*ban
Kinetic Sand Toys
https://www.youtube.com/watch?v=fvkk9OEX-9E

http://videosift.com/video/1959-Bel-Air-vs-2009-Malibu-crash-test

Crumple-zones (and other safety engineering) can be the difference between life and death.

And most crash tests (and star ratings) aren't for highway speeds.
http://www.iihs.org/iihs/ratings/ratings-info/frontal-crash-tests
so 40 mph and 35 mph. If you go up to 55,65,75 mph the difference in kinetic energy is exponentially higher (K=0.5mv^2)

there might have been a mythbusters episode about this...can't remember the result though.

I really want to do some UV night diving. It looks amazingly like Avatar to me.
Another video about how these effects are achieved here......
*related=http://videosift.com/video/Ultraviolet-Diving-with-Underwater-Kinetics-UV-Lights

Ultraviolet Diving with Underwater Kinetics UV Lights has been added as a related post - related requested by newtboy.

That was amazing. I only wish it had been a single long shot instead of multiple setups.
*promote *quality kinetic *art

Good question. I think that the entire device is unstable no matter what, its impossible to keep it straight no matter where the point of gravity as long as it needs to balance on a single tip. So zero speed would mean tip over in all cases unless you make a more stable tip (square) which would mean it cant spin very well which means you havent made a spinning top.

You can find more about the physics of the spinning top here:
http://hyperphysics.phy-astr.gsu.edu/hbase/top.html
and here:
http://physics.stackexchange.com/questions/271/why-dont-spinning-tops-fall-over

So what is the optimal distance of the center of gravity to the tip?

There are several things working at the same time. Most importantly is the energy. You need to store as much energy as possible so that the top can spin for as long as possible. When the top slows spinning the friction at the tip becomes larger (the precession becomes bigger) so it starts to lose more energy and slows the spinning even more. You store energy by adding weight with a center of mass that is further away from the tip. When the top then "falls" the center of gravity moves down and reduces potential energy. Due to energy conservation kinetic energy goes up meaning speed of precession or of spinning goes up and creates a force pushing the top back up.

Off course, more mass means more friction at the tip, so there is for sure an optimal here, most likely depending on mass, size and shape of spinning top, etc.

Last but not least, more rotation speed I assume also means more friction, so its a trade-off.

If you move the center of mass down below the tip, well, if you move it as far off as you would above, the energy you can save is about the same, but the entire thing would be harder to build and you would need to make sure the sides fit around the ground plateau. Also, when the precessions become bigger the sides will hit the plateau, meaning game over.

In the end you are better of keeping the center of gravity above the tip point.

Far out, that car is just beautiful.

I don't give a damn how it drives (I'll never get to drive one), as far as I'm concerned, it is kinetic sculpture.

From my experience, and physics class, that should only be true if your brakes suck. Physics don't lie, and I was taught that static friction is ALWAYS greater than kinetic friction...meaning rolling tires grip better than sliding tires every time. If your brakes are able to grip with more 'stopping' force than the sliding tire produces and still allow 'slip', they should be able to stop you faster than sliding.

Perhaps on ice that's true, or other slippery surfaces where the sensors get confused, or with really bad or broken ABS, but good ABS seriously reduces the distance to stop AND retains control by rapidly (thousands per second) pulsing the braking force up to maximum possible force without stopping the rotation. That should be more stopping force than locked tires can produce if the brakes are in good condition.

I've been in the car when an idiot friend decided to prove it to me, and slammed on his new BMW's brakes at about 70mph. The seatbelt hit so hard I had the wind knocked out of me, and we stopped ridiculously fast (WAY faster than when I've locked up non-abs cars on the freeway...repeatedly). Afterwards, he needed new brakes all around, because they gripped so hard and hot it warped the new rotors, but never locked up the tires...retaining static friction between the road and tire. Dumb...but informative.

Just trying to figure out how fast an Arrow looses kinetic energy, this is a neat site.

http://www.huntersfriend.com/carbon_arrows/hunting_arrows_selection_guide_chapter_5.htm

You can't blame the human. They're generally stupid and not prone to figuring out complex kinetic puzzles.

Wow, the *fear of how it's going to turn out is intense. Heli crashes usually go very bad. The rotor has so much kinetic energy it often destroys the airframe on even a minor crash.

I know. This is amazing. I assume they are professionals since they appear to have all the trappings, surroundings, etc. This is how I always played sports of any kind, and why my love of sports was socially-executed at an extremely young age (maybe 5th grade at most?)
I've always held that "it's only a game" is the mantra necessary for enjoyment of the event, but ever since I was repeatedly bitched out by teammates for being open when I fouled someone else in a sport, or otherwise being honest about how some interaction wasn't ultimately fair, I just lost interest (and faith in humanity, in general).
This was great to watch, but not even the first time I've seen it. What is it about table-tennis that makes the participants so eager to throw off their egos and have fun?
Also much respect for one of the most demanding examples of control over small-scale kinetics.

