Re: [pygame] Why does my ball vibrate?
Luke Paireepinart wrote: I don't pretend to understand the implications of this discussion, but it sounds to me like, at the speed of light, frequency is analogous to mass at lower speeds? Not really. The relationship between momentum and wavelength applies at all speeds, and to all particles, both massive and massless. Electrons have a wavelength too, related to their momentum in the same way as photons. Fast-moving electrons have a short wavelength, slow-moving ones have a long wavelength. Pass them through a diffraction grating and you get interference patterns. Sounds weird, but it really happens. In quantum mechanics, there's really very little difference between massive and massless particles. Also, the word particle doesn't exactly mean what it sounds like it means. Rather than a little ball, it's more like a mode of vibration in a string or a drumhead. As a vibration, it has a frequency, and there is energy associated with that vibration. The higher the frequency, the greater the energy. (Like all things in quantum mechanics, you shouldn't push that analogy too far. But the mathematics involved are very reminiscent of what you get from dealing with waves in a vibrating medium.) -- Greg
Re: [pygame] Why does my ball vibrate?
Ian Mallett wrote: On Dec 5, 2007 4:03 PM, Greg Ewing [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: Actually, it does -- a photon is an example of an object with no mass. Such an object always travels at the speed of light -- it doesn't even need a push to get it going. It's fundamentally incapable of standing still. Heh heh. Try hitting that with a paddle. While it has no mass, it does have both energy and momentum, both of which are proportional to its frequency. Momentum is defined as mass*velocity. If mass is zero, how does a photon have momentum? When a physicists say that a photon has zero mass, what they generally mean is that photons have zero /rest mass/ (a rather hypothetical notion since a photon can't be at rest). A photon that is moving (the only kind of photon there is) has a mass of *h/c**?*. Since particles have ever increasing mass as their velocity increases, approaching infinity as velocity approaches *c*. You can loosely imagine a photon as having it's rest mass multiplied by infinity, which would be m = 0*?, which doesn't help us much. The actual equations are: Energy of a photon is inversely proportional to wavelength* **?*, thats: *E = hc/?*, where *h* is Planck's constant We also have *E = mc^2*, so dividing both sides by *c^2*, we get mass: *m = h/c**?* Momentum is *p = mv = hv/**c**? = h/? *(because *v=c* since it is a photon) These are conserved in any collision, so when it bounces off a wall, the wall gains some momentum, just as it would if a massive particle with the same momentum bounced off it. And if the wall starts to move as a result, then it has also gained some energy, which must have come from the photon, so the reflected photon must be red-shifted slightly (longer wavelength = lower frequency = less energy). All this is true, but /how/ exactly does a massless particle have momentum? -- Greg Ian
Re: [pygame] Why does my ball vibrate?
On 12/4/07, Greg Ewing [EMAIL PROTECTED] wrote: That's true. Probably not something you need to take into account in a Pong game, but if you're simulating space flight or something like that, you certainly do! I always wondered how the ball in Pong could bounce indefinitely, and not accelerate when traveling downwards.
Re: [pygame] Why does my ball vibrate?
On Dec 5, 2007 9:59 AM, Ian Mallett [EMAIL PROTECTED] wrote: On 12/4/07, Greg Ewing [EMAIL PROTECTED] wrote: That's true. Probably not something you need to take into account in a Pong game, but if you're simulating space flight or something like that, you certainly do! I always wondered how the ball in Pong could bounce indefinitely, and not accelerate when traveling downwards. Well pong is viewed from above, so downward acceleration makes no sense. (ping pong) As for bouncing indefinitely, pong takes place in a magical universe where no energy is ever lost :) It's funny how you don't ever have to worry about going over the net. Why they didn't just name it hock (air hockey) we'll never know.
Re: [pygame] Why does my ball vibrate?
