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RWDboy Forums Junkie Location: South Australia Registered: July 2002	Re: Physics problem. Sat, 26 March 2005 03:21 Force is linearly related to the mass and the acceleration - correct? F = ma -> one of Newtons basic formula (I think it's Newton's) Acceleration is a result of the rate of change of velocity, not the actual velocity itself. In this case, one car will change it's velocity from 50km/h to 0km/h in some period of time - that period of time is determined by the rate at which the car's body can convert the kinetic energy of it's travel into heat/sound/whatever via crumpling. So the rate at which the car's body can convert kinetic energy is the key factor in the 'force' related to an accident. Thus the total kinetic energy is what is important in determining an accident's destructiveness...as the more kinetic energy there is, the more the car has to absorb. We can assume for this example that loss of mass (ie bits of glass flying off etc) is negligible and won't affect the argument/result. So we are basically focussing on the acceleration and thus the kinetic energy of the problem. The formula for kinetic energy is KE = 0.5 * m * v^2 Or as I stated above, kinetic energy is linearly related to mass, and linearly related to the square of velocity. In the two car collision, both cars have equal mass, and equal velocity. Whatever energy both cars contribute to the accident (ie kinetic) it will be divided between the two cars evenly (assuming that both cars are identical, generally even identical cars won't crumple exactly the same in any accident - but this is all theoretical anyway). So for one car, it will absorb it's own value of kinetic energy and convert it to heat via crumpling etc. As it absorbs it's own amount of kinetic energy, it accelerates (or 'decelerates') from it's original speed to stationary based solely on how it converts the energy - and how much energy it has to convert, and because mass isn't a factor here (it is 'constant') we can assume that the force applied to the one car is solely related to it's own original kinetic energy. If you were to look at the other car, the same result would apply, it will absorb kinetic energy equivalent to it's original value of kinetic energy, and come to a rest. The force it experiences will also be related to whatever it's original kinetic energy was. Now - we will look at a car hitting a wall. The wall has no kinetic energy at all, and will not absorb kinetic energy upon impact ie no deformation to it's structure, ho heat/friction generated. It will basically sit there and just reflect whatever energy is thrown at it because it (in this theoretical example) doesn't change it's state in any way during the collision. So whatever happens to the car (whatever energy it absorbs) must come from it's own stockpile of kinetic energy. The impact causes a force on the wall which is reflected back onto the car and it (the car) will absorb the energy related to that force (via crumpling). So you can see that whatever crumpling occurs to the car is done by it's own original kinetic energy. A similar argument follows on from the previous accident, the force of the accident will be determined mainly by the amount of original kinetic energy and (of course) the rate at which the car absorbs it. Now if you wanted to have equivalence between the two accident set-ups (ie car-car, and car-wall) in terms