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Help requested posted on 7th November 2013:

The weight of "flying objects" when loose in a car

Can anyone confirm the correct ratio of loose objects in a car at a set speed. I.E. do things increase by x30 in a collision? Does a box of tissues really weigh the same as a small dog? We are preparing a factsheet on this topic and would appreciate any assistance.

Maureen Riddle
Wiltshire Council

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Response posted on 7th November 2013 by:
Stuart Howarth
Rochdale Casualty Reduction Team
E: stuart.howarth@rochdale.gov.UK
T: 01706924605

weight ratio

I hope my maths is still correct
Velocity x mass = overall mass
So V 30mph x M 10 stone all coming to a sudden stop = OM 300 stone
So if a box of tissues is 250g then at 30 mph it would weigh 7.5 kilo’s about the size of a small dog


Response posted on 7th November 2013 by:
lianne darbinson
East Riding of Yorkshire Council
E: lianne.darbinson@eastriding.gov.uk
T: 014820395569

mass and momentum

Stuart has worked out the momentum involved :-
momentum = velocity x mass

'damage' is done by the transfer of energy from Kinetic energy (movement energy) into other forms of energy. Kinetic energy is given by

KE= 1/2 mv^2

there is quite a useful link here

http://www.differencebetween.info/difference-between-kinetic-energy-and-momentum

Momentum is conserved in a collision in a closed system, but Kinetic energy is not. Overall energy is conserved, the kinetic enrgy is converted into other forms of energy (eg sound, heat)

email me if you want a more detailed explanation - i'll need to do a bit of swotting up!


Response posted on 13th November 2013 by:
Robert Jones
Calderdale Metropolitan Borough Council
E: robert.jones@calderdale.gov.uk
T: 01422 264360

The weight of "flying objects" when loose in a car

The correct equation is:

Force = Mass x Acceleration (deceleration)

Force is given in newtons, mass in kg, acceleration metres/sec sqd.

It is the deceleration that determines the ultimate force.

A 'slow' speed crash and be just as harmful as a high speed crash depending on how quickly the car comes to rest i.e. decelerates.

As an example an object travelling at 1m/s comes to a dead stop in 1m, is the same as an object travelling at 10m/s coming to a dead stop in 10m.

Forces are equated to the force of gravity, therefore an object thrown forward with the equivalent force of its weight will slow down (decelerate) at 9.81m/s^2.

Consequently a 30g force is equivalent to decelerating at 294m/s^2

Basic assumptions are the vehicle is doing 30mph (48kph)

If it decelerates at 30g it will come to a dead stop in 0.45 seconds over a distance of 0.6m.


Response posted on 20th November 2013 by:
John Bullas
Atkins
E: john.bullas@atkinsglobal.com
T: 07946 317467

30g or bust.....

I passed on your question to a collision investigators forum I co-moderate, here are two replies to date:

I think that for promo purposes, it would be reasonable to state weight x crash velocity gives the rough answer?

Damian's points accepted. (below)

--

When they are flying they "weigh" nothing. It's when they collide with a vehicle that is in itself riding down at 30g that they "weigh" 30x their weight.

There are various presumptions in this statement - one is that the collision between the flying object and the vehicle is plastic - which is to say the box of tissues arrive at the car (or your head) and don't bounce off. The other is that whatever they hit is riding down with the bulk velocity of the car - not a wild presumption but not accurate for a flailing passenger on a moment-by-moment basis, though broadly accurate in aggregate throughout the crash.

If the above doesn't make sense then I haven't said it clearly and will have another go at more length. I'm trying to get out of the habit of writing long emails by default, though..


Response posted on 20th November 2013 by:
John Bullas
Atkins
E: john.bullas@atkinsglobal.com
T: 07946 317467

crush!

The last word:

I can't make sense of that at all!

Broadly speaking, the vehicle has a certain crush stiffness. Let's say it equates to 30g. If you crash at 10mph, you stop at 30g, but it doesn't take as long and crushes the car less. If you crash at 50mph, the car crushes at 30g until all the crush space is used up. Once the space is used up it doesn't really matter about the ride down deceleration because for the occupants, being crushed is being crushed however slowly you do it - refer to medieval history for more on that.

v^2=u^2+2aS, or u^2=2aS if the final velocity is zero and deceleration is defined as positive. The "a" gives the effective weight of the object which was previously flying but has now connected with something. The "S" is how far the vehicle crushes. "a" is a function of the crush stiffness of the car and not the speed.

In terms of the "effective weight of a flying object in the car", actually 30g is under-claiming it because if the path of the object is longer than the crush distance, the car has stopped by the time the object connects with it. In that case its effective weight is determined by its own crush stiffness - or, to be more accurate the series combination of your crush stiffness and its own crush stiffness. So a javelin doesn't weight much at all and takes quite a long time to stop when it connects with a stationary you, although that may not be much of a consolation.

Anyway, I think "30 times" is fine for a bit of PR fluff, but there's no speed dependency in it, IMHO. There's quite a lot of data out there on this, such as www-nrd.nhtsa.dot.gov/pdf/esv/esv16/98S4P16.PDF‎. Looking at this document it would seem like 20-25 is an upper limit in legislative barrier testing but the real barrier test crash pulses I've worked with often contain a number of spikes at 30g and above although they average 20-25g. Wiltshire council can rest assured that although the odd curmudgeon will warble on about it, "30 times" is entirely reasonable to quote.


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