...
document.getElementById('buttonID').onclick = null;

// set up the animation loop
var interval = window.setInterval(
function () {
flappy.calculatePosition();     // calculate the new position
flappy.move();                  // move there
},
1000 / flappy.fps                   // every 33 milliseconds
);
}

// if a key is pressed down...
window.onkeydown = function ( keyEvent ) {
// INPUT: read the key that was pressed...
var keyCode = keyEvent.which || keyEvent.keyCode;

if ( keyCode == 32 ) {  // if space bar is pressed
flappy.jumpUp();    // add a boost up
}
}
</script>
<style>
h2, p {
position:   absolute;
}
#flappyID {
position:   absolute;
width:      50px;
top:        50%;
}
#backgroundID {
position:   absolute;
height:     93vh;   /* set height to part of the viewport height to deal with scroll bars */
}
</style>