R turbo on the autobahn
Discussion
turbobloke said:
Like I said before, this is now not helpful and certainly not funny, the 'science' you quote is quite wrong but never mind, please feel free to continue thinking you're right. There are so many basic errors in your re-statements of non-science that I would be lost for a suitable place to start from, and as it's been done already - the posts are here for anyone with the interest, the knowledge and the patience to wade through - the diversion ought to hit a 'road closed' sign at this point.
Time for a U turn, back to the autobahn...
Time for a U turn, back to the autobahn...
Ah well, seems I've made loads of errors. Well since I'm always willing to learn I would be grateful if you would point them out. Which bits are non science. Feel free to choose just one error and go through that with me.
It's not going to happen though is it.
Oh, we're still here.
OK I'll try and do as much as possible in one go, to make it happen. Apologies to everyone else.
Where to start...for one thing I have never said "it isn't possible in general to have one quantity proportional to another, with different units" for example the pressure of a gas can be proportional to temperature and clearly there are different units in there. In such cases you will find there are other quantities involved, also with different units, e.g. volume.
However in a simple mechanics problem we only have three variables namely mass, distance (or position or displacement) and time. That's it, and from these you can derive others such as velocity, acceleration, momentum, kinetic energy and so on.
If you say 'force is proportional to energy' then you must also be able to write F = constant x Energy
Your constant of proportionality in this case ought to be a dimensionless number in which case the point I made earlier applies, namely that in this kind of mechanics situation the units of force and energy must tie up and they don't.
The alternative is to give the constant some units, but then you might as well ask, if we take the units of the right hand side as a whole and leave the constant as a number, what do the units look like? The answer is momentum divided by time, which is the same as rate of change of momentum, a phrase given by one of the website. Of course you're free to multiply both sides of an equation by any quantity you like but why would anyone do this and what physical meaning would it have? No reason, no meaning.
Another point is that collisions in mechanics aren't complex in terms of energetics, if kinetic energy is conserved then the collision is elastic (which happens with rigid i.e. inelastic objects) and if it's not conserved then the collision is inelastic (which happens with elastic objects to use the dodgy term from an earlier post, or deformable ones to be precise). In an inelastic collision it isn't pssible to follow kinetic energy and describe the changes by allocating some KE here and some KE there, since energy is absorbed when objects deform and this will be in a unique way for each and every collision. You're left with momentum which is always conserved no matter what the collision is. In collisions between a deformable car and a deformable pedestrian, energy is not conserved and is therefore a hopeless non-starter to use, you can't play with it in the way you're trying to, simply because it's not conserved. For example...
From the emotive road safety perspective, KE is useful simply because of the square law, regardless of the fact that the science is inapplicable. Take a typical car with mass 1000 kg and a pedestrian mass 50kg. Using some numbers from earlier, if the car is at 30 mph (13.4 metres per second) it has kinetic energy of 0.5 x m x v^2 which is 89780 joules. If it increases its speed to 40 mph (17.9 m/s) then from the same formula for KE it has kinetic energy of 160205 joules.
At either speed if this car hits a stationary pedestrian, the proportion of KE that will be transferred is very very small, and for sure the 'extra' KE at 40 compared to 30 is NOT transferred to the pedestrian. In going from 30mph to 40mph the extra KE gained is 70425 joules. If all this was transferred to a stationary 50kg pedestrian, then after impact the hapless person would be moving at a speed of 53.1 m/s which is 119 mph.
If anyone has ever had the misfortune to witness a pedestrian impact at about 40 mph they'll know the body is not propelled away from the impact at that speed. If you repeat the calculation for a car at 70mph and then 200mph and use KE by assuming KE transfer is relevant or important you will get some literally fantastic pedestrian velocities after impact.
