well i was the one to search the internet for hours to try and find some scientific study of kicks to satisfy the legit need of numbers and study results mighty hard to find believe it or not. I guess martial artist dont like being tested in fears of not being as great as they think they are. But i guess since i got such a warm welcome from you guys i could just give the link...... but then again even with the data i hear screams of no way arm movments actually make such a little difference. I say look it up yoursrlf or get tested yourself.

I say look it up yoursrlf or get tested yourself.

you just lost at bullshido.net dude.

is it just me or has this been happening a lot lately?

well you used an excuse of black eye and soreness which think is either a true and you are just cranky and had to take it out on some one attempting to be helpful or that the black eye and soreness is being used as an clever way of bragging something along the lines of "I'm sorry i actually get hit and what do u do..?" if you have to use either of those either you are egocentric or not use to getting hit.

i was implying that i misread what you said because i got smacked in the head and am tired. not angry, im always like this.

m = n * v, so no on the momentum. The balance makes the most sense to me since I used to swing my hand back all the time and now notice little difference in impact when I keep both in front. That being said, using the opposite hand as a sort of counter-weight to increase your rotational velocity may add power.

I think that feeling is just rotational energy being dumped into your arm. Forgive my use of psuedoscience, but if you stand on a spinning platform with your arms at your sides you will spin faster than if you extend them. That and the counterweight thing I wrote above.
You're really close as far as the mechanics go. Yes, the linear momentum does not really apply here, but angular momentum is very important. If we consider your body as whole during a kick, the net angular momentum is roughly the same, since your core is spinning on an axis toward the bag before you deploy the kick. But if we examine the upper and lower body (divided at the waist) separately, then the point of throwing the arm can be understood easily.

Long (if properly read, no can misunderstanding physics):

Because your net angular momentum is constant between the start of the kick and just before landing it on your target (neglecting friction on the ground, which if you're doing a nice pivot should be negligble), neither throwing your leg out further from your body nor throwing your arm out can affect the momentum quantity. It's conserved.

What does happen, though:
- you adjust your moment of inertia
- you transfer angular momentum between upper and lower body via the torque of your waist.

Things that adjust your moment of inertia:
- throwing your leg out for the kick. Originally, the kicking leg starts close to the support leg. It ends up fully extended far from the body.
- throwing your arm out (optional). Originally, it's in a guard position. It ends up straight out, or thrown out and then back, but in either case it's extended from the body.

L = Iw

L is the body's angular momentum, which is constant for this problem.
I is moment of inertia
w is rotational speed.

SECTION A.
Let's concentrate on the leg first. Since this is a kick, the leg extension will happen. Throwing your leg out increases your inertia, and by conservation, this decreases your rotational speed*. This is exactly what's going on when the ballerina does her open up arms/tuck up arms routine. When she opens them, her moment of inertia increases, so she starts spinning slower. When she tucks them, moment of inertia decreases, so she spins faster. Note that we can approximate L as constant because she's skating on ice, so friction doesn't degrade her motion that much.
Lesson: throwing out your leg strictly decreases your rotating speed, and strictly increases your moment of inertia.
*This is the rotational speed of your whole body, and not the leg itself. But because the leg has mass and its own speed, what happens is that the leg shoots out, and the rest of the body moves slower, but on the whole the system has a decreased rotational speed.

