On a typical A road, cars usually travel 60MPH or above, and frequently they pass quite close. Say 2 cars travelling on a road like this were doing 60 MPH and they crashed head-on (ie. 120MPH impact). If all passengers/drivers were wearing seatbelts, what do you think the chances of survival are for them ?

 

this is a serious question.

True. Not like hitting a brick wall at all. It's like hitting the other car! So in your example, for the Fiesta it's like hitting a truck and for the truck it's like hitting a Fiesta, oddly enough!

 

Which (and it's still paining me to say this so so so much) is what Nelson said originally.

Originally posted by andyduff

Two cars travelling towards each other do not have the same energy as one travelling at the combined speed. It is the energy that must be dissipated in a crash situation and this is what crumple zones are designed to do.

 

Energy=1/2xMASSxVELOCITYxVELOCITY - in other words velocity squared.

 

So your energy goes up with the square of the speed.

 

Say each car weighed 1 ton=1000kg. The velocity must be in m/s. 60mph is 25m/s.

 

Each car has 0.5x1000x25x25=312500 joules. Add these together and you get a total energy of 625000 Joules to be dissipated.

 

Now, if it was one car travelling at 120mph, the total energy would be 0.5x1000x50x50=1250,000 Joules.

Exactly double the energy.

 

So combined speeds in head on crashes are complete b0ll0x I'm afraid.

 

CheerZ,

 

Andy

 

 

Andy - this seems to make sense, but so does Duffy's argument. Can someone please shed light on this problems (yet again I've caused a commotion !;) :D )

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Think I've cracked it - the energy of 1 car is not equivalent to the energy of 2 cars - ie. there is half the MASS !

 

Because E = m (v squared), energy is not proportional to speed. Twice as fast has more than double the energy.

I think it was a coincidence that your equation (andyduff) was exactly half/double.

 

A 60-60 head on crash is equivalent to a 120MPH head-on into a stationary car. The actual result of the crash will probably be different tho', due to the cars being at different states (I think:confused: )

Originally posted by Mike Duffy

Nelson is right here unfortunately.

 

Steady on old boy - anyone would think you didn't like me !

The reason it was double is simple. What is 2 squared? 4! Ok we'd already doubled the speed but since this doubling is squared, the final answer is the original multiplied by 4 ;) I know what I mean lol.

Originally posted by andyduff

The reason it was double is simple. What is 2 squared? 4! Ok we'd already doubled the speed but since this doubling is squared, the final answer is the original multiplied by 4 ;) I know what I mean lol.

 

CORRECT.

By doubling the speed, you mulitplied the energy by 4 (V squared). Because you had halved the mass (2 cars became 1 - cue Spice Girls song!) the answer became TWICE (4/2) the original one. It should have been 4 times though.

Originally posted by b19rks

Got to agree with duffer on this one, you say its like hitting a brick wall at Xmph, but this is not true.

 

Say you are in a fiesta doing 50 mph and hit a truck doing 50mph for the truck it would be more like hitting a cardboard wall at 100mph, and for th fiesta more like hitting a lead wall at 100mph.

 

When two bodies collide and coalesce the resulting trajectory are governed by their momentum.

 

Momentum is a vector, equal to mass * velocity.

 

Two object of equal mass travelling at the same speed but meeting head on, and not bouncing off each other, will result in a single mass that is stationary. Both objects will have decelorated by the same amount.

 

Two objects of equal mass, one stationary and the other going twice as fast as the objects in the previous example, meet head on, and not bouncing off each other, will result in a single mass that travels at half the speed of the moving object, in the same direction as the moving object.

 

Note the moving object has decelorated by exactly the same amount as the previous case, and the stationary object also has decelorated by exactly the same amount as the previous case.

 

Note: acceloration is simply deceloration in the oposite direction.

 

Sorry but Andy is definitely wrong, (but will he admit it?)

The grammar may be terrible but you can't fault the man's physics! Couldn't have put it better myself.

My other hobby is sitting alone in the DARK!!!!:D

Sorry my other hobby is Research Chemistry

Originally posted by Zimon

When two bodies collide and coalesce the resulting trajectory are governed by their momentum.

 

Momentum is a vector, equal to mass * velocity.

 

Two object of equal mass travelling at the same speed but meeting head on, and not bouncing off each other, will result in a single mass that is stationary. Both objects will have decelorated by the same amount.

 

Two objects of equal mass, one stationary and the other going twice as fast as the objects in the previous example, meet head on, and not bouncing off each other, will result in a single mass that travels at half the speed of the moving object, in the same direction as the moving object.