On 12/5/07, Patrick Mullen [EMAIL PROTECTED] wrote: Well pong is viewed from above, so downward acceleration makes no sense. (ping pong) So it would be a case where the bounce is sideways, like hockey or soccer. As for bouncing indefinitely, pong takes place in a magical universe where no energy is ever lost :) And now, for your random amusement: Well, some energy must be lost from the ball, and transferred into the wall, (so the walls should move as well as the ball). That way, the energy would be temporarily lost to the ball, but then given back to it when it hits the other wall, which is moving towards it. Actually, that's sort of what happens. The energy of the ball goes into the Earth, accelerating it VERY slightly. If collisions were totally elastic, and friction was zero, the ball would bounce forever. Either that or the ball must be infinitely light, in which case, it would be accelerated to the speed of light by a touch. However, as long as the object it comes into contact with remains motionless, the bounce will just be a bounce, and not a speed change. (Velocity changes because the direction does). On the other hand, since the ball is massless, the ball's velocity can be changed instantaneously. Because the contact time is nonexistent, the speed of the paddle per unit of time is irrelevant, so it would always bounce and never accelerate. Even friction would have no effect. (Air friction, caused by air molecules, for example, would cause the ball to bounce rapidly between the atoms- no energy would even be lost as heat, or would it?). Note that these two cases only work if the ball has a set speed to begin with. If it was at rest, a single atom of gas would accelerate it to light speed. If it was going at light speed, there is no way to stop it because it would retain the speed of any collision. If it hit another atom of gas, it could not transfer energy because it does not have any mass to carry the energy. Or would it? Physics says that the same force is required to stop a particle as took to accelerate it. So identical atoms at identical speeds could both stop and start the ball? However, Physics, as far as I know, does not say what happens with a particle of no mass. Now, according to Wikipedia, the mass of a photon is zero. Yet we know that photons carry energy (shine this: http://www.youtube.com/watch?v=vXDYCGVEMac into your face (don't)). Since energy is mass (the conversion factor is the speed of light squared), photons must have effective mass. Yet they don't! Somehow, they can obviously be created and accelerated. So, there are your magic particles, but we don't often see them going slower than the speed of light, not to mention small, so they're useless for Pong. Of course, there is a paradox in all this, that the ball can retain kinetic energy, yet have no mass. It could be viewed that, by Physics, it could not impart energy with no mass, because energy transfer is proportional to the mass and speed of an object. But photons accelerate, at least, so ??? Incidentally, ask someone what (1/infinity)*(infinity) is. I want to know. Best not to analyse it too much. :-) Ian
Re: [pygame] Why does my ball vibrate?
Michael George wrote: I've thought of writing a game with physics operating at the quantum scale, but I couldn't think of anything interesting. Some things you might be able to exploit: * Non-commuting observables - an object that you can determine two different things about, but not both at the same time. Observing one makes the other indeterminate, so by making alternating observations you can change its state. * Interference - something can happen two mutually exclusive ways, but to achieve a desired outcome it has to happen both ways at the same time. You achieve this by closing your eyes and not watching what happens. * Entanglement - two distant objects are connected in such a way that observing something about one of them has an effect on the other. -- Greg
Re: [pygame] Why does my ball vibrate?
Ian Mallett wrote: However, Physics, as far as I know, does not say what happens with a particle of no mass. Actually, it does -- a photon is an example of an object with no mass. Such an object always travels at the speed of light -- it doesn't even need a push to get it going. It's fundamentally incapable of standing still. While it has no mass, it does have both energy and momentum, both of which are proportional to its frequency. These are conserved in any collision, so when it bounces off a wall, the wall gains some momentum, just as it would if a massive particle with the same momentum bounced off it. And if the wall starts to move as a result, then it has also gained some energy, which must have come from the photon, so the reflected photon must be red-shifted slightly (longer wavelength = lower frequency = less energy). -- Greg
Re: [pygame] Why does my ball vibrate?
On Dec 5, 2007 4:03 PM, Greg Ewing [EMAIL PROTECTED] wrote: Actually, it does -- a photon is an example of an object with no mass. Such an object always travels at the speed of light -- it doesn't even need a push to get it going. It's fundamentally incapable of standing still. Heh heh. Try hitting that with a paddle. While it has no mass, it does have both energy and momentum, both of which are proportional to its frequency. Momentum is defined as mass*velocity. If mass is zero, how does a photon have momentum? These are conserved in any collision, so when it bounces off a wall, the wall gains some momentum, just as it would if a massive particle with the same momentum bounced off it. And if the wall starts to move as a result, then it has also gained some energy, which must have come from the photon, so the reflected photon must be red-shifted slightly (longer wavelength = lower frequency = less energy). All this is true, but *how* exactly does a massless particle have momentum? -- Greg Ian
Re: [pygame] Why does my ball vibrate?
Ian Mallett wrote: On Dec 5, 2007 4:03 PM, Greg Ewing [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: Actually, it does -- a photon is an example of an object with no mass. Such an object always travels at the speed of light -- it doesn't even need a push to get it going. It's fundamentally incapable of standing still. Heh heh. Try hitting that with a paddle. While it has no mass, it does have both energy and momentum, both of which are proportional to its frequency. Momentum is defined as mass*velocity. If mass is zero, how does a photon have momentum? These are conserved in any collision, so when it bounces off a wall, the wall gains some momentum, just as it would if a massive particle with the same momentum bounced off it. And if the wall starts to move as a result, then it has also gained some energy, which must have come from the photon, so the reflected photon must be red-shifted slightly (longer wavelength = lower frequency = less energy). All this is true, but /how/ exactly does a massless particle have momentum? I don't pretend to understand the implications of this discussion, but it sounds to me like, at the speed of light, frequency is analogous to mass at lower speeds?
