simple physics

I'm kind of new to flash actionscripting. I want to start to include some physics into my flash. I'm starting out simply with a bouncing ball but my code doesn't work as it should.

Code:

---

onClipEvent(load)
{
        // 1 meter = 1 pixel
        aY = 9.8// Down is positive
        t = 0
        toff = 0 // offset
        vi = 0;
        vf = 0;
}
onClipEvent(enterFrame)
{
  if(this._y >= _root.ground._y)
  {
          trace("Hit Ground")
          toff = getTimer()/100; //Sets offset
          vi = -vf  //Reverse velocity
          this._y = _root.ground._x //Makes sure ball moves  to boundary
  }
  t = getTimer()/100 - toff; //Everytime ball bounces time resets
  this._y += vi*t +.5 * aY * t^2
  vf = vi + aY * t
}

---

Please see the attached .FLA.
Hope this is what you have asked.
Please see more Physics based movies at:
http://geocities.com/achyutveluvali/

Please post your comments

it says unexpected file format when I try to open it

I am able to open the attached .FLA without any problem.
I have used FLASH 8, FLASH PLAYER 8, and Action Script 2.

im only using flash 6

does my code look right?

Quote:

---

Originally Posted by jcafaro10

I'm kind of new to flash actionscripting. I want to start to include some physics into my flash. I'm starting out simply with a bouncing ball but my code doesn't work as it should.

Code:

---

onClipEvent(load)
{
        // 1 meter = 1 pixel
        aY = 9.8// Down is positive
        t = 0
        toff = 0 // offset
        vi = 0;
        vf = 0;
}
onClipEvent(enterFrame)
{
  if(this._y >= _root.ground._y)
  {
          trace("Hit Ground")
          toff = getTimer()/100; //Sets offset
          vi = -vf  //Reverse velocity
          this._y = _root.ground._x //Makes sure ball moves  to boundary
  }
  t = getTimer()/100 - toff; //Everytime ball bounces time resets
  this._y += vi*t +.5 * aY * t^2
  vf = vi + aY * t
}

---

---

typo, the code should say this._y = _root.ground._y //Makes sure ball moves to boundary
this just makes ball stay at the ground though, it won't bounce

http://www.flashkit.com/movies/Scrip...y-18/index.php

hth :)

I actually looked at that before and the code was way different than mine. I feel like my code should work, but it doesn't. I made a modification that I thought would fix it, changed the >= to >, but that didn't work.

I am sorry that you don't have FLASH8 installed. The .FLA attached by me has many extra features such as elasticity of the ball and the ground, air friction, and gravity, all of which can be varied by the user.
Anyhow, I studied your code, and found that it requires some modifications, which I have carried out. Now your code is working well. I am giving you below the modified code.
Note: I have set document size as width: 600 px, and height: 400 px.
__________________________________________________ ______________

onClipEvent(load){
this._x=300;
this._y=100;
yb=this._y;
g = 9.8;
t = 0
vi = 0;
vf = 0;
groundy=350;
}
onClipEvent(enterFrame){
if(this._y > groundy){
trace("Hit Ground")
vi = -vf //Reverse velocity
yb= groundy //Makes sure ball moves to boundary
t=0;
}
this._y = yb+vi*t +0.5 * g* t*t;
vf = vi + g*t;
t+=0.1;
}

I am glad that you have chosen to use physics formulas:
v=u+a.t
s=u.t+0.5.g.t.t
I have developed another throw ball movie using the above formulas which you can view at:
http://www.flashkit.com/movies/Scrip...9323/index.php

im sorry but I don't quite understand the difference between your mods and the original code. It looks like you just gave things different variable names and bumped up time by .1s each frame...i changed some things around (it still doesn't work)

Frame 1:

Code:

---

_global.g = 9.8;
_global.ground = ground._y;

stop();

---

Ball1:

Code:

---

onClipEvent(load)
{
        aY = g;
        t = 0;// time
        toff = 0;// offset for time
        v0 = 0;// initial velocity
        v = 0;// current velocity
}
onClipEvent(enterFrame)
{
  if(this._y > ground)
  {
          toff = getTimer()/100; //Sets offset
          v0 = -v;  //Reverse velocity
          this._y = ground; //Makes sure ball moves to boundary
  }

  t = getTimer()/100 - toff; //Everytime ball bounces time resets
  this._y += v0*t + 1/2 * aY * t^2;
  v = v0 + aY * t;
}

---

Your code as modified by me is working fine. (Set frame rate to 12 fps).
Your code is not working mainly because you are using getTimer() function
to increment time, whereas I have substituted it with t+=0.1;
I can't suggest any further solution to this problem.

what's wrong with using get timer?

I don't think its the getTimer() that's messing the code up because I just got rid of it and used the t+=.1 and I get the same affect, just a little slower. I think the problem is with my use of this._y. In you're code I see that sometimes you refer to "yb" instead of this._y and while I don't see why this should make a difference, it seems to. With yB in place, it behaves better, but the ball still bounces higher than it should and keeps going higher after each bounce

this._y, I think refers to dynamic state of the instance, where as yb refers to ground._y, which is very definite and specific.