Actually I'm about as English as they come but crucially I spent my advanced academic career studying Philosophy and rhetoric (lamentably only to Hons. due to laziness) and consequently have an ingrained habit of arguing around a problem rather than relying on established parameters (not always entirely helpful when discussing more day to day matters as I'm sure you've by now gathered but it is essential to working with advanced epistemological problems and so serves me well none the less). I'm also prone to poor punctuation and odd patterns of grammar when I'm not going back over everything I write with a fine tooth comb which has likely not helped. (A consequence of learning to describe tangent after tangent when trying to thoroughly encapsulate some conceptual problems with language alone)

That said, while I may have gone around the houses so to speak I think my conclusion is entirely compatible with what I now understand your own to be.

I didn't want to describe my original counter-point by simply working with the idea that weight is lower on the moon relative to the earth (though I did not try to refute this either) because that would not illustrate why a 2-300kg man in a space suit still takes some shifting (relatively speaking) even if there were no gravity at all. (Would have been faster to just crunch some numbers but that's not what I specialise in)

Sure you could move anything with any force in 0G (which I do understand is technically relative as every object in the universe with mass exerts gravitational forces proportionately (and inversely proportional to the distance between)) but the resulting velocity is directly proportional to mass vs force applied. Weight here then, can be seen as another competing force in the equation rather than the whole thing which it can be convenient to treat it as for a simple calculation (which is what I think you are doing).

To put that another way I was applying a different/deeper linguistic/descriptive paradigm to the same objective facts because that's what we philosophers do. Single paradigm approaches to any subject have a dangerous habit of making one believe one possess such a thing as truly objective facts rather than interpretations only (which are all that truly exist).


In other terms weight alone isn't the whole story (as I assume you well know). Overcoming inertia due to mass scales up all by itself, then gravity comes along and complicates matters. This is why rocket scientists measure potential thrust in DeltaV rather than Watts, Joules etc. right? The mass of the object dictates how much velocity a given input/output of energy would equal.

Gravity and thus the force in newtons it induces (weight) in these terms is an additional force which depending upon the direction in which it is acting multiplies the required DeltaV to achieve the same effect. Moreover when concerning a force of inconstant nature (such as pushing up/jumping or a brief burn of an engine) brings duration into play also. (the foundations of why rocket science gets its fearsome reputation for complexity in its calculations)


Man on the moon lies on the ground and pushes off to try and stand back up.
This push must impart enough DeltaV to his body to produce a sufficient velocity and duration to travel the 2 meters or so needed to get upright so he can then balance the downward gravitational force with his legs&back and successfully convert the chemical/kinetic energy from his arms into potential energy as weight (the energy he uses to stand up is the same energy that would drag him down again right?).

One could practically speaking reduce this to a simple calculation of weight and thrust if all one wanted was a number. Weight would be the only number we need here as it incorporates the mass in it's own calculation (weight = mass x gravity)

But where's the fun in that? My way let's one go round all the houses see how the other bits of the paradigm that support this basic isolated equation function and inter-relate.

Plus (and probably more accurately) I've been playing loads of Kerbal Space Programme lately and have ended up conditioning myself to think in terms of rocketry and thus massively overcomplicated everything here for basically my own amusement/fascination.


Basically few things are more verbose and self indulgent than a bored Philosopher, sorry .


Re: Your challenge. (And I'm just guessing here) something to do with your leg muscles not being able to deliver the energy fast/efficiently enough? (as your feet would leave the ground faster/at a lower level of force?). This is the only thing I can think of as it's easier to push away from things underwater and it certainly looks difficult to push away hard from things when people are floating in 0g.

So lower resistance from gravity = less force to push against the floor with?

Warm? Even in the Ballpark? (Regardless I'm really pleased to discover you weren't the nut I originally thought you to be! (though I imagine you now have some idea what a nut I am))


If I got any of that wrong I'd be happy for you to explain to me why and where (assuming you can keep up with my slightly mad approach to syntax in the 1st place). I'm an armchair physicist (not that I haven't studied it in my time but I'm far from PHD) I'm always happy to learn and improve.

Geez is is too much to ask that these people do a little research before they write their scripts?

HEAT = High Explosive Anti Tank, not High Energy. (high energy rounds, or kinetic energy (as they're more commonly called) use an entirely different method of penetrating armor (basically they go really, really fast))

That they got something so basic wrong it makes me doubt they put any effort into the rest of it.

4mph difference in initial speed is still going to be more than 4mph at impact. Also kinetic energy is speed^2.

So yah. The accident is obviously not the fault of the guy speeding. His speed didn't make the accident inevitable, he just made it worse. Would an ad campaign urging drivers to be more cautious have any effect? No. People don't stop making lapses in judgement/mistakes because you ask them politely. Dangerous drivers perhaps believe that their driving is good enough and, again, are unlikely to drive more safely. You can attempt to address the causes of these errors - tiredness, inebriation etc.

I think perhaps the heavy handed tactic to combat speeding backfires somewhat. People, from their own experiences, know that when they go a bit faster they don't immediately have a crash, and they also probably don't believe that 5mph is going to be the difference between little Timmy buying it in most cases - and I think their probably right. So this message that a bit of extra speed equates to disaster is rejected; it comes across as inaccurate and emotionally manipulative. Instead I might try to communicate why maintaining and policing a speed limit is important - that the sheer volume of people that go that 5mph faster makes a statistically significant impact on the number of injuries and fatalities in accidents.