Re: [pygame] Why does my ball vibrate?
a while ago someone said that the ball speeding up and slowing down randomly was because it was based on the time elapsed every frame and that time varies because if other tasks are being run then the cpu is executing both threads so it doesn't spend all of its time in your game. But then what workaround is there for physics simulations that are based on time?
Re: [pygame] Why does my ball vibrate?
Ian Mallett wrote: However, the thing that will change significantly is gravitational pull. The further an object is away, the less pull it exerts by an inverse square function. (On the roof, you do weigh slightly less). That's true. Probably not something you need to take into account in a Pong game, but if you're simulating space flight or something like that, you certainly do! -- Greg
Re: [pygame] Why does my ball vibrate?
Ian Mallett wrote: If the ground were mobile, that gravity force down would cause the earth to move down with that same force. Since the earth can't move, it pushes back with the force of gravity. But the Earth does move. If you drop a 1kg. ball from a height of one meter, then both objects (the ball and the Earth) accelerate towards each other. The reason we see the ball fall is because 1), we move with it, and 2) the Earth is more massive and so is accelerated very little by the force. (In fact, the Earth accelerates: (1 kg.)/(5.9736×1024 kg.) = .1674^ -1024th as much. Not even a trillion trillion trillionth of a millimeter. Still much smaller than even that. As soon as I get a minute, I'll do some more demos... This is less than the Planck length of 1.6 * 10 ** -35 meters so becomes moot. The Planck length would make a natural cutoff for any game physics. :-) -- Lenard Lindstrom [EMAIL PROTECTED]
Re: [pygame] Why does my ball vibrate?
Lenard Lindstrom wrote: This is less than the Planck length of 1.6 * 10 ** -35 meters so becomes moot. The Planck length would make a natural cutoff for any game physics. :-) I've thought of writing a game with physics operating at the quantum scale, but I couldn't think of anything interesting. Maybe your character gets to resolve quantum choices but you lose if your improbability goes too high or something (like a life meter).
Re: [pygame] Why does my ball vibrate?
So, if our object masses 1kg., to move the Earth 1m/s., the object must move 199,120,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 times the speed of light! That's 199120*1trillion^84! Had to do this mostly by hand, my calc doesn't handle this sort of thing, so it might not be terribly accurate... Ian
Re: [pygame] Why does my ball vibrate?
Hi Ian, On Dec 4, 2007 6:18 AM, Ian Mallett [EMAIL PROTECTED] wrote: So, if our object masses 1kg., to move the Earth 1m/s., the object must move 199,120,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 times the speed of light! That's 199120*1trillion^84! Had to do this mostly by hand, my calc doesn't handle this sort of thing, so it might not be terribly accurate... Ian :) This is a python related mailing list, ya know.. /usr/lib/python2.6/site-packages/IPython/Extensions/path.py:32: DeprecationWarning: the md5 module is deprecated; use hashlib instead import sys, warnings, os, fnmatch, glob, shutil, codecs, md5 /usr/lib/python2.6/site-packages/IPython/iplib.py:58: DeprecationWarning: the sets module is deprecated from sets import Set IPython 0.8.2 [on Py 2.6a0] [~]|1 a = 199120 [~]|2 c = 84 [~]|3 b = 1 [~]|4 a * pow(b, c) 4 199120 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 0L Which is considerably larger than the figure you gave.
Re: [pygame] Why does my ball vibrate?
On Dec 4, 2007 7:02 AM, David Gowers [EMAIL PROTECTED] wrote: Hi Ian, On Dec 4, 2007 6:18 AM, Ian Mallett [EMAIL PROTECTED] wrote: So, if our object masses 1kg., to move the Earth 1m/s., the object must move 199,120,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 times the speed of light! That's 199120*1trillion^84! Had to do this mostly by hand, my calc doesn't handle this sort of thing, so it might not be terribly accurate... Ian :) This is a python related mailing list, ya know.. /usr/lib/python2.6/site-packages/IPython/Extensions/path.py:32: DeprecationWarning: the md5 module is deprecated; use hashlib instead import sys, warnings, os, fnmatch, glob, shutil, codecs, md5 /usr/lib/python2.6/site-packages/IPython/iplib.py:58: DeprecationWarning: the sets module is deprecated from sets import Set IPython 0.8.2 [on Py 2.6a0] [~]|1 a = 199120 [~]|2 c = 84 [~]|3 b = 1 [~]|4 a * pow(b, c) 4 199120 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 0L Which is considerably larger than the figure you gave. Oops, of course it is. silly me. My earlier point stands, though.
