Video: 50mph BMW 535i vs. Volvo 960

[DueyT]

Ouch!

Interestingly, if the vehicles have the same mass, then conservation of energy and momentum says this would be the same as slamming into a stationary wall at 60mph as well... Neither is a very interesting prospect! =:-o

Duey

[shortbuss2336]

thats a cool video, one of my favorites!

[Craig]

Ouch!

Interestingly, if the vehicles have the same mass, then conservation of energy and momentum says this would be the same as slamming into a stationary wall at 60mph as well

I think you mean 120mph. Even worse.

[DueyT]

I think you mean 120mph. Even worse.

No, 60 mph relative to the inertial frame of reference within wich the vehicles are being considered. While it is true that their closing speed relative to each other is 120 mph, this is not at all equivalent to one vehicle standing still, and the other hitting it at 120 mph.

Conservation of energy proves this. Let's say v (velocity) = 60 mph, and both cars equally weigh mass, m. Total kinetic energy just before collision is:

Car 1: K.E. = 1/2 m * v^2
Car 2: K.E. = 1/2 m * v^2
Total: K.E. = m*v^2

If someone said that's like one car hitting a stationary car at 120mph (2 x 60mph or 2v) the energies would be:

Car 1: K.E. = 1/2 m * (0)^2 = 0
Car 2: K.E. = 1/2 m * (2v)^2 = 1/2 m * 4v^2 = 2m*v^2
Total: K.E. = 2m*v^2

m*v^2 does not equal 2m*v^2, so the 120mph effective impact (Energy wise) is not a valid construct.

To make the conversation more intersting, when do you steer for a wall and when do you still hit the oncoming car?

Conservation of momentum (product of mass times velocity, mv...not kinetic energy, 1/2mv2) makes us consider the head on vice wall crash carefully. If you have greater momentum than the other car, then you are better off hitting it, than a concrete wall. Your greater momentum means you will slow down less than the other vehicle...which would actually reverse direction of travel. The difference may be slight, for example, say your car weighs 3500lbs and the oncoming vehicle weighs 2500 lbs. Calculating conservation of momentem, you would slow from 60 to 10mph, a -50 mph change, while the other guy would slow from 60 to 0 then be pushed backwards by you at 10mph opposite to his original travel...overall change of 70mph vs. your 50mph change...ouch!

Now, if the vehicles have the same mass... then both come to 0 mph...exactly as if you had hit a wall (interesting!)

OK, what if you have less mass (against say, a Brinks armoured truck full of coin!)...now you're heading backwards and you would have been literally better off hitting a brick wall.

Interestingly enough, a wall (or other unyielding structure) is actually a fully variable force structure...if strong enough (so it's structuraly integrity is not compromised) it will "push back" against the car with "just enough" force to counter the car's energy...perfectly! No less (or it would yield, crumble, chip, etc... and the car would continue forward) and no more (otherwise the wall would actually "push" the car backwards after the collision.)

Food for thought...


Cheers,
Duey

[Brian C.]

Luckily I just take photos at the crashes I go to. The Traffic Investigation Guys are the ones that do all sorts of skid and vehicle weight calculations. There's actually one old guy who's been doing it for 30 years who still uses a slide rule! Hey.....if it works....

Brian C.

[Hector]

I remember COE and COM being shoved down my throat cuz I think the prof had a relative who analyzed distance traveled of totaled cars in traffic accidents and such. So we ate this stuff 3 times a week for a month!! I can still taste it :o)

No, 60 mph relative to the inertial frame of reference within wich the vehicles are being considered. While it is true that their closing speed relative to each other is 120 mph, this is not at all equivalent to one vehicle standing still, and the other hitting it at 120 mph.

Conservation of energy proves this. Let's say v (velocity) = 60 mph, and both cars equally weigh mass, m. Total kinetic energy just before collision is:

Car 1: K.E. = 1/2 m * v^2
Car 2: K.E. = 1/2 m * v^2
Total: K.E. = m*v^2

If someone said that's like one car hitting a stationary car at 120mph (2 x 60mph or 2v) the energies would be:

Car 1: K.E. = 1/2 m * (0)^2 = 0
Car 2: K.E. = 1/2 m * (2v)^2 = 1/2 m * 4v^2 = 2m*v^2
Total: K.E. = 2m*v^2

m*v^2 does not equal 2m*v^2, so the 120mph effective impact (Energy wise) is not a valid construct.

To make the conversation more intersting, when do you steer for a wall and when do you still hit the oncoming car?

Conservation of momentum (product of mass times velocity, mv...not kinetic energy, 1/2mv2) makes us consider the head on vice wall crash carefully. If you have greater momentum than the other car, then you are better off hitting it, than a concrete wall. Your greater momentum means you will slow down less than the other vehicle...which would actually reverse direction of travel. The difference may be slight, for example, say your car weighs 3500lbs and the oncoming vehicle weighs 2500 lbs. Calculating conservation of momentem, you would slow from 60 to 10mph, a -50 mph change, while the other guy would slow from 60 to 0 then be pushed backwards by you at 10mph opposite to his original travel...overall change of 70mph vs. your 50mph change...ouch!

Now, if the vehicles have the same mass... then both come to 0 mph...exactly as if you had hit a wall (interesting!)

OK, what if you have less mass (against say, a Brinks armoured truck full of coin!)...now you're heading backwards and you would have been literally better off hitting a brick wall.

Interestingly enough, a wall (or other unyielding structure) is actually a fully variable force structure...if strong enough (so it's structuraly integrity is not compromised) it will "push back" against the car with "just enough" force to counter the car's energy...perfectly! No less (or it would yield, crumble, chip, etc... and the car would continue forward) and no more (otherwise the wall would actually "push" the car backwards after the collision.)

Food for thought...


Cheers,
Duey