Safe Speed Forums

Sorry, but I'm not sure what forum this belongs on...

I remember from my O-Level physics class at school that M1*V1 = M2*V2 or in other words if I hit a car on my pushbike my energy wont move the car very much but if he hits me I'll move quite a lot.

So then, if the destructive power of an object is a product of the mass and velocity you could argue that lighter transport is safer to go faster on our roads. If you expand on this further you could have a sliding scale so for instance, a Smart car should be allowed to go faster than a big 4x4 with bull bars.

Many a truth spoken in jest. I assume this is why lorries have speed limiters and I get stuck on the outside of the M56 for ten minutes while Mr55.1mph is trying to overtake Mr55mph. Grrrr

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

My bold

This is a non sequiter. Destructive power is related to the rate of change of velocity, not the velocity itself. Also, your hypothesis can only work for a 'total' collision - for any other tpe of collision, you need to be able to define the degrees of mass involved.

The reasons for lower speed limits on HGV is more related to overcoming the inertial dynamics of stopping (and steering) a high mass vehicle.

IOW, for any given velocity, the force required on a mass to reduce the velocity to any lower value is directly proportional to mass.

I beg to differ...

If a HGV is travelling at a constant 55mph when it ploughs into the back of stationary traffic that energy is transferred, with devastating effect, into many cars.

This has happened on our motorways when the driver has fell asleep. Velocity is speed with a direction, not acceleration.

If a car does the same to a line of HGVs, the first HGV will hardly move.

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

This is (simply put ) due to the fact that a 44t truck will release 44 times more energy at a set speed than a 1t car.

M1=44T, V1 = 44FT/SEC
M2=1T,V2=44FT/SEC.

iN ITS SIMPLEST FORM, neglecting any energy used to move the stationary vehicles.

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lets bring sanity back to speed limits.
Drivers are like donkeys -they respond best to a carrot, not a stick .Road safety experts are like Asses - best kept covered up ,or sat on

This is conservation of momentum, although in itself probably isn't the best indicator of destructive energy.
The thing I would be worried about is the rate of change of the MV of the people involved.
It could be argued that a light bike should be allowed to go really fast as the damage it will inflict upon others (in vehicles) would have been less, but when the rider wipes out (for whatever reason) the lightness of the bike won’t matter to the survivability of the rider.

There is an old but related thread here:
Mass x Speed kills not just speed

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

I meant it just as a bit of light-hearted banter but with an element of truth although maybe I've stumbled on something.

I understand the point about stopping heavier vehicles but it is also true that if the same vehicle were to go at 70mph the energy and potential destructive force will increase exponentially.
It ties in with Einstein's famous E=MC squared. An object as small as an atom at the speed of light has the power of something larger but which is travelling slower. This also holds true for transport.

Help me Paul, I'm drowning

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

This is one that's been debated in the past on the old Cumbria Safety Camera Partnership board (as a result of them getting all zealous about making HGVs do 40 on single carriageway NSL roads)!

The M1V1 and M2V2 bit only works where momentum is conserved (like two snooker balls) If the snooker balls have crumple zones, it all gets more complicated. It is, nevertheless true that the heavier something is and the faster it goes, the more destructive power it has.

It's probably better to look at kinetic energy rather then momentum (1/2MV^2) for destructive power. So it goes up with the square of the velocity but only lineraly with the vehicle's weight. The Pro-scamera faction liked this idea...

...until we did the sums to show that a 40 ton artic at 56MPH had about the same kinetic energy as a 1 ton TVR at about 300MPH!

Thanks guys. I think this is all very interesting.

It's the relationship between weight and speed I was focusing on and the power which increases exponentially with speed. This is why they say little Johnny has an exponentially greater chance of living from being knocked down at 30mph than at 35mph, alegedly.

That said, we have recently seen a girl, in Rehab, who was hit by a bus at 50mph!

Needless to say I can't name names but I'm very pleased to say she's doing well and has gone home

Figure that one out if you will

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

I don't think it's exponential - I think it follows a "squared" relationship. Also how did anyone know the girl was hit by the bus at 50? Obviously, I'm glad she's getting better but I am always very wary of figures like that.

There was an item on the BBC 'Trauma' series, where they shadowed the London Air Ambulance, when the Dr. arrived on-scene at a car Vs motorcycle interface situation (can't remember, are they 'accidents, incidents, crashes, or whatever these days? ).

"Always a bad sign when the vehicles and the rider are in close proximity" or words to . . .