This shows that KE is a hopeless, pointless, useless tool for describing impacts between cars and pedestrians. The 40mph free travelling speed impact I saw resulted in a very very small but unknown proportion of the car's KE being transferred to the drunk who had stepped out inside the driver's thinking distance. They went into the air to a height of about 5 metres (rotating slowly) and moved forward by a short distance, measurable in metres. Their problem wasn't KE transferred, in one sense it was the fact that not enough KE was transferred. Some kinetic energy had been lost when the car's A-pillar was moved into a V-shape by their head, which itself then had a very different shape. This fatal set of changes will have required a very very small proportion of the car's KE, as was visible since the car continued moving at essentially the same speed of 40 mph after impact as before. Braking started, I would imagine, when the driver heard a loud bang and saw their windscreen shatter.
The collision could be followed successfully in terms of momentum and force. The force applied by the vehicle to the pedestrian was precisely equal to the loss of momentum of the car divided by the time the car and pedestrian remained in contact, which was probably a large fraction of a second or a bit more. In reality it would be absolutely impossible to determine the force applied in any other way, for example by trying to use energy - for one thing there is no way of knowing how much was lost in deforming the two objects, for another there is no applicable equation to use that doesn't reduce to force = change in momentum divided by time.
KE as a quantity really is a non-starter for following or describing such collisions. It certainly is important in terms of stopping distance to avoid a collision - but that's another matter - and in head-on impacts between slab fronted vehicles when their total KE has nowhere else to go. Otherwise it's an unhelpful view to take, unless you;re trying to con people, for example into believing that 'speed kills'.
If the description of the collision offends anyone than I apologise, but it's necessary to view what happens and understand the roles of energy and momentum in such an impact.
OK I'll try and do as much as possible in one go, to make it happen. Apologies to everyone else.
Where to start...for one thing I have never said "it isn't possible in general to have one quantity proportional to another, with different units" for example the pressure of a gas can be proportional to temperature and clearly there are different units in there. In such cases you will find there are other quantities involved, also with different units, e.g. volume.
However in a simple mechanics problem we only have three variables namely mass, distance (or position or displacement) and time. That's it, and from these you can derive others such as velocity, acceleration, momentum, kinetic energy and so on.
If you say 'force is proportional to energy' then you must also be able to write F = constant x Energy
Your constant of proportionality in this case ought to be a dimensionless number in which case the point I made earlier applies, namely that in this kind of mechanics situation the units of force and energy must tie up and they don't.
The alternative is to give the constant some units, but then you might as well ask, if we take the units of the right hand side as a whole and leave the constant as a number, what do the units look like? The answer is momentum divided by time, which is the same as rate of change of momentum, a phrase given by one of the website. Of course you're free to multiply both sides of an equation by any quantity you like but why would anyone do this and what physical meaning would it have? No reason, no meaning.
Another point is that collisions in mechanics aren't complex in terms of energetics, if kinetic energy is conserved then the collision is elastic (which happens with rigid i.e. inelastic objects) and if it's not conserved then the collision is inelastic (which happens with elastic objects to use the dodgy term from an earlier post, or deformable ones to be precise). In an inelastic collision it isn't pssible to follow kinetic energy and describe the changes by allocating some KE here and some KE there, since energy is absorbed when objects deform and this will be in a unique way for each and every collision. You're left with momentum which is always conserved no matter what the collision is. In collisions between a deformable car and a deformable pedestrian, energy is not conserved and is therefore a hopeless non-starter to use, you can't play with it in the way you're trying to, simply because it's not conserved. For example...
From the emotive road safety perspective, KE is useful simply because of the square law, regardless of the fact that the science is inapplicable. Take a typical car with mass 1000 kg and a pedestrian mass 50kg. Using some numbers from earlier, if the car is at 30 mph (13.4 metres per second) it has kinetic energy of 0.5 x m x v^2 which is 89780 joules. If it increases its speed to 40 mph (17.9 m/s) then from the same formula for KE it has kinetic energy of 160205 joules.
At either speed if this car hits a stationary pedestrian, the proportion of KE that will be transferred is very very small, and for sure the 'extra' KE at 40 compared to 30 is NOT transferred to the pedestrian. In going from 30mph to 40mph the extra KE gained is 70425 joules. If all this was transferred to a stationary 50kg pedestrian, then after impact the hapless person would be moving at a speed of 53.1 m/s which is 119 mph.
