|
Post by joris85 on Jul 28, 2016 12:09:49 GMT -6
So where were we, @chrisclement? Oh yeah, here! And isn't the conservation of momentum in direct correlation to the kinetic energy (in other words, the speed) just before impact? You're probably talking about the fact that you want to accelerate just before impact as that's what you do when you essentially power clean through the hit, but doesn't that just mean you increase the speed at impact? Again, this is not affected by weight (except for muscle mass), so I think my argument still stands. No it's an important distinction because conservation of momentum is, first of all, a scalar quantity, and second allows for the transformation of energy. The exponent on the velocity also means there are some cheats for its conservation that make it difficult to use energy as a means of determining forces in a collision. Outside of billiards there's really no such thing as an elastic collision. Conservation of momentum is a vector concept and is directly proportional to both mass and velocity, and it doesn't allow for transformation to other forms of energy. The only way to create or destroy momentum is by applying force or cancelling with another vector. Put it this way: if the outcomes of collisions were determined by conservation of energy and not momentum, the NFL would be full of 5'7" players. Well, it's been a while since I ran through my books of dynamics (is that the accurate word in English?), but I don't recall using the term "conservation of momentum" when studying it, or can't think of any translation that i'd expect to be accurate (I speak dutch as a native language). How would you define conservation of momentum as accurately as you could? Would you say the segment of the initial movement that still moves into the "same" direction after impact? Would that make a football tackle or hit a spring/dampener system, with both the tackler and tacklee having a spring, a dampener and a mass? (where the mass is the mass, the dampener stands for the dispersive ability of the football player, and the spring would sort influence the actual "conservation of momentum"?) It would be a little more complex, but interesting to dissect (for my inner geek at least). Mods, if you want me to take this down, I'd understand and will take it to PM (if @chrisclements wants that). While not entirely linked to coaching football, I like the theoretical approach to football hits and why some are harder than others with all other things being equal.
|
|
|
Post by fantom on Jul 28, 2016 13:48:24 GMT -6
Once Chris starts talking physics it might as well be Dutch to me.
|
|
|
Post by Chris Clement on Jul 28, 2016 13:52:42 GMT -6
It appears the Dutch word is "impuls" nl.wikipedia.org/wiki/ImpulsIn any collision linear momentum must be conserved unless and until some force outside the system is applied. Let us consider the situation of a stationary car being hit along its long axis by a truck mc = 1,000 kg (small car) mt = 25,000 kg (fully loaded tractor-trailer) vc = 0 m/s (stationary) vt = 20 m/s (72 km/h or about 45 mph) pc = 1,000 kg * 0 m/s = 0 kg*m/s pt = 25,000 kg * 20 m/s = 500,000 kg*m/s Ec = 1,000 kg * 0 m/s * 0 m/s = 0 J Et = 25,000 kg * 20 m/s * 20 m/s = 10,000,000 J = 10 MJ These kinds of collisions are known as "sticky" collisions, because the two objects tend to stick to one another and in the second part of the problem we can treat them as one object. mf = 1,000 kg + 25,000 kg = 26,000 kg po = pc + pt = 0 kg*m/s + 500,000 kg*m/s = 500,000 kg*m/s by conservation of linear momentum we know that pf = po = 500,000 kg*m/s pf = mf * vf 500,000 kg*m/s = 26,000 kg * vf vf = 19.23 m/s so we know that immediately after the collision the combined car/truck will be going 19.23 m/s. How much of the original kinetic energy is conserved? Ef = 26,000 kg * 19.23 m/s * 19.23 m/s = 9.6 MJ, so we have a collision that is 96% efficient, because it's a huge truck going quite fast hitting a tiny car that is stationary, and the efficiency of these collisions is given by m1/(m1+m2)
|
|
|
Post by joris85 on Jul 28, 2016 14:20:49 GMT -6
Yeah, impuls, that makes sense! Great explanation in the example above l, although I think the energy generated by the truck is half of what you calculated. (v/2)
|
|
|
Post by Chris Clement on Jul 28, 2016 14:24:31 GMT -6
Right I forgot the 1/2 on both energy calculations, so 5 and 4.8 MJ
|
|
|
Post by cqmiller on Jul 28, 2016 14:38:17 GMT -6
My wheel-house... the math/physics behind things. My boy Chris Clement always jumps in to answer these things. I'm much more familiar using pool as my examples of conversation of momentum in my physics class, but cars works as well! joris85 ... The physics can get really crazy when you start getting 3 dimensional geometry involved (like on a football field) but if you just use objects in the same plane with one being stationary. Basically conservation of momentum is like a cue-ball traveling 5 m/s toward the 8-ball which is stationary and striking it head-on. The cue-ball stops at the collision and the 8-ball starts to move. In an elastic collision (no loss of energy or momentum) the 8-ball will be traveling at exactly 5 m/s after the collision as both balls weigh the same amount so the mass x velocity = mass x velocity must match speeds if the masses match. If the objects are not the same mass (like Chris explained above). Then the speeds will be different for the object. If a 5 g ball moving 20 m/s hits a 10 g ball that