Re: [pygame] Why does my ball vibrate?
At those speeds you need to use relativity to get the right answer anyway. A 1kg mass needs to be moving at roughly the speed of light to accelerate the earth to 1m/s. Ian Mallett wrote: So, if our object masses 1kg., to move the Earth 1m/s., the object must move 199,120,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 times the speed of light! That's 199120*1trillion^84! Had to do this mostly by hand, my calc doesn't handle this sort of thing, so it might not be terribly accurate... Ian
RE: [pygame] Why does my ball vibrate?
#ok, I think I have it broken down into pieces now... check out this code:
#
import sys, pygame, mathpygame.init()pixelspercm=10.0gravity=9.80 * 10.0 #
meter per second per second (juiced up a little :) )framerate=50.0 # per
secondlossperbounce=0.33 # lose energy per bounceframestosilence=3#if a bounce
occurs AGAIN within the next 3 frames, consider the ball dead
class Ball(object):def __init__(self, id,pos=(0,0)):self.speed=0.0
# meter per secondself.diameter=0.025 # meterself.pos=pos
self.previous=0#count how many frames ago we had the previous bounce
self.name=id#just some name to show for feedback
image=pygame.Surface((int(self.diameter*2*pixelspercm*100),int(self.diameter*2*pixelspercm*100)))
image.set_colorkey((0,0,0))image.fill((0,0,0))
pygame.draw.circle(image, (255,255,255),
(int(self.diameter*pixelspercm*100),int(self.diameter*pixelspercm*100)),
int(self.diameter*pixelspercm*100))self.image=imagedef
updatepos(self,ms):if not (self.speed==0 and self.pos[1]==300):
distance=self.DeltaX2(self.speed, gravity,ms/1000.0)if
distance+self.pos[1]300:
self.speed=self.FinalVelocity1(self.speed,gravity,ms/1000.0)
self.pos=(self.pos[0],distance+self.pos[1])if self.previous0:
self.previous=self.previous-1print
self.name, 'normal cycle',self.previouselse:print
self.namedis2=300-self.pos[1]# distance to ground
print 'distance to ground=',dis2
ms2=self.MsTillGround(dis2,self.speed,self.FinalVelocity1(self.speed,gravity,ms/1000.0))
#ms till groundprint 'ms till ground=',ms2
endvel=self.FinalVelocity1(self.speed, gravity, (ms-ms2)/1000.0)#speed at
groundspeedatground=-endvel*(1-lossperbounce)#lose energy
print 'speed at ground=',speedatground
dis3=self.DeltaX2(speedatground, gravity,(ms-ms2)/1000.0)#deltaX after all ms
print 'deltaX after all ms=',dis3
endvel2=self.FinalVelocity1(speedatground, gravity, (ms-ms2)/1000.0)#speed
after timeprint 'speed after time=',endvel2if
self.previous1:self.pos=(self.pos[0],300)
self.speed=0.0print 'self.speed=',self.speed
print 'self.pos=',self.posprint 'SILENCE!'
else:self.speed=endvel2
self.pos=(self.pos[0],300+dis3)print
'self.speed=',self.speedprint 'self.pos=',self.pos
self.previous=framestosilence
def MsTillGround(self, Dx, Vi, Vf):#Dx=1/2*(Vi+Vf)*t
#t=Dx/(1/2*(Vi+Vf))t=Dx/(0.5*(Vi+Vf))return t*1000.0 # t is in
seconds, but we want the amount of millisecondsdef
FinalVelocity1(self,Vi,A,t):# ### 1# Vf=Vi+A*t
# Final Velocity = Initial Velocity + Acceleration * time#
meter/second = meter/second + meter/second/second * secondreturn Vi+A*t
def AverageVelocity(self,Vi,Vf):# ### 2#
Va=(Vi+Vf)/2# Average Velocity = ( Initial Velocity + Final Velocity
)/2# meter/second = ( meter/second + meter/second )/2return
(Vi+Vf)/2def DeltaX1(self,Vi,Vf,t):# ### 3#
Dx=1/2*(Vi+Vf)*t# DeltaX = 1/2 * ( Initial Velocity + Final Velocity )
* time# meter = 1/2 * ( meter/second + meter/second )* second
return 0.5*(Vi+Vf)*tdef DeltaX2(self,Vi,A,t):# ### 4
# Dx=Vi*t+ 1/2*a* t^2 # notice: t^2 in python is: t**2# DeltaX =
Initial Velocity * time+1/2*Acceleration* time*time# meter =
meter/second * second+1/2*meter/second/second * second*secondreturn
Vi*t+0.5*A*t**2def FinalVelocity2(self,Vi,A,Dx):# ### 5
# Vf^2=Vi^2+2*a*Dx# Final Velocity^2=Initial
Velocity^2+2*Acceleration*DeltaX# meter/second*meter/second=
meter/second*meter/second+2*meter/second/second*meterVf2=Vi**2+2*A*Dx
return math.sqrt(Vf2)
screen = pygame.display.set_mode((200, 400), 0, 32)balls =
[]balls.append(Ball('Ball 1',(78,15)))balls.append(Ball('Ball
2',(12,158)))balls.append(Ball('Ball 3',(122,56)))newtime=oldtime=0
while True:newtime=pygame.time.get_ticks() # milliseconds since start of
programif newtime-oldtime(1000.0/framerate)*2:print 'LAGGING!'
