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Old 18 September 2007, 03:57   #21
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Volvo 940 with 2 over 60's in it.... when have you ever seen one of these going over 55mph... bet they where in the middle lane too.
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Old 18 September 2007, 08:50   #22
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Quote:
Originally Posted by alexm View Post
It might be true to say that from the perspective of one car the impact is equivalent to hitting an immovable mass at 70 mph however in the example of two cars each travelling at 70mph the relative closing speed IS 140 mph... that is to say that two cars heading towards each other at 70mph each will create the same impact as one car travelling at 140mph hitting the other car when it is stationary.

No no no!!!

Think about it - you hit a concrete block at 70mph - you stop dead. You hit another car both doing 70mph - you stop dead. Now if you are doing 70mph and the car you hit is doing 80mph then you go backwards at 10mph.

It's called an inelestic collision. Lets assume a car has 20,000 units of energy at 70mph. When it stops it has 0 units of energy. It loses 20,000 units very rapidly. The other car has 20,000 units of energy - it also loses all it's energy. 20,000 hitting 20,000 = 0. If the car was doing 140mph it would have 40,000 units of energy.
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Old 18 September 2007, 09:23   #23
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Police closing motorways again

--------------------------------------------------------------------------------

http://news.bbc.co.uk/1/hi/wales/6998191.stm

This was an extremely nasty accident - a car drove the wrong way down the motorway and hit another car head on - 5 dead.

The Police were chasing the Volvo before it turned the wrong way down the motorway - the Police then stopped the chase - bit late methinks. 2 innocent people in a Mondeo killed.


Codprawn,
try reading the bbc report again and see if you got the story right.
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Old 18 September 2007, 09:29   #24
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Police closing motorways again

--------------------------------------------------------------------------------

http://news.bbc.co.uk/1/hi/wales/6998191.stm

This was an extremely nasty accident - a car drove the wrong way down the motorway and hit another car head on - 5 dead.

The Police were chasing the Volvo before it turned the wrong way down the motorway - the Police then stopped the chase - bit late methinks. 2 innocent people in a Mondeo killed.


Codprawn,
try reading the bbc report again and see if you got the story right.
Initially they did say it was the volvo they were chasing - now it turns out it was the Mondeo.
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Old 18 September 2007, 10:15   #25
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Originally Posted by codprawn View Post
No no no!!!

Think about it - you hit a concrete block at 70mph - you stop dead. You hit another car both doing 70mph - you stop dead. Now if you are doing 70mph and the car you hit is doing 80mph then you go backwards at 10mph.

It's called an inelestic collision. Lets assume a car has 20,000 units of energy at 70mph. When it stops it has 0 units of energy. It loses 20,000 units very rapidly. The other car has 20,000 units of energy - it also loses all it's energy. 20,000 hitting 20,000 = 0. If the car was doing 140mph it would have 40,000 units of energy.
Yes yes yes!!!

Actually if it was doing twice the speed it would have FOUR times the energy as kinetic energy is proportional to the square of the velocity, however... I think we need some proofs!... If you don't like physics you should look away now...




Let's make a few assumptions:

1) Car 1 is identical to Car 2 and each does 10m per second and weighs 1000kg...
2) The collision is perfectly inelastic, i.e. they do not 'bounce' apart, rather they crumple and 'stick' together

Scenario 1) Car 1 hits Car 2, both are doing 10 m/s toward each other

Total kinetic energy = 2 x (0.5 x 1000 x 10^2) = 100kJ

As the cars are identical and the forces in any collision are equal and opposite (Newton III) the energy absorbed by each car will be the same. We can also summise that due to conservation of momentum that the two cars will be stationary after the collision. Hence each car will have to absorb 50kJ of energy.



Scenario 2) Car 1 is doing 20m/s and car 2 is stationary

Total kinetic energy = 0.5 x 1000 x 20^2 = 200kJ

This is double the kinetic energy of scenario 1.... BUT conservation of momentum means that after the collision the mangled ball of wreckage of the two cars will still be moving!