of 'destructiveness' you would merely need to work out the amount of kinetic energy you want the car to absorb (and convert). In terms of kinetic energy only ONE car is involved in the equation of the car-wall accident, so we have half the mass of the two-car collision. The mass of the wall? Not important, it's velocity is zero - thus it has no kinetic energy. So if you crunch the numbers, you will find that you have to times the velocity by 1.404 (the square root of two) in order to come up with an equivalent level of kinetic energy for an accident with half the mass. Now - finally - you are mostly going wrong with your interpretation of relative velocity. If you are travelling at 50km/h and someone is coming in the opposite direction also travelling at 50km/h, then yes, you are approaching them at 100km/h, but they are also approaching you at 100km/h! So when you 'collide' you are trying to say that there is suddenly 2 * accidents that are 100km/h? Definitely not! There is only 2 * 50km/h accidents. [Report message to a moderator]

	Subject	Poster	Date
	Physics problem.	Squid	Thu, 24 March 2005 03:43
	Re: Physics problem.	Corona RT142	Thu, 24 March 2005 03:46
	Re: Physics problem.	Squid	Thu, 24 March 2005 03:50
	Re: Physics problem.	Corona RT142	Thu, 24 March 2005 03:50
	Re: Physics problem.	Norbie	Thu, 24 March 2005 03:51
	Re: Physics problem.	Squid	Thu, 24 March 2005 03:58
	Re: Physics problem.	Indi	Thu, 24 March 2005 04:28
	Re: Physics problem.	STR8 2.8	Thu, 24 March 2005 04:32
	Re: Physics problem.	86DRFT	Thu, 24 March 2005 12:27
	Re: Physics problem.	Norbie	Fri, 25 March 2005 07:14
	Re: Physics problem.	Intensevil	Thu, 24 March 2005 04:25
	Re: Physics problem.	Corona RT142	Thu, 24 March 2005 04:31
	Re: Physics problem.	Squid	Thu, 24 March 2005 04:34
	Re: Physics problem.	Indi	Thu, 24 March 2005 04:42
	Re: Physics problem.	BlackMR2	Thu, 24 March 2005 04:32
	Re: Physics problem.	white86	Thu, 24 March 2005 04:44
	Re: Physics problem.	white86	Thu, 24 March 2005 04:51
	Re: Physics problem.	Mookie	Thu, 24 March 2005 04:58
	Re: Physics problem.	bbaacchhyy	Thu, 24 March 2005 05:08
	Re: Physics problem.	IRA11Y	Thu, 24 March 2005 05:39
	Re: Physics problem.	Dust	Thu, 24 March 2005 05:34
	Re: Physics problem.	Norbie	Thu, 24 March 2005 05:39
	Re: Physics problem.	skellator	Thu, 24 March 2005 05:57
	Re: Physics problem.	BlackMR2	Thu, 24 March 2005 05:58
	Re: Physics problem.	IRA11Y	Thu, 24 March 2005 06:04
	Re: Physics problem.	IRA11Y	Thu, 24 March 2005 06:07
	Re: Physics problem.	EnFlaMEd	Thu, 24 March 2005 06:33
	Re: Physics problem.	SPRINTAH!	Thu, 24 March 2005 06:30
	Re: Physics problem.	skellator	Thu, 24 March 2005 06:43
	Re: Physics problem.	SPRINTAH!	Thu, 24 March 2005 07:47
	Re: Physics problem.	IRA11Y	Thu, 24 March 2005 08:13
	Re: Physics problem.	skellator	Thu, 24 March 2005 08:57
	Re: Physics problem.	skellator	Thu, 24 March 2005 06:33
	Re: Physics problem.	RWDboy	Thu, 24 March 2005 06:42
	Re: Physics problem.	skellator	Thu, 24 March 2005 06:54
	Re: Physics problem.	RWDboy	Thu, 24 March 2005 06:49
	Re: Physics problem.	RWDboy	Thu, 24 March 2005 07:20
	Re: Physics problem.	skellator	Thu, 24 March 2005 07:44
	Re: Physics problem.	RWDboy	Thu, 24 March 2005 13:42
	Re: Physics problem.	CrUZsida	Thu, 24 March 2005 08:39
	Re: Physics problem.	CrUZsida	Thu, 24 March 2005 08:59
	Re: Physics problem.	NeiGH	Thu, 24 March 2005 09:51
	Re: Physics problem.	skellator	Thu, 24 March 2005 10:01
