Visibility, predictability, anticipation, driver error

Lumen

rp wrote: »

That is true, but on the other hand, the impact energy of a bike at 35kph is the same as a car at less than 8kph - leaving out the fact that the car has a better chance of hitting cuz of its size and has lots more nasty sharp and hard things to complicate injuries.

If I had to choose, I'd take the two tonne SUV at walking pace over the bike at 35kph, thanks.

In fact, I'll let you drive my car at me at 8kph first if I can then hit you at 35kph with your bike.

Your physics is somewhat problematic.

rp

Lumen wrote: »

I'll let you drive my car at me at 8kph first if I can then hit you at 35kph with your bike.

Dodging allowed?

Your physics is somewhat problematic.

could be teecher, i did it like this:

double impactEnergy (double massKg, double speedKph)
{
    double velocity = ((speedKph * 1000.0) / 60.0 / 60.0);
    return 0.5 * massKg * Math.pow ( velocity, 2);
}

Lumen

rp wrote: »

Dodging allowed?

could be teecher, i did it like this:

double impactEnergy (double massKg, double speedKph)
{
    double velocity = ((speedKph * 1000.0) / 60.0 / 60.0);
    return 0.5 * massKg * Math.pow ( velocity, 2);
}

Your maths assumes that all kinetic energy of the vehicle is dissipated into the victim.

In other words, that after the collision the vehicle will be at rest.

This is not the case, since momentum must also be conserved. You do the maths

rp

Lumen wrote: »

This is not the case, since momentum must also be conserved. You do the maths

Ah, I see what you mean: wee granny won't absorb all the energy of the car, she'll also be catapulted across the road, under the wheels of the number 10 bus.

Lumen

rp wrote: »

Ah, I see what you mean: wee granny won't absorb all the energy of the car, she'll also be catapulted across the road, under the wheels of the number 10 bus.

No, no, no.

If you hit an object at rest with another object travelling at 8kph, the absolute worst collision outcome (or best, if you're trying to kill them) is that they get catapulted off at 8kph.

Consider that from a classical mechanics perspective there is no difference between you hitting a wall and the wall hitting you. i.e. if you want to know what it's like being hit by a car at 8kph, just run into a stationary car at that speed. It won't hurt much.

Raam

Lumen wrote: »

if you want to know what it's like being hit by a car at 8kph, just run into a stationary car at that speed. It won't hurt much.

I'd like to hear the outcome of this experiment, so report back to us.

DirkVoodoo

Its a bit of a wild oversimplification alright, maybe true of particles in a vacuum, but you are neglecting a lot of real world variables that would render such a hyopthesis null and void.

dub_skav

Lumen wrote: »

No, no, no.

If you hit an object at rest with another object travelling at 8kph, the absolute worst collision outcome (or best, if you're trying to kill them) is that they get catapulted off at 8kph.

Consider that from a classical mechanics perspective there is no difference between you hitting a wall and the wall hitting you. i.e. if you want to know what it's like being hit by a car at 8kph, just run into a stationary car at that speed. It won't hurt much.

Ah but you're using Classical Mechanics. Using quantum mechanics it is clear that you cannot know the position and momentum of the car at the same instant.
So, if you know the momentum of the car precisely then you cannot know the position, therefore the car may or may not be on that road at all and might or might not make any contact at all.

311

If a bike hits me when I'm unaware ,the most that would happen is I'd be badly brused.

If a car hits me at 8kmph unawares ,the force could well shatter a lower leg bone.

Civilian_Target

311 wrote: »

If a bike hits me when I'm unaware ,the most that would happen is I'd be badly brused.

I dunno about that... if a bike hits you at 35kmph you will end up in hospital...

But what are the odds... most cyclists going at 35 kmph have the wind in their faces and are extremely alert. Any collision is likely to be preceded by a decent panic brake. A motorist going at 35kmph is quite likely to be drinking coffee, adjusting makeup, talking on the phone, shushing the kids, whatever, in a lovely warm car, and probably has nothing close to the same reaction time.

All this aside... it's funny this thread came up today. I ran into the side of a guy changing lanes in traffic this morning. No indicator, nothing, he just decided he wanted into the other lane 4m in front of me. Managed to take it from about 25-30kmph to almost stopped, and went shoulder first into the side of his Twingo. He got one hell of a shock!