As the energy involved has to be absorbed somewhere - ie the rider

Yes this for for perfectly eleastic collisions.

The full equation for conservation of momentum is M1*U1 + M2*U2 = M1*V1 + M2*V2.

In road collisions, where there are crumple zones, usually the 2 vehicles "join together", ie a perfectly plastic collision, and travel at the same speed.

So if a vehical ploughs into a vehical at rest.
M1*U = (M1 + M2 )V

This means that with crumple zones a 44t lorry doesn't shoot a 1t car off at 44 times the speed, but both continue off at 44/45 the speed. Unfortunately this is quite a large acceleration but tiny for the lorry, so it goes to show that it's always safer to be in the heavier vehicle.


Watch out using the word exponentially. This implies that for the same increase each time, the other factor doubles. EG 10mph, energy = 1x, 20mph energy 2x, 30mph energy 4x.

This is actually a squared relationship. 10mph = 1x 20mph = 4x, 30mph = 9x, 40mph = 16x

Eh? Exponential means that the exponent is constant, so

n^x

e.g.

1^2=1
2^2=4
3^2=9

'square law' is thus a specific case of exponential.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

But which occupant is safer if they both hit something at a given speed?

Depends at least partly on what they hit. Into a bridge parapet, the 4x4 with its separate chassis might not crumple as much and might therefore give the ocupants a big deceleration. Obviously if it's going fast enough, the Smart car will crumple too much and squash them.

Not sure about that. You could easily be right and I have to admit that my memory is hazy here but I always thought "exponential" meant "logarithmic" - so, unlike a squared (or higher power) expression, the value climbs very steeply but then never gets beyond a particular value. I converges on it increasingly slowly but wouldn't get there until it reached infinity.

???

What's "U"?

When she was 12 years old, one of my cousins was "hit" by a van at 40 mph, and escaped with concussion, cuts and bruising.
What actually happened was she ran across the road without looking, and ran into the side of the van wing, and was flung back across the road.

Year before last, a boy at my son's school ran across a zebra crossing from behind a stationary van, and a woman driver passing the van struck him square on, and he was flung thirty feet, but escaped with just bruises. Is this because their lightness (relatively) causes them to be propelled so easily that their body yields to the force, rather than resisting it?

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Time to take responsibility for our actions.. and don't be afraid of speaking out!

I'm no expert in accident mechanics - although I'd love to know more because it interests me greatly! I think it's true to say that it is very complex and depends on so many factors. Younger, healthy people generally recover better from any kind of trauma than older ones - just because they're younger. The precise impact characteristics play a huge part too. Dpending on the shape of the impacting vehicle and the size / weight / weight distribution of the pedestrian, the outcomes can be very different. The "pedestrian friendliness" of the car's nose obviously plays a big part too.

I'm assuming the woman was in an ordinary car? I'm also making assumptions here about the boy but if the initial point of contact between him and the car occurs above his centre of gravity, he's likely to go under it. Big 4x4s are a problem in this respect - especially if they have bull bars and the shape of the bull bars is such that they make the point of contact higher up than it might have been without them. If the initial point of contact was below his centre of gravity, it is more likely that he would have been scooped up on to the bonnet. What happens next depends on a variety of factors. Depending on his height, the most important thing is that his head might hit something hard. If he's tall enough and the car is low enough and / or has a short enough bonnet, his head might hit the base of the windscreen (which is generally bad news). If he's luckier, it might hit a "soft" area of bonnet towards the back of the engine bay so that there isn't anything hard immediately underneath it. That's obviously much better. Depending on the geometry of everything, it's possible, I guess, that he'd get a "ride" on the bonnet for a number of yards while the car comes to a halt and then roll off. This might possibly account for the 30 feet.

Then, of course, there are the real freak accidents where he goes right over the top of the car without his head hitting any of it and lands on the road behind it - or any number of other "odd" scenarios that he probably wouldn't be able to replicate if he got run over another 100 times! (although he should have learned his lesson by now)!

One last bit of trivia I could throw in is that (apparently) most serious head injuries to pedestriants are inflicted when they hit the rod or pavement with their head and NOT by the car. I was told this at an accident investigator's seminar at Loughborough university once so I guess they knew wha tthey were on about!

Initial velocity, in this case the speed of the lorry about to plough and crumple into our stationary test car.



The base (n is constant) x is the variable.

EG if a car loses 10% every year:

value of car = initial value - 0.9^x
where x is the number of years.

This book is just what you want then ! And most is in imperial !