	Re: Physics problem.	Alex120	Fri, 25 March 2005 11:49
	Re: Physics problem.	86DRFT	Thu, 24 March 2005 12:22
	Re: Physics problem.	skellator	Fri, 25 March 2005 01:42
	Re: Physics problem.	RWDboy	Fri, 25 March 2005 08:16
	Re: Physics problem.	b1gb3n	Fri, 25 March 2005 02:37
	Re: Physics problem.	skellator	Fri, 25 March 2005 08:40
	Re: Physics problem.	RWDboy	Fri, 25 March 2005 09:10
	Re: Physics problem.	skellator	Fri, 25 March 2005 09:22
	Re: Physics problem.	gabe	Sat, 26 March 2005 00:46
	Re: Physics problem.	skellator	Sat, 26 March 2005 00:58
	Re: Physics problem.	RobertoX	Wed, 30 March 2005 05:40
	Re: Physics problem.	skellator	Wed, 30 March 2005 06:35
	Re: Physics problem.	RWDboy	Fri, 25 March 2005 10:42
	Re: Physics problem.	Alex120	Fri, 25 March 2005 11:45
	Re: Physics problem.	RWDboy	Fri, 25 March 2005 12:40
	Re: Physics problem.	skellator	Fri, 25 March 2005 22:50
	Re: Physics problem.	RWDboy	Sat, 26 March 2005 01:47
	Re: Physics problem.	skellator	Sat, 26 March 2005 02:16
	Re: Physics problem.	gabe	Sat, 26 March 2005 02:59
	Re: Physics problem.	RWDboy	Sat, 26 March 2005 01:53
	Re: Physics problem.	RWDboy	Sat, 26 March 2005 03:21
	Re: Physics problem.	IRA11Y	Sat, 26 March 2005 03:29
	Re: Physics problem.	Squid	Tue, 29 March 2005 03:48
	Re: Physics problem.	RWDboy	Sat, 26 March 2005 03:29
	Re: Physics problem.	RWDboy	Sat, 26 March 2005 03:33
	Re: Physics problem.	skellator	Sat, 26 March 2005 03:41
	Re: Physics problem.	RWDboy	Sat, 26 March 2005 04:13
	Re: Physics problem.	skellator	Sat, 26 March 2005 04:24
	Re: Physics problem.	RWDboy	Sat, 26 March 2005 04:47
	Re: Physics problem.	IRA11Y	Sat, 26 March 2005 05:04
	Re: Physics problem.	skellator	Sat, 26 March 2005 05:26
	Re: Physics problem.	skellator	Sat, 26 March 2005 06:06
	Re: Physics problem.	tom210	Sat, 26 March 2005 06:52
	Re: Physics problem.	skellator	Sat, 26 March 2005 06:56
	Re: Physics problem.	RWDboy	Sat, 26 March 2005 07:02
	Re: Physics problem.	skellator	Sat, 26 March 2005 07:05
	Re: Physics problem.	tom210	Sat, 26 March 2005 07:19
	Re: Physics problem.	jackel	Sat, 26 March 2005 07:23
	Re: Physics problem.	skellator	Sat, 26 March 2005 07:39
	Re: Physics problem.	tom210	Sat, 26 March 2005 07:28
	Re: Physics problem.	Norbie	Sat, 26 March 2005 09:47
	Re: Physics problem.	skellator	Sat, 26 March 2005 10:10
	Re: Physics problem.	Norbie	Sat, 26 March 2005 10:12
	Re: Physics problem.	skellator	Sat, 26 March 2005 10:21
	Re: Physics problem.	EldarO	Sat, 26 March 2005 11:37
	Re: Physics problem.	Norbie	Sun, 27 March 2005 02:02
	Re: Physics problem.	EldarO	Tue, 29 March 2005 16:41
	Re: Physics problem.	tom210	Sat, 26 March 2005 12:46
	Re: Physics problem.	RWDboy	Sat, 26 March 2005 12:53
	Re: Physics problem.	skellator	Sun, 27 March 2005 02:36
	Re: Physics problem.	RWDboy	Sun, 27 March 2005 03:15
	Re: Physics problem.	RWDboy	Tue, 29 March 2005 04:41
	Re: Physics problem.	skellator	Tue, 29 March 2005 05:50
	Re: Physics problem.	RWDboy	Tue, 29 March 2005 12:07
	Re: Physics problem.	skellator	Tue, 29 March 2005 21:05
	Re: Physics problem.	79rollaboy	Tue, 29 March 2005 21:20
	Re: Physics problem.	RobertoX	Wed, 30 March 2005 00:19
	Re: Physics problem.	RWDboy	Wed, 30 March 2005 00:42
	Re: Physics problem.	skellator	Wed, 30 March 2005 04:40
	Re: Physics problem.	RobertoX	Wed, 30 March 2005 04:49
	Re: Physics problem.	skellator	Wed, 30 March 2005 04:57
	Re: Physics problem.	RWDboy	Wed, 30 March 2005 13:47

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