SECTION B.
A setup example for what's about to be explained:
This is just for you to visualize. If you can imagine it correctly, you may actually have to do it.
Take two identical pencils. Wrap a rubber band around the two at the middle. Twist the pencils against each other, so that the rubber band is being wound up and is holding one pencil (around the middle of it) in each end loop. Do this a lot, so the rubber band is really wound up. Hold one pencil in each hand, so they are barely separated, and parallel to each other. Consider our pencils+rubber band contraption the system. The system obviously has 0 angular momentum, since it's not spinning.
Let go of both pencils and clear your hands. What happens? The system starts to fall, and the two pencils begin rotate in opposite directions. What may not be obvious is that the net angular momentum is still 0. But it is. At any moment, one pencil is spinning counterclockwise at some rotational speed, and the other other pencil is spinning clockwise at the same rotational speed. Each pencil has its own angular momentum, but they exactly cancel out. The energy stored by winding up the rubber band is released as torque between the two pencils. For one pencil to start spinning though, it needs to be able to apply torque to the other pencil. Pencil A needs to 'ground itself' against pencil B. That might be a little weird since it makes it seems like the rubber band is just for pencil A. But of course it's working both ways at the same time.
Thought experiment (the dorkiest sounding, yet handiest thing from physics):
What if one pencil is really fat? Answer: The fat pencil will spin slowly, and the thin one will blaze. Because L = 0, the canceling out of momenta still holds. In the fat pencil, because of its large moment of inertia, it gets a slow speed. In the thin one, it gets a fast speed, since its moment of inertia is relatively smaller.
Though experiment 2:
Will the thin pencil be spinning faster (relative to us) than when its counterpart was the same size? Answer: Yes. Imagine if we taped one pencil to the wall, aka has near-infinite moment of inertia. The free pencil would be going as fast as the rubber band can spin it, with no rotational speed going into the wall pencil (well a tiny bit).

SECTION C: the arm comes into play
The upper body is one pencil. The lower body is another pencil. The muscles at your waist are the rubber band. Your waist muscles can obviously apply different amounts of torque depending on your body's posture and positioning, but we'll have to assume that it's constant over the duration of the kick, across the range of body positions you execute.
When you throw out your arm, you're increasing the moment of inertia in your upper body. It's becoming the fat pencil. So the torquing action of your waist will now cause your lower body to cancel out this change in angular momentum. Because your leg is always thrown out, moment of inertia for your lower body doesn't change. What must change is rotational speed. Voila -- that's the basic result -- throwing your arm out allows you to twist your lower body faster, and by association, your leg.
If you keep your arm tucked, your upper body doesn't become the fat pencil, and some of your waist torque will act to spin your upper body away from the target, to do the proper canceling out against the lower body.

Stepping away from the physics:
How you train is how you kick. One weak assumption is the constant torque of your waist muscles. Obviously if you train by throwing your arm out, you'll actually get more torque from that posture. And the same if you hold your arm in.

also the race car driver comparison is cute scientist improve cars so race car drivers can go faster safer and to drive you must understand the rules of the road which usually found in a book of some sort.

I guess after reading the book on the rules of the road, you're all ready to go racing in a cute scientist improved car that can go from 0 - 60 in less time then you can unzip your pants and turn corners at breakneck speeds on the race track.....

ATleast Meng Mao came to almost the same conclusion with its how you train and actually using a intellectual arguement. Arm position in guard or down you learn to use your body like that and the number I gave support that the actual increase and decrease in force is neglible, but that there are actually numbers. also crapple agrument can support why the kickers had decrease in power because they dont train like that so it is awkward for them and not sure how to generate power and keep balance. also to crapple your pic has "software engineer" under it and if you are i take your arguement better than these others cause when i went to school for that it involved alot of physics and calculus.

also to crapple your pic has "software engineer" under it and if you are i take your arguement better than these others cause when i went to school for that it involved alot of physics and calculus.

A lot of physics in software engineering school? What kind of physics did you study in software engineering school?

I found this to determine the use of hand position for a roundhouse kick not sure if it applies much to a front kick or side kick but it might.

http://ezinearticles.com/?Increasing-The-Power-of-Your-Roundhouse-Kick&id=301656

Who the **** is Kirkham, and why would I take his word on kicking over Vut Kamanark's?

A lot of physics in software engineering school? What kind of physics did you study in software engineering school?
Calculus and sometimes physics are used as weed-out courses for first-year undergrad. You're also not going to enjoy any decent graphics classes without multivariable calculus.

YOUR LOGIKS IS WRONG ITS NOT W ITS OMEGA, LOL jk

Sorry I am still taking College Physics.