Momentum before = momentum after

1000 x 20 = 2000 x velocity_after :. velocity_after = 10m/s

So the kinetic energy of the wreckage = 0.5 x 2000 x 10^2 = 100kJ

So, the energy absorbed by both vehicles in the crash = total kinetic energy before - kinetic energy of the wreckage

200kJ - 100kJ = 100kJ, so the energy absorbed by each car is still 50kJ



Scenario 3) Car 1 is doing 10m/s and hits a brick wall

This situation is entirely different as the wall will absorb no energy and, like scenario 1, the car is going to come to a complete halt so it will have to absorb all the energy.

Total energy = 0.5 x 1000 x 10^2 = 50kJ

The car must still absorb 50kJ of energy


So I say again... two cars heading towards each other at 70mph each will create the same impact as one car travelling at 140mph hitting the other car when it is stationary.
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Old 18 September 2007, 10:33   #26
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but what if cods was in a 260 tonne marble bath at 70mph
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Old 18 September 2007, 15:04   #27
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Quote:
Originally Posted by alexm View Post
Yes yes yes!!!

Actually if it was doing twice the speed it would have FOUR times the energy as kinetic energy is proportional to the square of the velocity, however... I think we need some proofs!... If you don't like physics you should look away now...




Let's make a few assumptions:

1) Car 1 is identical to Car 2 and each does 10m per second and weighs 1000kg...
2) The collision is perfectly inelastic, i.e. they do not 'bounce' apart, rather they crumple and 'stick' together

Scenario 1) Car 1 hits Car 2, both are doing 10 m/s toward each other

Total kinetic energy = 2 x (0.5 x 1000 x 10^2) = 100kJ

As the cars are identical and the forces in any collision are equal and opposite (Newton III) the energy absorbed by each car will be the same. We can also summise that due to conservation of momentum that the two cars will be stationary after the collision. Hence each car will have to absorb 50kJ of energy.


Scenario 3) Car 1 is doing 10m/s and hits a brick wall

This situation is entirely different as the wall will absorb no energy and, like scenario 1, the car is going to come to a complete halt so it will have to absorb all the energy.

Total energy = 0.5 x 1000 x 10^2 = 50kJ

The car must still absorb 50kJ of energy


So I say again... two cars heading towards each other at 70mph each will create the same impact as one car travelling at 140mph hitting the other car when it is stationary.

Nothing wrong with your maths - you have just PROVED I was right!!! You say a car colliding head on absorbs 50kj - you also say one hitting a concrete block will absorb 50kj - that's the SAME amount of energy - if the car was travelling at 20m/s and hit a concrete block the energy absorbed would be much higher!!!
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Old 18 September 2007, 15:34   #28
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Originally Posted by codprawn View Post
Nothing wrong with your maths - you have just PROVED I was right!!!


I think what we have here... is failure to communicate!

Ed2 said:

Quote:
Originally Posted by ed2 View Post
Two cars hitting head on. Both probably doing around (at least) 70mph. Therefore probably 140mph at least impact..
You said:

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Originally Posted by codprawn View Post
Thats a very common misconception. The impact is NOT 140mph.

I said:

Quote:
Originally Posted by alexm View Post
in the example of two cars each travelling at 70mph the relative closing speed IS 140 mph... that is to say that two cars heading towards each other at 70mph each will create the same impact as one car travelling at 140mph hitting the other car when it is stationary.
You said:

Quote:
Originally Posted by codprawn View Post
No no no!!!
My proof shows without a shadow of a doubt that two cars each travelling at 70mph will suffer the same impact as one car travelling at 140mph and hitting a stationary car which is what it appeared you were 'no no no'ing at!

I never had any issue with your concrete block statement.
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Old 18 September 2007, 16:07   #29
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Please do not prove codders wrong. It would be the first time in his life and he may have a heart attack; and you would be responsible!!!!!
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Old 18 September 2007, 16:29   #30
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When I am wrong I always admit it - there has been the very rare occasion - the reason being I make sure of my facts BEFORE I post anything!!!
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