If anyone has ever had the misfortune to witness a pedestrian impact at about 40 mph they'll know the body is not propelled away from the impact at that speed. If you repeat the calculation for a car at 70mph and then 200mph and use KE by assuming KE transfer is relevant or important you will get some literally fantastic pedestrian velocities after impact.
This shows that KE is a hopeless, pointless, useless tool for describing impacts between cars and pedestrians. The 40mph free travelling speed impact I saw resulted in a very very small but unknown proportion of the car's KE being transferred to the drunk who had stepped out inside the driver's thinking distance. They went into the air to a height of about 5 metres (rotating slowly) and moved forward by a short distance, measurable in metres. Their problem wasn't KE transferred, in one sense it was the fact that not enough KE was transferred. Some kinetic energy had been lost when the car's A-pillar was moved into a V-shape by their head, which itself then had a very different shape. This fatal set of changes will have required a very very small proportion of the car's KE, as was visible since the car continued moving at essentially the same speed of 40 mph after impact as before. Braking started, I would imagine, when the driver heard a loud bang and saw their windscreen shatter.
The collision could be followed successfully in terms of momentum and force. The force applied by the vehicle to the pedestrian was precisely equal to the loss of momentum of the car divided by the time the car and pedestrian remained in contact, which was probably a large fraction of a second or a bit more. In reality it would be absolutely impossible to determine the force applied in any other way, for example by trying to use energy - for one thing there is no way of knowing how much was lost in deforming the two objects, for another there is no applicable equation to use that doesn't reduce to force = change in momentum divided by time.
KE as a quantity really is a non-starter for following or describing such collisions. It certainly is important in terms of stopping distance to avoid a collision - but that's another matter - and in head-on impacts between slab fronted vehicles when their total KE has nowhere else to go. Otherwise it's an unhelpful view to take, unless you;re trying to con people, for example into believing that 'speed kills'.
If the description of the collision offends anyone than I apologise, but it's necessary to view what happens and understand the roles of energy and momentum in such an impact.
tony.t said:
turbobloke said:
tony.t said:
You can't argue with the physics of travelling at 200mph. The KE of a car increases with the square of the speed and hence the higher the speed the correspondingly much more difficult it is to contain the energy of the car if the unfortunate does occur.
True to a point but KE is only important to consider primarily if there is no stopping distance available, if no braking can take place before the KE is transferred, or if the (assumed) collision is a head-on between slab-sided objects of similar mass. In most real world situations, braking even of the oh sheeeeeeet kind (and crumple zone deformation) absorb most of the KE relatively harmlessly and dissipate it as heat in the brakes. If the point about KE refers to the unlikely possibility of impact with a pedestrian - we are talking motororways here - then KE is even less important, we should consider momentum as the collision will be inelastic. If not, and KE was conserved, you could be looking for the bodies of motorway suicides beyond the next junction.
KE is important where there is a stopping distance. It takes twice as long to stop at 40 as it does at 30 as a result of the square law relating to KE. Also speed is not lost in a linear fashion. Using the 30/40 analogy above after half the braking distance from 40 has been reached you're still doing 30.
All of which is fine if there is adequate stopping distance and braking is controlled. However when not in control the higher KE does present a problem; A car is more likely to become airbourne for example or skids/slides will be much longer and much more difficult to recover.
KE transference to pedestrians are not inelastic as you well know . BTW the force on the pedestrian is directly proportional to the change in KE.
Hi Tony,do these figs bare out what your saying?
And in the real world the braking distances and times for the 997 Turbo are;
30-0mph----8.4m----8.4secs
50-0mph----23m-----23secs
70-0mph----44.4m---44.4secs
Figs from Autocar.
Regards Mark.
Just read the entire thread ! mass = Thread = What ?! 
Adam T if you are still around, nice car, I think I read your story about the build some time ago, glad she is still going strong.
To decanter, OMG are you just bored ? to the intelligent gentleman (you know who you are) late one was it ?
Adam T if you are still around, nice car, I think I read your story about the build some time ago, glad she is still going strong.
To decanter, OMG are you just bored ? to the intelligent gentleman (you know who you are) late one was it ?
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