if newtime-oldtime1000.0/framerate:for event in pygame.event.get():
if event.type == pygame.QUIT:exit()
screen.fill((0, 0, 0))for ball in balls:
ball.updatepos(newtime-oldtime)for ball in balls:
Re: [pygame] Why does my ball vibrate?
On Dec 4, 2007 12:45 AM, Greg Ewing [EMAIL PROTECTED] wrote: To do this calculation properly you'd need to use the appropriate relativistic formulas. No doubt you could get enough kinetic energy if the object was moving at a speed close enough to c, but the more likely result would be vaporisation of the object together with a sizeable chunk of the earth... -- Greg Not to mention that you'd have to modify the size of the ball every frame, and somehow manage the changing looks of the ball too, aie
Re: [pygame] Why does my ball vibrate?
Hi, On Nov 30, 2007 9:30 AM, Greg Ewing [EMAIL PROTECTED] wrote: Matt Smith wrote: I can't quite get my head around WHY the ball sinks into the ground if I don't set ypos back to 384 but it certainly works. It's probably because you're applying gravity to the ball even when it's touching the ground. So it repeatdly gets accelerated downwards for one time step, then moved back up to ground level. In real life, when the ball is touching the ground, the downward force of gravity is balanced by an upward force from the ground, so the net acceleration is zero. You can model that by only applying gravity if the ball is above ground level. I believe this is the most reliable solution. (AFAIK there is no upward force though; it's more to do with the pressure the object exerts being insufficient to penetrate the ground. So for a simple physics simulation, this solution is also more correct.) Matt: I assume you didn't mean to use a suggestive subject line, Every time I look at my mail I see this conversation, read the subject line rather differently than you intended, and delete it. And then someone posts another reply, so it appears again.. ._.
Re: [pygame] Why does my ball vibrate?
On Dec 2, 2007 7:00 PM, David Gowers [EMAIL PROTECTED] wrote: I believe this is the most reliable solution. (AFAIK there is no upward force though; it's more to do with the pressure the object exerts being insufficient to penetrate the ground. So for a simple physics simulation, this solution is also more correct.) But there IS an upward force, at least as far as my memory of the physics I learned. The whole every action has a reaction business. If the ground were mobile, that gravity force down would cause the earth to move down with that same force. Since the earth can't move, it pushes back with the force of gravity. It has been quite a while since I was in physics, but that's how I remember it. Matt: I assume you didn't mean to use a suggestive subject line, Every time I look at my mail I see this conversation, read the subject line rather differently than you intended, and delete it. And then someone posts another reply, so it appears again.. ._. I never once read the subject line in that way, but now I can't help it. Thanks a lot. LOL
Re: [pygame] Why does my ball vibrate?
Douglas Bagnall wrote: FLOOR = 384 if ypos = FLOOR and velocity 0: overstep = ypos - FLOOR ypos = FLOOR - overstep * bounce velocity = -velocity * bounce I tried using this code and I go back to the situation where the ball continues to bounce when it should have come to rest. I would imagine this is because each time the ball passes below the floor we are giving it a little bit of upwards velocity.
Re: [pygame] Why does my ball vibrate?
Thanks for all the help. I have modified the code for determining the position of the ball so that the pixel co-ordinates are always integers: ypos = ypos + int(((velocity + newvelocity) / 2) * time_passed * 160) This has cured the vibrating problem and the ball comes to rest quite realistically. I tried consolidating the code checking for the floor but found that the ball sinks into the floor unless I always correct an over shoot of the floor to the co-ordinates of the floor. # Reverse velocity taking into account bounciness if we hit the ground if ypos = 384 and velocity 0: # Avoid the ball sinking into the ground ypos = 384 velocity = -velocity * bounce I can't quite get my head around WHY the ball sinks into the ground if I don't set ypos back to 384 but it certainly works. I am interested by the idea of separating the physics calculations from the frame rate calculations but I really don't know where to start. Thanks again, Matt
Re: [pygame] Why does my ball vibrate?
