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14-02-2011, 06:10 PM | #31 | ||
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Which car will end up better off? A stationary f150 pick up truck or a fiesta hitting it at 150km/h
Clearly the f150 is double its mass? |
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14-02-2011, 06:19 PM | #32 | |||
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- Prado - Merc M-class - Kluger - Toureg While not real ones they are in the same class - Territory - Santa Fe - Sorento - BMW X5 - Volvo XC 90 - Audi Q5 Funny enough the Nissan Patrol is the only one under 4 stars (even the great wall is 4 stars).
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14-02-2011, 06:21 PM | #33 | |||
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14-02-2011, 06:25 PM | #34 | |||
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if you do the numbers on yours, the smaller vehicle has suffered a great momentum change than the larger vehicle, which isnt in accordance with laws of physics which predicts that the momentum changes should be equal in magnitude and opposite in direction. Its also impossible to predict the speeds of the vehicles after the collision unless you know the elasticity of the materials involved, the characteristics of the crumple zones of the two vehicles etc. |
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14-02-2011, 06:42 PM | #35 | ||
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In the worst case, a head on between two vehicles travelling at 100 kph will only ever reduce
the forward velocity to 0 kph, this is the maximum deceleration that can be applied to either vehicle, any remaining momentum is then expressed as either a forward or negative velocity after impact. |
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14-02-2011, 06:44 PM | #36 | |||
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Consider car A going to the right versus car B going to the left each doing 100km/h Car A is brought to a halt by a force of magnitude F acting on it in the opposite direction to its travel, this force to the left comes from car B Similarly car B's motion to the left is arrested by a force F acting to the right, from car A. Equal and opposite forces, or sum of the magnitude of the two forces = 2F. Consider car A travelling right into wall at 100km/h Car A experiences a force to the left of force F to arrrests its motion, the change in speed is exactly the same as above (100 to whatever speed they bounce of each other) hence the force is the same. The wall experiences a force of F from the car pushing it to the right Mag of total forces = F + F =2F the forces are exactly the same, though the amount of damage is clearly double when two vehicles are involved. |
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14-02-2011, 06:44 PM | #37 | ||
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Oh NC Lane, what a can of worms this topic has opened up in the past
There are many things to consider but I think a major factor is that injury to the occupants of the cars is largely a result not of the forces the vehicles are subjected to but how and how much of that force is transferred into the bodies of the occupants. One way of looking at it is if you put an egg (representing a passenger) into a small box made of inch thick steel (representing a car) and hurl the box into a wall, the box may remain completely unmarked but the egg inside will be smashed. Safety features such as air bags could be simulated by lining the box with cotton wool, thus diminishing the amount of force transferred into the egg (person). Occupant safety really comes down to how much of a collisions impact force is absorbed or negated by things such as crumple zones, airbags, pre tensioning seat belts, impact absorbing materials, etc. That being said there's also the possibility of the 4WDs high center of gravity resulting in a roll over at 100kph which is a situation where I don't think any amount of mass is going to do anything other then carry the vehicle further and possibly through more rolls. I think it should also be noted however that the star rating is a guide only and isn't the be all and end all of safety, as the difference between 4 and 5 stars has in the past been something as little as having a passenger seat belt warning light on the dash. Here's a couple of threads that have gone into the question (relating to an older bigger car) in much more detail. http://www.fordforums.com.au/showthread.php?t=11305690 http://www.fordforums.com.au/showthr...6&page=1&pp=30 |
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14-02-2011, 06:49 PM | #38 | |||
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14-02-2011, 07:33 PM | #39 | |||
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In order to bounce as you say, the forward velocity must have been zero at one point. In any case the RTA saying that the impact is the same as a 200kph SVA into a brick wall is incorrect. You would have to hit a train or a Semi fully laden head on for that to equal a brick wall travelling 100 kph. |
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14-02-2011, 07:48 PM | #40 | |||
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Yes in order to bounce back the forward velocity must have come to zero at some time, but I didnt dispute that. You claimed that in a head on that the maximum deacceleration occurs when the vehicles velocity is reduced to zero. If the vehicle goes in the opposite direction the change in velocity is greater, hence the acceleration is greater(assuming all other factors such as the time of collision remains equal), clearly the maximum deaccleration does not occur when the speed goes to zero. |
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14-02-2011, 07:54 PM | #41 | |||
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If the deceleration in a car crash from 100 kph to 0 happens at XGs, then any momentum left over propels the car in reverse direction at XGs. There is no way the Gs approach that of a car hitting a brick wall at 200 kph, do you agree yes or no? |
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14-02-2011, 07:56 PM | #42 | ||
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Since no one has yet mentioned that video of that test between a Fiat 500 (rated Euroncap 5-star) vs an Audi SUV (rated Euroncap 4-star), here it is http://www.youtube.com/watch?v=6pVF1Wr7GLQ
I think that confirms what a few users here have been saying about bigger cars being better off in a head-on with a smaller car. |
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14-02-2011, 08:42 PM | #43 | ||
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Yes passenger cell intrusion is a very nasty thing.