is stationary, the 10 g ball will only move at 10 m/s after the collision because it is twice the mass. Which is why a 150 lb WR running really fast into a 275 lb LB that is stationary can work, but if the WR does 'win' the LB isn't gonna go flying anywhere very quickly because of the LB's large mass. Obviously in real-world, there is no such thing as 'elastic', and you have to start keeping track of + and - numbers because sometimes the forces add together and sometimes they subtract from each other. Head-on collision between 2 cars going 10 mph each will cause more damage than a car going 90 hitting a car going 85 from behind. The 10's add in the head-on collision and the 90 and 85 subtract from each other in the second. Once you start getting to other angles you now have to throw sin, cos, tan into the math and things get really crazy when you have players taller than others as well. Center of mass, height of center of mass, as well as the impulse the player provides (driving legs & feet on contact) also affect the result of collisions. I'm trying to stay away from too many numbers since a lot of guys seem to not like them, but I've been accused by parents at times of teaching "football" during my physics class because it seems to matter more to a classroom full of high school kids why a 250 lb LB wins if he hits a 150 lb RB than Car A hitting Truck B. I love all this stuff... but I'm commonly referred to as the "NERD" on every staff I've ever been on.
|
|
|
Post by joris85 on Jul 28, 2016 15:52:37 GMT -6
Its all coming back, thanks fellas.
The vectors are no issue for me, im working with those almost on a daily basis.
Just entirely forgot about those "impulses". Not that difficult, Just gotta remember is all...
Sounds like the football nerd crew is growing :-)
|
|
|
Post by blb on Jul 28, 2016 16:05:42 GMT -6
I am not a Math or Science guy, but here's what I get from this thread:
It is better to be the hitter than the hittee.
So teach your kids how to protect themselves and use proper fundamentals to be the former rather than the latter.
|
|
|
Post by coachwoodall on Jul 28, 2016 16:22:17 GMT -6
To what extent do the soft tissue structures of the body and the multi-rotational range of motion different joints have, and the relative strength/training of said body structures play an impact on the transfer of energy from one body to another?
|
|
|
Post by blb on Jul 28, 2016 16:24:45 GMT -6
Its all coming back, thanks fellas. The vectors are no issue for me, im working with those almost on a daily basis. Just entirely forgot about those "impulses". Not that difficult, Just gotta remember is all... Sounds like the football nerd crew is growing :-)
"Request vector, over..."
"What's our vector, Victor?"
|
|
|
Post by Chris Clement on Jul 28, 2016 16:52:47 GMT -6
I've been working on calculating the Cd of a football, I'll let you know if I publish it.
|
|
|
Post by cqmiller on Jul 28, 2016 17:05:13 GMT -6
To what extent do the soft tissue structures of the body and the multi-rotational range of motion different joints have, and the relative strength/training of said body structures play an impact on the transfer of energy from one body to another? 90 degree levers allow for maximum torque to be applied by the muscle that pulls that lever... any other angle between 0 and 90 gives a fraction of the force in the intended direction. It becomes a fun math problem where every time you account for something, the equation gets bigger, which is why we as coaches try to do things to maximize the power a player can use by getting those joints bent at the proper angle (hit position), keeping center of gravity low (low pad level), increasing the time you can apply your force during a collision (drive legs on contact), make collisions completely inelastic (wrap up), etc... All of the coaching buzz words that tend to increase performance match a mathematical/physics principle. It is just easier to tell the kid to lower his pad level to win in a 1on1 situation than to explain why having a lower center of mass will improve all of the leverage your body can exert on another person. I'm by no means an expert on the anatomy and the limits of joints/muscles specifically, but I know that as players get bigger and faster, all of the physics gets a larger and larger result as mass and velocity are major components of all of them... the body cannot evolve joints at the rate that we have increased mass (food & weights) or length of levers (leg length a.k.a. height, and arm length a.k.a. wingspan) which is putting more torque and strain on the joints. From a mathematical standpoint, if weight restrictions were introduced into the NFL and College football, the collisions would become much safer and you would probably see a regression in not only joint/bone/muscle injuries, but concussions as well. Taking a 325 lb OL down to 280 lbs would reduce that players force at the same speed to only 86% of what it is at 325... How many NFL lineman do you see that retire and immediately lose 40-50 lbs? The concussion issue scares me because I love this game and think that the statistics are being misrepresented and used to feed fear. Driving a car is still more dangerous than playing any sport. I'll end now before I go off on a concussion rant.
|
|
|
Post by coachwoodall on Jul 28, 2016 18:43:36 GMT -6
If I'm hanging by my toes 3 feet off the ground and get dropped on my pumpkin; does the same amount of damage result as if I land on my arse?