I actually got interested in the problem, and tried to recreate your code, even though you practically gave it to me. Had to sleep though :) I'll finish when I get home. Anyway, as long as the physics time engine is based on your fps, any point when the window is moved will cause a drop in the fps. I recommend timing the loop and getting an average speed going. Then, pause the simulation if the loop time is longer than that. That way, when you release-click, the ball will move from where it started. You could just forget timing and estimate, which I don't recommend if you want accuracy. Ian
Re: [pygame] Why does my ball vibrate?
For your amusement (and envy), here's some beautiful simulations. On their bouncing ball: the ball is rendered in 3D and actually deforms noticeably and oscillates after bouncing. I think it was here: http://www.gamedev.net/community/forums/topic.asp?topic_id=465499 You'll have to go to the project page and download the demo(s), but it's worth it. Ian
Re: [pygame] Why does my ball vibrate?
Greg Ewing wrote: Matt Smith wrote: I can't quite get my head around WHY the ball sinks into the ground if I don't set ypos back to 384 but it certainly works. It's probably because you're applying gravity to the ball even when it's touching the ground. So it repeatdly gets accelerated downwards for one time step, then moved back up to ground level. In real life, when the ball is touching the ground, the downward force of gravity is balanced by an upward force from the ground, so the net acceleration is zero. You can model that by only applying gravity if the ball is above ground level. I have a slightly different take on it: assuming an instantaneous bounce, the ball should have travelled up in proportion to the distance it would have sunk below, like so: FLOOR = 384 if ypos = FLOOR and velocity 0: overstep = ypos - FLOOR ypos = FLOOR - overstep * bounce velocity = -velocity * bounce This is unrealistic, of course, since a ball spends a period of time deforming before bouncing back up, but at least it is consistent, while just truncating gives you a different error each time. But, anyway, the reason it sinks is it always goes down further than it can come up, because bounce 1 and gravity points down, so the upward velocity is always less than the downward. For a period towards the end of the journey, the upward velocity will not be enough to lift the ball above the floor before it turns to drop again. Rounding might exacerbate this. douglas
[pygame] Why does my ball vibrate?
Hi,
I am beginning to learn Pygame and I have written a short program to
simulate a bouncing ball. The motion of the ball is pretty realistic
until it has almost come to rest. The ball continues to vibrate long
after you would have expected it to come to a complete rest (or be
moving less than 1 pile each time as it will never stop moving 100%).
Also, the ball can start to bounce higher again if I click somewhere
else on the desktop. I can't work out why this happens so can anyone
shed some light on it and suggest how I can prevent it.Here's my code:
#! /usr/bin/python
import sys, pygame
pygame.init()
xpos = 92
ypos = 0
gravity = 9.8
velocity = 0
# How much of the velocity of the ball is retained on a bounce
bounce = 0.8
screen = pygame.display.set_mode((200, 400), 0, 32)
# The ball is a 16px sprite
ball = pygame.image.load('ball.png')
clock = pygame.time.Clock()
# The main loop
while True:
# Test for exit
for event in pygame.event.get():
if event.type == pygame.QUIT:
exit()
# The physics
# Reverse velocity taking into account bounciness if we hit the ground
if ypos == 384 and velocity 0:
velocity = -velocity * bounce
time_passed = clock.tick(60) / 1000.0
newvelocity = velocity + (gravity * time_passed)
# Use the average velocity over the period of the frame to change
position
ypos = ypos + (((velocity + newvelocity) / 2) * time_passed * 160)
# Prevent the ball from sinking into the ground
if ypos = 384:
ypos = 384
velocity = newvelocity
# Update the screen
screen.fill((0, 0, 0))
screen.blit(ball, (xpos, ypos))
pygame.display.update()
Thanks for looking.
Matt
Re: [pygame] Why does my ball vibrate?
On Wed, Nov 28, 2007 at 08:26:19PM +, Matt Smith wrote:
Hi,
I am beginning to learn Pygame and I have written a short program to
simulate a bouncing ball. The motion of the ball is pretty realistic
until it has almost come to rest. The ball continues to vibrate long
after you would have expected it to come to a complete rest (or be
moving less than 1 pile each time as it will never stop moving 100%).