I did point out earlier that the ANCAP system is based on many things and not just structural rigidity. But consider what's possible through safer design after seeing what happens when one of those itsy bitsy Smart cars goes head on into a Merc with twice its mass here. That they can plough it into a concrete wall like that and still open the door amazes me. http://www.youtube.com/watch?v=8RAN4gTtc4s The heavier Volvo in this clip does appear to win the inertia battle in this one but look closer. http://www.youtube.com/watch?v=emtLLvXrrFs Last edited by WMD351; 14-02-2011 at 08:50 PM. |
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14-02-2011, 08:52 PM | #44 | |||
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that thread was largely about 'old v new' not 'new v new'. if both cars have modern crash protection, an extra 500kg+ will help you out ancap testing assumes the cars are being hit by a similar mass. |
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14-02-2011, 09:25 PM | #45 | ||||
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Please take the time to look at the link I posted in post #26. Mythbusters took 4 identical cars. 1 was crashed into a solid wall at 50 mph 1 was crashed into a solid wall at 100 mph 2 were crashed head on into each other travelling at 50 mph each. It's pretty conclusive as far as this scenario is concerned.
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14-02-2011, 09:29 PM | #46 | ||||
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the former creates a higher deacceleration(assuming collision time is equal) since acceleration is the rate of change of velocity for both the car and the occupant, assuming of course they are wearing a seat belt. Claiming "excess" momentum is converted into deacceleration is nonsense. Momentum is not converted into other quantities, it is conserved/transferred shared amongst other bodies in the collision. Quote:
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14-02-2011, 09:37 PM | #47 | |||
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14-02-2011, 09:48 PM | #48 | |||
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balls are different to cars? they are masses and both have different levels of elasticity, the same principles of motion apply |
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14-02-2011, 10:01 PM | #49 | ||||
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My point that the acceleration can be larger when the vehicles rebound after the collision stands. Sorry to turn this into a he said/she said folks and really its sidetracking the original question of the thread, enough from me, goodnight all. |
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14-02-2011, 10:07 PM | #50 | ||||
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The ball doesn't bounce of the wall faster than it hit it, unless of course you thought the movie "Flubber" was actually a documentary.
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14-02-2011, 11:45 PM | #51 | ||
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OK what if Im in My Mack truck and dog
fully legally loaded at 45 Tonne gross and I hit a small hatch with 5 star rating at 1 tonne gross head on @100Ks Im maybe gonna get a sore knee or ribs (no airbags at all) what is going to happen to the passengers in the hatch I hope it never happens as I wouldnt be able to live with it but work it out me doing 100Ks and them doing 100Ks Im sorry but no amount of hi tech stuff will save them thats just physics if an asteroid weighing a thousand tonnes hit the earth and you were prepared and were living in a bunker (airbag technology sort of) what would happen with a direct hit we all know that weight will always win try hitting a nail with a hammer now try hitting a hammer with a nail I rest my case |
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15-02-2011, 12:52 AM | #52 | |||
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15-02-2011, 01:16 AM | #53 | ||
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forces are equal on both cars no mater how big the mass (newtons third law - this is the basics guys), its the change of momentum or impulse which is different as the weights are different.
pet hate of mine when people throw around and talk about physics laws in the wrong way but that's probably the engineer in me coming out haha
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15-02-2011, 03:25 AM | #54 | |||
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as for all the brick wall talk, are we talking single brick or double brick wall
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15-02-2011, 07:46 AM | #55 | |||
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I said the acceleration will be more if the body ends up travelling in the opposite direction to its original motion, not that the acceleration entering the wall would be less than when leaving, that would mean some type of explosion on impact. Do you understand how the force varies as the body is compressed on collision? There are two things to look for in a collision, the actual amount of deacceleration and how long that acceleration lasted. You can easily survive 3000G's for a nanosec or so, you survive small bumps and knocks everyday that have high G values but dont last long, but not over the average time of a car collision. Clearly bouncing also causes the acceleration to continue for longer, not desired. Last edited by sudszy; 15-02-2011 at 08:04 AM. |
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15-02-2011, 07:51 AM | #56 | |||
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Last edited by sudszy; 15-02-2011 at 08:03 AM. |
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15-02-2011, 09:29 AM | #58 | |||
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15-02-2011, 09:43 AM | #59 | ||||
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How about you activate your PM (private message) ability and we'll continue this discussion between us and stop hijacking the thread. You're just not getting it and I've got the feeling it will take a bit of back and forth. I don't mind people having opinions, as there is no right and wrong. This however is physics, so there is. Now one of us is obviously wrong, so how about we work out which so the other can learn.
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15-02-2011, 09:49 AM | #60 | |||
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fesses impact 50kph impact energy f150 25kph impact energy
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