Impact Speed

[Licramite]

You THINK you know this from parachuting and judo. What's that to do with cycling - or more importantly one's own style of cycling?

I ride and have come off a motor bike at speed. I adopted (and I'm probably as right as you) a completely different configuration as I grounded. The trick is to roll and slide a far as you can to dissipate the energy gradually. In that case you pull everything in and assume the heavy duty helmet will take care of that particular extremity and the leather and other padding will aid the sliding.

When riding along to the shops on my Brompton between 4-8 mph then the opposite applies. You should have time to anticipate (people who have cleated experiences will know how time is very slow ) and prepare for the relative simple exercise of cushioning your fall. Hands & shoulders can do a fine job. People who do rough downhills at speed or TT racing on dual carriageways may need to do other things. It may not be an accident (sic) that countries with the highest percentage of utility cycling have the lowest use of helmets. Is it just because, as a group, they realise they just don't need them?

The UK's relatively high use of helmets may not be unconnected with a tradition of more extreme cycling. And then the peer (and even judicial) pressure for ordinary cyclists to wear them?

I did say - of course it depends on the severity of the fall - and I never once mentioned a helmet.
parachuting and judo - falling is the big thing so they teach you how to doit properly-
I came down on my MTB on the weekend , failed to unclip and got the handlebar in my chest, as I was practically stationary when the bike got stuck in the mud I put my hand down, but if I had been hurtling down a hill I would have gone for a shoulder crunch instead.

your right , for that situation, and ys I wear loves for just that situation

 

[Licramite]

Your fall distance may be 5.2m, but that is in an arc; your I think the torpedo guy meant 'head first' rather than 'the speed of a torpedo'. If you drop someone into a sandpit from 1.5m, they'll make a certain dent in the sand. But if someone is running and doing the long jump, if their maximum height from the ground is 1.5m, their dent will be the same depth but longer.
I was going to give a bullet example, but these guys beat me to it:
http://physics.stackexchange.com/questions/10773/how-does-gravity-affect-bullets
With the key catchphrase: 'the equations of motion in one axis of a coordinate system do not have any effect on motion in any orthogonal axis'
So your velocity towards the ground is unaffected by your forward velocity. At least not in a spacesuit in a be-vacuumed shooting range in the north pole.
I'm just trying to make this more sciencey so Dara O'Briain will come along and say 'airrrrrr'. Which he does, a lot.
Airrrrrr!

But we are looking at impact speed, - yes gravity pulls the bullet down and it speed decreases as it looses velocity from drag. - but if you fire a rifle bullet 3000ft/per second it will travel 5miles before it hits the ground (roughly) you drop the same bullet from the same height, it hits the ground fist - because Gravity will not effect the bullet untill it falls below the earths escape velocity. - the bullet will defie gravity until it slows enough to be effected were as the dropped bullet will be effected immediately.

but we are looking for an impact speed, - to keep to thread- if you drop something it will increase in speed due to to mass and gravity over the distance its falling.
but velocity adds distance , so although your falling 1.5m vertically your traveling on the hypotonus of the arch 5.2m so gravity has that long to increase your speed - the difference between 0.5 and 1.1 secounds so you have an extra 0.6 of a second to fall and increase speed by the effect of gravity. Parachutists don,t drop verically yet they reach terminal velocity - about 150mph - so the longer you fall the faster you get . till terminal.
So I stand by my calc. -

 

[Twilkes]

I don’t think the first paragraph is correct – gravity is gravity, no matter how fast you’re going. As soon as you fire a bullet from a gun it is being pulled to the earth, and you could drive off a cliff at 10,000mph but as soon as your wheels leave the ground, you’re going down. The only thing that would stop this would be a force that would push you upwards, or something aerodynamic to stop you falling as fast. So maybe, rather than helmets, cyclists should wear collapsible wingsuits that are only activated by velocitous proximity to the ground.

The calc you made assumed that all the forward speed was being aimed at the ground, whereas it’s not, it’s aimed forwards. If I cycled at a wall at 20mph and was unlucky enough to fall off just as I reached the wall, my impact speed with the wall would be 20mph (horizontal direction), and my impact speed with the ground would be about 10mph (vertical direction).

Unless this is just goading me onto filming myself cycling into a wall just to prove a point, airrrrr……

 

[Licramite]

the bullet thing, a 7.62mm weighs 150.5grammes dropped from 1.5m - takes 0.5second (though I think thats a computer minimum)
Fire a 7.62 @2750f/ps - has an absolute range of 5miles - takes 9 seconds to hit the ground.
the bullet flys absolutely straight, no drop for 0.32 seconds - 300yds

as for velocity - 20mph forward is exactly the same as 20mph at a slope of 30degrees. just because your falling over sideways doesn't mean your traveling any slower forward. yes you would loose a bit of speed as you stop pedalling and travel under momentum only for the 3.7m it takes to ground zero. so I put it at 15mph.