Also, the ball can start to bounce higher again if I click somewhere
else on the desktop. I can't work out why this happens so can anyone
shed some light on it and suggest how I can prevent it.Here's my code:
I have two suggestions, see below:
#! /usr/bin/python
import sys, pygame
pygame.init()
xpos = 92
ypos = 0
gravity = 9.8
velocity = 0
# How much of the velocity of the ball is retained on a bounce
bounce = 0.8
screen = pygame.display.set_mode((200, 400), 0, 32)
# The ball is a 16px sprite
ball = pygame.image.load('ball.png')
clock = pygame.time.Clock()
# The main loop
while True:
# Test for exit
for event in pygame.event.get():
if event.type == pygame.QUIT:
exit()
# The physics
# Reverse velocity taking into account bounciness if we hit the ground
if ypos == 384 and velocity 0:
Here you probably want if ypos = 384 ... rather than checking the
precise pixel position, since your ball could be falling fast enough to
skip over pixel 384. I notice you have a check for that later, but why
not put it here?
velocity = -velocity * bounce
time_passed = clock.tick(60) / 1000.0
newvelocity = velocity + (gravity * time_passed)
# Use the average velocity over the period of the frame to change
position
ypos = ypos + (((velocity + newvelocity) / 2) * time_passed * 160)
Your velocity is a floating point number, and floating point math has
limited precision. Because your ball sprite is being displayed only at
integer pixel coordinates, you ought not to let your xpos and ypos have
decmals. I suggest you change the above line to this:
ypos = ypos + int(((velocity + newvelocity) / 2) * time_passed * 160)
This throws away the decmal on the ypos, but preserves it for your
velocity. This fix alone, even without the ypos == 384 thing I mentioned
above, is enough to solve your bouncing problem.
# Prevent the ball from sinking into the ground
if ypos = 384:
ypos = 384
velocity = newvelocity
# Update the screen
screen.fill((0, 0, 0))
screen.blit(ball, (xpos, ypos))
pygame.display.update()
Thanks for looking.
Matt
You are welcome :)
---
James Paige
Re: [pygame] Why does my ball vibrate?
When I click and hold on the window of the demo, the simulation
pauses. This accounts for the ball bouncing high into the sky.
Since the game has paused for a while, the framerate has gone wy
down, resulting in a very large time_passed amount. When put into the
rest of the system, this very large time_passed makes the new velocity
very large, resulting in it shooting high above. A better method of
handling physics, is to run a certain number of physics steps
depending on how much time has passed, so that variable time doesn't
cause issues such as this. So it would be like:
def physics_step(blah):
Physics to be run for 1/30th of a second
update_velocity()
collision_detection()
As to the bouncy problem, you see this happen in even established
physics engines such as ODE. What you have to do is have some sort of
threshold in which to stop the physics and freeze the moving
objects, when they collide with something at some small amount of
speed. The inaccuracies have to do with floating point math, which is
never going to be exactly 0, in addition to your allowance of the ball
falling through the floor before calculating the velocity. Here's my
quick and dirty modification:
# The physics
# Reverse velocity taking into account bounciness if we hit the ground
newvelocity = velocity + (gravity * time_passed)
# Use the average velocity over the period of the frame to change position
ypos = ypos + (((velocity + newvelocity) / 2) * time_passed * 160)
if ypos = 384:
ypos = 384
newvelocity = -newvelocity * bounce
if velocity 1: #Alter this value for the freeze to
happen at different speeds
ypos = 384
newvelocity = 0
# Prevent the ball from sinking into the ground
velocity = newvelocity
On Nov 28, 2007 12:26 PM, Matt Smith [EMAIL PROTECTED] wrote:
Hi,
I am beginning to learn Pygame and I have written a short program to
simulate a bouncing ball. The motion of the ball is pretty realistic
until it has almost come to rest. The ball continues to vibrate long
after you would have expected it to come to a complete rest (or be
moving less than 1 pile each time as it will never stop moving 100%).
Also, the ball can start to bounce higher again if I click somewhere
else on the desktop. I can't work out why this happens so can anyone
shed some light on it and suggest how I can prevent it.Here's my code:
#! /usr/bin/python
import sys, pygame
pygame.init()
xpos = 92
ypos = 0
gravity = 9.8
velocity = 0
# How much of the velocity of the ball is retained on a bounce
bounce = 0.8
screen = pygame.display.set_mode((200, 400), 0, 32)
# The ball is a 16px sprite
ball = pygame.image.load('ball.png')
clock = pygame.time.Clock()
# The main loop
while True:
# Test for exit
for event in pygame.event.get():
if event.type == pygame.QUIT:
exit()
# The physics
# Reverse velocity taking into account bounciness if we hit the ground
if ypos == 384 and velocity 0:
velocity = -velocity * bounce
time_passed = clock.tick(60) / 1000.0
newvelocity = velocity + (gravity * time_passed)
# Use the average velocity over the period of the frame to change
position
ypos = ypos + (((velocity + newvelocity) / 2) * time_passed * 160)
# Prevent the ball from sinking into the ground
if ypos = 384:
ypos = 384
velocity = newvelocity
# Update the screen
screen.fill((0, 0, 0))
screen.blit(ball, (xpos, ypos))
pygame.display.update()
Thanks for looking.