 

Note the moving object has decelorated by exactly the same amount as the previous case, and the stationary object also has decelorated by exactly the same amount as the previous case.

 

Note: acceloration is simply deceloration in the oposite direction.

 

Sorry but Andy is definitely wrong, (but will he admit it?)

 

Zimon

 

Although I totally agree with your explaination, I think Andy was also trying to expain the resulting force of impact.

 

In simple terms, this is how I understand it....god 'elp me.

 

If a car travelling at 50mph hits a stationary car (same model of car, equal mass etc) we can safely say the the result will be a "50mph" impact.

 

If 2 cars both travelling at 50mph (same cars again) hit head on then the result is a "200mph" as oposed to a "100mph" impact.

 

Please correct me is this is wrong..

 

Andy

Ok. It's wrong.

Nicely explained there, Mike. ;)

As Mike Duff said this is wrong BUT in your first sentence you ask to explain the resulting force, this is shall do.

 

Force is Mass * acceleration

 

Since the mass of the two cars in both situations is constant and equal, (I am ignoring the bits of the car that fall off!), then the force is directly proportional to the deceleration.

 

In the first scenario the car hits a brick wall and decelerates from 50 mph to a complete stop.

 

In the second scenario the car hits another car and again decelerates from 50 mph to a complete stop.

 

Therefore the two situations are equivalent and so someone in the car will in fact be subjected to exactly the same force, whether they hit a brick wall or another car!

 

Oh and Mike D I not intoxicated at the moment and so I think I have got the grammar and spelling right. Watch out I am after your crown.

:p

Hey Zimon mate! You never saw my posts when I was paralytic did you? Don't worry, I've never seen anyone degenerate to that sad state of affairs under the influence, so my crown's badly in need of some polishing.

Originally posted by Zimon

I not intoxicated

 

:o This should have been "I'm not intoxicated", so it looks like your crown is safe - I'm just going to have to try a lot harder.;)

Are you quoting yourself Zimon?! Isn't that a bit like masturbation?? :D :eek: :D

" " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " "

 

Done.

'

Done

Just caught up with this one, I know its a bit late, but totally agree with Zimon on this one, the two cases give an equivalent impusle, which is the product of a force and the time for which it acts, i.e. impulse = change in momentum ( 50mph to zero)

 

momentum is mass X velocity, as has already been said, therefore for equal mass and speed the impulse must be the same for both cases.

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Ok had a few long drives to work this week and had a chance to think about this. IF the other car was a solid object with infinite mass, then closing speed would be appropriate, BUT it isn't, so you can't. If two cars hit each other at the same time it is more like hitting a wall at that speed, not the combined speed.

You're not going to be persuaded on this one are you mate?!

 

To get your head round it you have to stop thinking of speed as being measured relative to the Earth. You can measure a velocity relative to any other point. So you could measure the velocity of each car relative to the Earth, or you could measure the velocity of one car relative to the other. (Disclaimer: I know this doesn't take into account frictional factors and wind resistance but you physicists out there know where I'm coming from!)

 

I'm bowing out of this one now anyway in case anyway starts calling me names!

Originally posted by andyduff

Ok had a few long drives to work this week and had a chance to think about this. IF the other car was a solid object with infinite mass, then closing speed would be appropriate, BUT it isn't, so you can't. If two cars hit each other at the same time it is more like hitting a wall at that speed, not the combined speed.

 

I love the concept of someting of infinite mass moving, infinite mass = infinite gravity - therefore nothing could move.

 

I love the phrase "if two cars hit each other at the same time" - I would like to see them hit each other at different times.

 

I'm sorry andy but you are just plain wrong.

 

Have you ever been in a train stopped at a station with another train next to you. You look at the other train and you see movement - and just for a moment you cannot tell if the train you are in is moving or the other train is moving. Well the only way to tell is to look at another frame of reference - the earth, and then you can tell what is happening.

 

All velocities are relative to a frame of reference, we tend to think of the earth as stationary, but of course it is not, when we stand still all that is happenning is that we have the same velocity around the sun / galaxy / universe as the earth under our feet. The relative velosity is zero so we think we are still - but we are not relative to another frame of reference.

 

But all these words will not convince you. If I get a little time I will put together some calculations to explain what is going on - but life is a bit busy at the moment so do not hold your breath!

anyone see 5th gear last night? They crashed 2 cars head on at 60.

That must have smarted a bit.