I have a few clarifying questions, sorry if it sounds like my idea of physics is wrong (which is why it needs clarifying)

When you say that the angular momentum of both systems (the fat pencil and the thin one) must be equal to zero and this is because the angular momentum of each system are in opposite directions. (oh whoops isn't momentum a scalar quantity? I forgot I must be thinking of angular acceleration)

When you say that the momentum equals to zero, are you saying that both systems must have the same angular momentum. (is that right?) If it is, then that means we want to increase the momentum of one side and thus forcing the other side to do the same. The torque exerted on both sides are the same.

My question is if you increase the moment of inertia on one side, it will have to decrease the angular velocity thus having the same amount of momentum. If both sides of the momentum are the same how does the thinner pencil move faster if the fatter pencil COMPENSATED its speed in order to keep the same momentum? If this does happen then how does increasing the moment of inertia and lowering its speed, increase the momentum? and thus increasing the speed of the thinner one? won't it ultimately depend on the torque?

I know I am wrong somewhere... cause this does not happen with actual pencils and rubber bands and it sure as hell is not true when I round kick. Also all this must be true only if the pencils are moving at opposite directions, so which direction is your arm suppose to go? horizontal or down?

Thai round kick goes up and then turns in horizontal at the last moment in one smooth motion. Does our arm move in a almost diagonal way? (it does for me when I do round kicks (same with buakaw)


Help a Brotha out.....

A lot of physics in software engineering school? What kind of physics did you study in software engineering school?
Yeah I dunno either. In college I took 1 physics class, and that was in E&M. You don't really need much physics to do software engineering.
All the mechanics I discussed in the monster post were from high school classes.


your pic has "software engineer" under it and if you are i take your arguement better than these others cause when i went to school for that it involved alot of physics and calculus.
I'm guessing the schools you went to didn't have a lot of spelling, grammar, or writing?
What school did you go to that offers a degree in software engineering? I can't think of ANY that do at the undergrad level, and at the grad level, no schools would require physics and calculus -- too basic.

When you say that the momentum equals to zero, are you saying that both systems must have the same angular momentum. (is that right?) If it is, then that means we want to increase the momentum of one side and thus forcing the other side to do the same. The torque exerted on both sides are the same.

Yes, except the torques to each side/half have opposite 'directions,' or orientations.



My question is if you increase the moment of inertia on one side, it will have to decrease the angular velocity thus having the same amount of momentum. If both sides of the momentum are the same how does the thinner pencil move faster if the fatter pencil COMPENSATED its speed in order to keep the same momentum? If this does happen then how does increasing the moment of inertia and lowering its speed, increase the momentum? and thus increasing the speed of the thinner one? won't it ultimately depend on the torque?

I know I am wrong somewhere... cause this does not happen with actual pencils and rubber bands and it sure as hell is not true when I round kick. Also all this must be true only if the pencils are moving at opposite directions, so which direction is your arm suppose to go? horizontal or down?

The two halves' momenta cancel out, but they actually increase over time as the rubber pencil adds more energy to the system. More detail to come.



Thai round kick goes up and then turns in horizontal at the last moment in one smooth motion. Does our arm move in a almost diagonal way? (it does for me when I do round kicks (same with buakaw)


Help a Brotha out.....
i'll get back to you. gotta go to some tgiving dinner.

My question is if you increase the moment of inertia on one side, it will have to decrease the angular velocity thus having the same amount of momentum. If both sides of the momentum are the same how does the thinner pencil move faster if the fatter pencil COMPENSATED its speed in order to keep the same momentum? If this does happen then how does increasing the moment of inertia and lowering its speed, increase the momentum? and thus increasing the speed of the thinner one? won't it ultimately depend on the torque?
Elaborating on what I said before.
net L = 0 (this is a vector quantity, but for simplicity, consider negative as one orientation, and positive as the other)

net L = L_upper + L_lower

L_upper = L_up(t), a function in time
L_lower = L_lo(t)
L_up(t) = - L_lo(t)

At any time t, the magnitude of L_up can be anything. It can change almost instantaneously, which happens when you throw out your arm and leg. However, by conservation, L_lo must match L_up in magnitude, so that the net L remains 0 under any change for L_up.