If I was good at math, I'd be an engineer. However, I was thinking in terms of how Kevlar stops a bullet; it doesn't at like a steel plate but rather disperses the energy of the projectile quickly so that there isn't enough energy to penetrate.
So it isn't so much as the mass x speed = force, as to the type of mass x speed / angle of the joint = the effective force applied to the same variables?
|
|
|
Post by cqmiller on Jul 28, 2016 18:50:03 GMT -6
Your body can distribute force using ankles, knees, hips if you land on your feet. Neck and vertebrae can't absorb anywhere near that shock. Brain isn't really strapped in either which means nothing can really stop it from slamming into the inside of your skull.
|
|
|
Post by coachwoodall on Jul 28, 2016 19:22:14 GMT -6
right so it isn't so much as to how hard you hit, it's where the hard hit goes/comes
|
|
|
Post by Chris Clement on Jul 28, 2016 21:14:00 GMT -6
Essentially yes, some body parts handle it better than others.
Joints with large muscles can be somewhat prepared and insulated against injury by making them stronger, allowing them to respond to forces applied against them.
Bones you can't do much with, lifting and diet can improve bone density but they're basically classic statics problems.
Brains are a problem just the way CQMiller described. Helmet accelerometers can help but we're still learning how to calibrate them. A skull accelerometer would be ideal.
|
|
|
Post by John Knight on Jul 28, 2016 21:49:01 GMT -6
|
|
|
Post by utchuckd on Jul 29, 2016 5:25:05 GMT -6
"Theoretical physics can prove that an elephant can hang from a cliff with its tail tied to a daisy."
|
|
|
Post by blb on Jul 29, 2016 5:47:02 GMT -6
Once Chris starts talking physics it might as well be Dutch to me.
Thread is like being in an episode of "The Big Bang Theory."
|
|
|
Post by joris85 on Jul 29, 2016 7:40:57 GMT -6
Essentially yes, some body parts handle it better than others. Joints with large muscles can be somewhat prepared and insulated against injury by making them stronger, allowing them to respond to forces applied against them. Bones you can't do much with, lifting and diet can improve bone density but they're basically classic statics problems. Brains are a problem just the way CQMiller described. Helmet accelerometers can help but we're still learning how to calibrate them. A skull accelerometer would be ideal. This IS where you can make a model of a human body with springs, masses and dampeners. the brain just being dampened by the liquid around your Brains and having an awfully low spring constant (or how's that in english?) I remember the formulas to calculate those are unnessecarily long, but can be calculated comparable to Electric circuits. Boy, talking physics in a different language is harder than it seems! :-D
|
|
|
Post by Chris Clement on Jul 29, 2016 7:55:48 GMT -6
yes, those are mostly the correct words, although "spring constant" is usually used for solid springs, and only within their linear range.
In this case it's a fluid dynamics problem, and those can be a mess because there are a lot of parameters, like CS fluid viscosity and Reynolds number (those two may actually be linked sometimes), but there's also a biology component, where does the brain receive the impact? and since the brain is squishy it will deform on impact, but because of its irregular shape we can't necessarily model that without knowing how all the parts are moving.
|
|
|
Post by cqmiller on Jul 29, 2016 15:53:52 GMT -6
Fluid Dynamics... for the love of God... let's not start that again
|
|
|
Post by Chris Clement on Jul 30, 2016 8:30:21 GMT -6
We had previous discussions on fluid dynamics? Why did nobody invite me?
|
|
|
Post by silkyice on Jul 30, 2016 8:59:09 GMT -6
We had previous discussions on fluid dynamics? Why did nobody invite me? Missed that one also.
|
|
|
Post by blb on Jul 30, 2016 9:03:50 GMT -6
We had previous discussions on fluid dynamics? Why did nobody invite me?
Somebody said "Don't tell Sheldon!"
|
|
|
Post by 33coach on Jul 30, 2016 11:46:20 GMT -6
This is a discussion I wish I could get in on. But my understanding of physics comes from computer science... Meaning its all applied physics.
Springs are my favorite topic by far
|
|