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Simulating Gravity (Or Any Attractive Force)
Author: Darren Hundt

Page 4
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Step 3

Next I added the following code to the sun, ball1.

onClipEvent (load) {
 xvel=.0;
 yvel=.05;
}
onClipEvent (enterFrame) {
 this.xvel+=xa*.05;
 this.yvel+=ya*.05;
 this._x+=xvel;
 this._y+=yvel;
 if (this._x>400 || this._x<0) {
 xvel*=-1;
 }
 if (this._y>300 || this._y<0) {
 yvel*=-1;
 }
 distanceobject=this.distance(_root.ball2);
 r=distanceobject.distance;
 theta=Math.atan(distanceobject.disty/distanceobject.distx);
 a=240/r/r;
 xa=Math.abs(Math.cos(theta))*a*distanceobject.directionx;
 ya=Math.abs(Math.sin(theta))*a*distanceobject.directiony;
}

This code does the following:

Sets the x velocity and y velocity to initial values.
On enterFrame adds the x component of acceleration to the x velocity and does the same with y (this.xvel+=xa*.05) I will explain the *.05 later.
Adds the velocity values to the position values of this clip (this._x+=xvel).
Checks boundary conditions to see if we are flying off the stage.
Then the real math!

First I called the distance formula on ball1 with ball2 as the argument.
I set r=the distance between the two balls.
I found the angle of the distance between the two balls.
I set the accleration to 240(a constant) divided by r^2(since gravity is inveresly proportional to distance)
I found the x and y components of the acceleration by using cos and sin of the angle between the balls and multiplying it by directionx or direction y to give the acceleration the right sign(+/-).

Then I added this code to ball2

onClipEvent (load) {
 xvel=0;
 yvel=-1;
}
onClipEvent (enterFrame) {
 this.xvel-=_root.ball1.xa;
 this.yvel-=_root.ball1.ya;
 //this.xvel*=.99;
 //this.yvel*=.99;
 this._x+=xvel;
 this._y+=yvel;
 if (this._x>400 || this._x<27) {
 xvel*=-1;
 }
 if (this._y>300 || this._y<27) {
 yvel*=-1;
 }
}

This is pretty much the same as ball2 except we subtract the acceleration from velocity instead of adding it
I have the comments this.xvel*=.99; in case you want to add friction to the equation.

Finally, the reason I multiplied the sun's acceleration by .05 is because it is more massive than the earth and should be affected less by the gravity. In reality, instead of .05 we would use a much smaller number so that the suns motion would be imperceptible.
You can play around with this value, the xvel and yvel in the onClipEven (load) statements, and the 240 in the code that determines acceleration and see what happens!

This is my first tutorial and I am not sure if I am doing this right. Any comments would be appreciated. Hopefully I will have a tute on gravity with more than 2 objects soon.

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» Level Intermediate

Added: : 2002-04-11 Rating: 6.27 Votes: 80 Hits: 3350

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Engineering student who lieks flash.

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