I didn't give enough detail during my pencil example. Consider two sets:
system A, which has 2 identical pencils.
system B, with one of the pencils replaced with a fatter pencil.

A(t), the rotational speed of a pencil from A at time t, in seconds.
B(t), the rotational speed of a skinny pencil from B.

Obviously, A(2) > B(0.1). That is, at different points in time, you can't really compare the speeds A and B. What I'd wanted to say in the original post is that B(t) > A(t), for any time t. System A and B are different, whereas the act of extending one's arm during a kick represents a single, changing system.

Now to address your question "If both sides of the momentum are the same how does the thinner pencil move faster if the fatter pencil COMPENSATED its speed in order to keep the same momentum?"
Imagine if one of the pencils from system A were to suddenly change to a fat pencil, so that the whole system were to turn into B. Call the component L for the changing pencil L_up. Right after the magical change, L_up is gonna spike. To compensate, L_lo must increase in magnitude to oppose L_up and to conserve net L at 0. When you were thinking about the fat pencil, you linked its component L with an L from system A. This deosn't hold. IF you are going to change from A to B, you can't expect L_up to remain the same, because even if the speed doesn't change (this is an arbitrary rule, since you're doing a magic swap in the first place), the fat pencil has more mass and thus a different moment of inertia. So...since L_up changes (pencil:skinny->fat or arm:tucked->extended), you don't need to think about somehow conserving just the L_up part of the net angular momentum.

"won't it ultimately depend on the torque?"
Yes, actually, I think I glossed over this too much, and made a bad model. Let's say your waist can provide 10 ft lbs of torque, and any more than that, the muscles involved will snap (ouch). If you're using the max 10 before you extend your arm, you're gonna break your waist muscles once you do extend your arm.

Conservation of physical quantities is not magic -- some process is required to do the conserving. In this problem, the only way to make sure that L_up and L_lo have matching but opposite momenta is via the waist. Extending your arm momentarily increases L_up, and your whole body system can resolve it in 2 ways:
- Maintain the old L_up and L_lo, and not use any more waist effort. Thus both parts will slow down.
- Manage the increase L_up. To do this, the waist needs to jack up its torque to 'transfer' some of the deltaL into the lower body, to increase (but opposite dir.) L_lo. This isn't free though. Increasing L_up and L_lo thusly means that their spinning speed with respect to each other increases. And to get that speed boost, against larger moments of inertia in both upper and lower body, your waist needs to put out more torque. If you can't provide this extra torque, the kicking leg won't move as fast, and thus won't be as powerful.

You can feel this sometimes when you throw a 'lazy' kick. If you just half-assedly throw your arm out and don't exploit the extra moment of inertia to really power your hips through the kick, then your leg will just hit the bag with no oomph. This is because you're not mustering up the increase in torque you need to drive your lower body.

Feign the kick and step through with a punch? :P

Honestly I'm very much from the "keep your hands" up school, however disregarding oppertunities such as an extended arm for more power etc would be silly.

When you say that the angular momentum of both systems (the fat pencil and the thin one) must be equal to zero and this is because the angular momentum of each system are in opposite directions. (oh whoops isn't momentum a scalar quantity? I forgot I must be thinking of angular acceleration)
To be exceedingly clear (you need that for physics, i guess):
L = Iw
L and w are vector quantities. I is scalar. Linear momentum is a vector, too, BTW. Vector variables are bolded, when you can't put the nice arrow over them.

Picture a record playing on a record player on a table. The record spins clockwise. Its L vector points straight up. If you were to spin the record CCW at the same rate, L would point down, and have the same length. It's an arbitrary and somewhat confusing definition, but it works for the kinds of problems you work with with rotation.


Thai round kick goes up and then turns in horizontal at the last moment in one smooth motion. Does our arm move in a almost diagonal way? (it does for me when I do round kicks (same with buakaw)
Now we're making the problem more realistic but a lot more complex. We'd have to mix linear and angular momentum to capture the effects of the leg and arm not quite rotating in the same plane the whole time. But, I'm gonna go ahead and say that those effects are not essential to understanding the original point, of extending the arm.