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Dan’s gravity expression, creeping object
This is for Dan if he’s around and not on vacation.
I’m using your gravity expression from this earlier post, and I would have swore it worked before, when I first copied it down, but now it’s very weird. The object never stops moving, it just creeps along the floor (x direction), the only way I can get it to stop is by setting the initial x velocity (VxO)to 0. In other words it keeps that initial velocity forever. Am I doing something wrong? I have AE6.5, is this an issue over expression for an older version? Dazed and confused.
Dan Ebberts
Date: Oct 10, 2003 at 12:03:22 am
Subject: Re: expression question with gravity and ‘the ground’Vy0 = 500; //initial y velocity (pixels/second)
Vx0 = 100; // initial x velocity (pixels/second)
g = 2500; // gravity (pixels/second/second)
floor = 400;
e = .85; //elasticityb = floor – position[1];
h = b + Vy0*Vy0/(2*g);
T = Vy0/g + Math.sqrt(2*h/g);if (time < T){ y = Vy0*time - g*time*time/2 + b; }else{ Vy = -(Vy0 - g*T); while (true){ Vy *= e; t = T; T += 2*Vy/g; if (time < T){ t = time - t; y = Vy*t - g*t*t/2; break; }else if (T - t < thisComp.frameDuration){ y = 0; break; } } } [position[0] + Vx0*time, floor - y] PS.I was going to drop this post on the CottonBall question just below, but can't get it to work right, and before I saw he's trying for a 3D.
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4 Replies
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Mike,
I doesn’t look like the expression has any friction or damping for motion in the x direction – as you’ve discover it just keeps moving at a constant velocity. Let me give it some thought.
Dan
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Try changing the last line to this:
[position[0] + Vx0*(-Math.pow(2,-time) +1), floor – y]
You may have to crank up the initial x velocity.
Dan
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In this setup, Vx0 becomes (in pixels) how far the object will move in the x direction. The first parameter of Math.pow() determines how quickly it will get there, with 1.0001 being very slow and 10.0 being very fast (I think).
Dan
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Dan, I don’t understand about the 1st parameter (or any parameter for that matter) of the Math.pow(), but Thanks, for the new toy to play with. I tought I was going crazy there for awhile with my perpetual motion object.
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