Matt
Re: [pygame] Why does my ball vibrate?
A somewhat simple fix to get the ball to stop bouncing would be like this
(put after velocity is set to newvelocity):
if abs(velocity) 0.01:
gravity = 0
velocity = 0
This is just a quick fix though (and alters gravity, which is bad), you
probably want to use Patrick's or James'.
On Nov 28, 2007 1:57 PM, James Paige [EMAIL PROTECTED] wrote:
On Wed, Nov 28, 2007 at 08:26:19PM +, Matt Smith wrote:
Hi,
I am beginning to learn Pygame and I have written a short program to
simulate a bouncing ball. The motion of the ball is pretty realistic
until it has almost come to rest. The ball continues to vibrate long
after you would have expected it to come to a complete rest (or be
moving less than 1 pile each time as it will never stop moving 100%).
Also, the ball can start to bounce higher again if I click somewhere
else on the desktop. I can't work out why this happens so can anyone
shed some light on it and suggest how I can prevent it.Here's my code:
I have two suggestions, see below:
#! /usr/bin/python
import sys, pygame
pygame.init()
xpos = 92
ypos = 0
gravity = 9.8
velocity = 0
# How much of the velocity of the ball is retained on a bounce
bounce = 0.8
screen = pygame.display.set_mode((200, 400), 0, 32)
# The ball is a 16px sprite
ball = pygame.image.load('ball.png')
clock = pygame.time.Clock()
# The main loop
while True:
# Test for exit
for event in pygame.event.get():
if event.type == pygame.QUIT:
exit()
# The physics
# Reverse velocity taking into account bounciness if we hit the
ground
if ypos == 384 and velocity 0:
Here you probably want if ypos = 384 ... rather than checking the
precise pixel position, since your ball could be falling fast enough to
skip over pixel 384. I notice you have a check for that later, but why
not put it here?
velocity = -velocity * bounce
time_passed = clock.tick(60) / 1000.0
newvelocity = velocity + (gravity * time_passed)
# Use the average velocity over the period of the frame to change
position
ypos = ypos + (((velocity + newvelocity) / 2) * time_passed * 160)
Your velocity is a floating point number, and floating point math has
limited precision. Because your ball sprite is being displayed only at
integer pixel coordinates, you ought not to let your xpos and ypos have
decmals. I suggest you change the above line to this:
ypos = ypos + int(((velocity + newvelocity) / 2) * time_passed * 160)
This throws away the decmal on the ypos, but preserves it for your
velocity. This fix alone, even without the ypos == 384 thing I mentioned
above, is enough to solve your bouncing problem.
# Prevent the ball from sinking into the ground
if ypos = 384:
ypos = 384
velocity = newvelocity
# Update the screen
screen.fill((0, 0, 0))
screen.blit(ball, (xpos, ypos))
pygame.display.update()
Thanks for looking.
Matt
You are welcome :)
---
James Paige
--
This, from Jach.
How many programmers does it take to change a light bulb?
None. It's a hardware problem.
How many Microsoft programmers does it take to change a light bulb?
None. Microsoft just declared darkness as the newest innovation in
cutting-edge technology.
Re: [pygame] Why does my ball vibrate?
On Nov 28, 2007 3:38 PM, Kevin [EMAIL PROTECTED] wrote: A somewhat simple fix to get the ball to stop bouncing would be like this (put after velocity is set to newvelocity): if abs(velocity) 0.01: gravity = 0 velocity = 0 I'm not sure that would work. At the top of a parabolic arc, Physics says that the ball's velocity is temporarily zero. So if you're starting from rest, it won't fall at all. If it starts with some velocity, it will stop at the maximum of the parabola. A solution would be to have it also check for the ground level. When I wrote Marble Madness, ( http://www.pygame.org/project/471/), for example, exploding particles hung disturbingly in the air until I checked for the ground level (mercifully simple, because it was flat). Incidentally, I noticed your parameter for gravity was 9.8. As in 9.8m/s. I'm probably stating the obvious, but I'm guessing you're using the framerate to calculate to motion because it's a simulation, which requires accurate time. Also, I suggest that you change the collision detection to: if ypos = 384 and velocity 0: velocity = -velocity * bounce (== becomes =) so that you don't have the extra code. Hope this helps, Ian