I understand but setting this._y to equal ground and then bumping up this._y by its own value (ground) + physics equation should be the same as setting yB to equal ground and then bumping up this._y by yB (ground) + physics equation. Even when I do copy your code exactly, it still doesn't work correctly. After a few bounces it becomes obvious that the ball is getting higher each time.

getTimer() will give you a random number I'd rather use something that produces a more constant number . like setInterval or just an integer like you said t+=.1 is good

Yes, this code has the problem of the ball bouncing to higher height with each bounce.
I have found another code in my library, which seems to be not having this problem! But the code does NOT directly use the physics equations! :)
__________________________________________________ ____________
onClipEvent(load){
velocity = 0;
gravity = 1;
floory=350;
}
onClipEvent(enterFrame){
velocity += gravity;
_y += velocity;
if(_y>floory){
_y = floory;
velocity=-velocity;
}
}

that code works, I'm just not comfortable using it because I want to use actual physics equations and I'm a little uneasy about the fact that they won't work

The problem may not be with the formulas used or the code.
The problem I think is with the iterations, during which the ball bouncing height slightly increases with each bounce.
Hope some FLASH expert can give a better explanation, and suggest a solution.

yes that would be nice, any experts out there?

i have this working in a 3 frame movie, running at 30FPS
movieclip instance - ball - across all 3 frames
button instance - start_btn - across all 3 frames
button instance - reset_btn - across all 3 frames

Code:

---

// frame#1 -
var t:Number = 0;
var d:Number = 0.6;
var g:Number = 9.8; // Acceleration of gravity
var e:Number = 0.1; // Energy absorption
var y:Number = ball._y;
var gr:Number = 300; // ground

btn_start.onRelease = function() {
gotoAndPlay(2);
};

btn_reset.onRelease = function() {
y = ball._y = 70;
gotoAndStop(1);
};

stop();

---

Code:

---

// frame#2 -
ball._y = Math.min(y+(g/2)*t*t, gr+2);
    if (ball._y >= gr) {
      d = -d;
      y += e*(gr+2-y);
      t = Math.sqrt((gr-y)*2/g);
      if (y >= gr) {
        gotoAndStop(1);
      }
    }

---

Code:

---

// frame#3 -
t += d;
gotoAndPlay(2);

---

can you make use of the logic and apply to your onClipEvent method ?

mx2004 fla

I'm not really understanding what's happening. You're setting the ball's position to be the minimum of what it would be according to free fall or the height of the ground + 2. That makes sense except for the +2. Then if the ball hits the ground you're setting d=-d, i'm assuming d's the velocity, and y becomes it self +some energy times what you have. I don't understand what you're doing with time with sqrts or adding d. If you could, could you look at my code and see what I'm doing thats not right?

Quote:

---

// frame#2 -
ball._y = Math.min(y+(g/2)*t*t, gr+2);
if (ball._y >= gr) {
d = -d;
y += e*(gr+2-y);
t = Math.sqrt((gr-y)*2/g);
if (y >= gr) {
gotoAndStop(1);
}
}

---

I'm going to attach my .fla so you guys can see what my code does. Also note that theres a problem where, if I place the ground at (0,460) ground._y will return 475 for some reason I think. See for yourself

Hey! This code seems to be working OK!
//__________________________________________________ _________
onClipEvent(load)
{
aY = 9.8;
t = 0;
v0 = 0;
v = 0;
ground=350;
yb=40;
yt=40;
this._y=yb;

}
onClipEvent(enterFrame)
{
trace(this._y);
if(this._y<yt){//Stops the ball moving to a height <yt.
yb=yt;
t=0;
v0=0;
}

if(this._y > ground)
{
t=0;
v0 = -v;
yb = ground;
}

this._y = yb+v0*t + 1/2 * aY * t*t;
v = v0 + aY * t;
t+=0.1;

}

Thanks, it works but its sloppy. The ball is forced to stop so it doesn't look real. Also, I'm still not sure why its telling me the ground is at 475 when its not.

In the .FLA you have the ground._y according to the property box as 475.3
Please check this up.

When I click on the movieclip which is the ground, the property box shows Y: 460.0. I don't see 475.3 anywhere except for the console window that pops up

In scene 1, Layer 1, world
Properties:
W:500.0 X: 235.6
H: 40.0 Y: 475.3
__________________________
I have again checked up.
The above data is indeed shown!

movieclip - ground is at y:460 on stage
but if you open the clip, the centre point is set at - y:-15.3

460 + 15.3 = 475.3

sloppy huh :D

set the clip at zero and use -

_global.floor = ground._y - ground._height;
trace(floor); // 420
trace(ground._y) // 460

why would I want the floor to be at 420 if the top of the ground is at 460?

ps, thanks for catching that, I forgot about where the shape coordinates are

pps, im starting to not like physics!

I think i see where the problem lies. If you watch the animation, the ball doesn't hit the ground at the same spot everytime so therefore there will be times when it will fall a greater distance than others. I'm not exactly sure though how to correct that. The ball is accelerating so its distance is not linearly increasing

Quote:

---

Originally Posted by jcafaro10

Thanks, it works but it’s sloppy. The ball is forced to stop so it doesn't look real. Also, I'm still not sure why its telling me the ground is at 475 when its not.

---

This is my analysis:
When we make a program combining physics and animation I think, we should remember that during animation the movement is controlled by iterations of time (t), which gets added or subtracted during each iteration. Since the computer uses binary method of calculation, this sometimes leads to errors due to lose of significant digits.
_______________________________
//See the following example:
var a=1.23456E20;
var b=1E5;
var c;
c=a-b;
trace(c);
_______________________________
The trace statement shows 1.23456E20, which is obviously not correct!
This has happened because the computer stores only about 15 significant digits!
Anyhow this is what I think. Let other computer experts offer their comments.
Don't hate physics. Understand how the binary system works.
To overcome this problem to some extent, we have to fix the boundary conditions for the ball at the top, and bottom positions.

ah ok well how do we do this