I think 21mph is probably right but of course your hitting at an angle , so the two forces are not exactly joined up, if you hit something that stopped your movement, like the base of a tree you would get the full 21mph, but if you skid along the ground its going to be more like 11mph impact with 10mph of travel on the ground - or scrape.

of course as Stuart G pointed out quite rightly its unlikey your head is the first thing to hit the ground - unless you have left the bike and are making like a torpedo. - its more likely you already have slowed down quite a bit from the braking effect other bits hitting the ground first.

I wouldn,t like to do it without a helmet on though -

 

[Licramite]

look up escape velocity - its about 11.2ft/ps its the speed were your kenetic energy minus gravitaional pull = 0

not that this applies to cyclists.

 

[Dan B]

Gravity will not effect the bullet untill it falls below the earths escape velocity. - the bullet will defie gravity until it slows enough to be effected were as the dropped bullet will be effected immediately.

Again, not in Physics World

 

[green1]

But we are looking at impact speed, - yes gravity pulls the bullet down and it speed decreases as it looses velocity from drag. - but if you fire a rifle bullet 3000ft/per second it will travel 5miles before it hits the ground (roughly) you drop the same bullet from the same height, it hits the ground fist - because Gravity will not effect the bullet untill it falls below the earths escape velocity. - the bullet will defie gravity until it slows enough to be effected were as the dropped bullet will be effected immediately.

Nothing defies gravity, and bullets never get near the earth's escape velocity (~11km/s I believe). Drop 2 bullets from the same height they'll hit the ground at the same time, it doesn't matter about the horizontal component of their travel, the vertical component will be exactly the same.

 

[Twilkes]

So to summarise - if I got shot, but I was wearing a helmet, I'd be fine?

 

[green1]

So to summarise - if I got shot, but I was wearing a helmet, I'd be fine?

Yes, but only if you were shot in the foot, You'd have a limp for a while but hey ho.

 

[Licramite]

ok - so whats your answer to the original question of whats your impact speed - I haven,t seen it yet.

 

[deanE]

Assuming you don't scrub any speed losing the wheels you will hit the ground at ~22.5mph. you won't start slowing down from 20 mph in the forward direction until you hit the ground.

I think that Green1 is probably right, although he didn’t really explain how he reached his conclusion. Others may have also reached this conclusion but I couldn’t be bothered to read all the posts It is all about vectors as someone has already stated. Imagine a right angled triangle with the forward speed of 20 mph represented by the long side and , say, 10 mph the vertical speed of your head, by the sort side. The hypotenuse represents the speed that your head will hit the tarmac, which is the square root of the sum of the squares of 20 and 10. ie 400 + 100. Square root of 500 is about 22.3. I might be totally wrong as it is a long time since I failed to pay attention at school, but seems logical to me. However, if you pay attention to your riding, rather than trying to solve questions like this when you are out cycling, then you probably will never find out whether I am right or not.

 

[green1]

I think that Green1 is probably right, although he didn’t really explain how he reached his conclusion. Others may have also reached this conclusion but I couldn’t be bothered to read all the posts It is all about vectors as someone has already stated. Imagine a right angled triangle with the forward speed of 20 mph represented by the long side and , say, 10 mph the vertical speed of your head, by the sort side. The hypotenuse represents the speed that your head will hit the tarmac, which is the square root of the sum of the squares of 20 and 10. ie 400 + 100. Square root of 500 is about 22.3. I might be totally wrong as it is a long time since I failed to pay attention at school, but seems logical to me. However, if you pay attention to your riding, rather than trying to solve questions like this when you are out cycling, then you probably will never find out whether I am right or not.

I worked out the vertical speed using the equations of motion then added that to the horizontal speed as you suggested.

 

[Twilkes]

I think what I really was looking for was the size of the impact with the ground, in some newtonjoules type force unit, and was confusing myself by thinking about speeds. But I'm not even going to start to try and calculate that.

In any case, I did manage to carry out a few experiments on this - the first was a high-speed fall from a low height, which didn't hurt too much, and I did actually manage to remount my bike afterwards, so no harm done; and the second was a slower fall from a greater height, and this is where gravity's acceleration was more marked, and also resulted in more of a 'bounce' than the first fall, which quickly turned itself into a roll. See for yourselves: http://tinyurl.com/btlpzc9

 

[Licramite]

Ok I got the bullet one wrong - but even the science is difficult to prove in practice terms as the scientists admits in the article.

but my speed pretty well tally's with green 1.

the trouble with trying to work out the force involved, is how do you measure it in real terms , joules energy is just math, you can,t physically feel it.
I had the same problem trying to work out pedal resistance and load on a gearing design, without physically building a mock up. (which I am having a devil of a job getting the non standard parts )
which for me is not so bad but I don't fancy playing crash dummy. - I will just stick to - dummy.

 

[Dan B]

the trouble with trying to work out the force involved, is how do you measure it in real terms , joules energy is just math, you can,t physically feel it.

Again, and not withstanding anything your crystal healer may tell you to the contrary, force and energy are two different things. Newtons are a pretty good unit in which to measure force. Pounds and ounces will do in a pinch if you're old-school