Safe Speed Forums

Some say that we need speed limits because if speeds get too high there's too much kinetic energy. Sounds reasonable doesn't it?

KE = 1/2 m.v^2

So a 40 tonne truck has a KE of 62,720 units at 56mph.

How fast would a 1 tonne sports car have to be travelling to have a matching KE?

354mph

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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

Oh dear, another meaningless observation/comparison.

If a single pound of explosive in a hand-grenade is enough to kill you, there is nothing to be gained from pointing out that an atomic bomb contains as much destructive power as millions of hand-grenades. You'll either be dead in one or two pieces or absolutely, completely and utterly disinegrated...for the unfortunate concerned it amounts to pretty much the same thing.

A small car can possess enough KE to kill you at low speeds. A big truck will just make the job more efficient.

You're right - except the point has considerable value in dismissing one particular flawed argument.

I heard the argument on the telephone earlier today and thought I'd make the straightforward logical dismissal available to others.

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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

This is another one of those spurious arguments so beloved of scamerati and co.
If it held any water whatsoever, one might equally say that a bus travelling at 10mph is as dangerous as a car doing 30mph, and therefore we should limit buses to 5mph.

But that argument would (sadly) be equally spurious.

The truth of the matter is, the only kinetic energy of any consequence is that which is dissipated by the collision. In a car/pedestrian collision the energy dissipated is that which is required to accelerate the pedestrian to the speed of the car, being a tiny proportion of the total kinetic energy possessed by the car (and a much tinier proportion in the case of a lorry or bus). After the collision the car is still possessed of almost all its original kinetic energy, as it's still travelling at virtually the same speed if the driver doesn't brake.
Actually, the kinetic energy dissipated in a collision between a pedestrian and a car doing 20mph is about the same as that dissipated if a runner doing 20mph collides with the front of a stationary car - the injuries will be about the same.
In a collision between a bicycle and a pedestrian the bicycle will likely come to a stop as a result, so the total kinetic energy will be dissipated in the collision - and, assuming the cyclist and the pedestrian are more-or-less equal weight, the pedestrian will absorb half the kinetic energy, and be accelerated to around half the speed of the bicycle. So a bicycle hitting a pedestrian at 30mph is about the same as a car hitting the pedestrian at 20mph - except that handlebars and things can inflict a lot more damage.

Regards
Peter

KE is only relevant in some cases. I think conservation of momentum is more relevant in the case of two colliding cars. (Newton’s 3rd law)


http://theoryx5.uwinnipeg.ca/physics/mom/node3.html

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Speed camera policy Kills

The best general physics reference I've found on the net is HyperPhysics

Here's the HyperPhysics index to convervation laws

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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

I think Pete's right.

Force is rate of change of momentum, and energy is force x disatnce.

So you could work out the energy transferred by taking the area under a graph of force against distance.

Force x Distance is the formula for WORK isn't it?

I'm not too clear on the difference between energy and work. I know they're similar though.

Yes, that is why it is better to crash with a truck than with a bike, at least when you are in the truck.

Yes they are related. A split second before the crash it is still energy, a split second after the work has been done. Energy is potential work. Work is energy used.