Hitting harder: physics made easy

Introduction

Most martial artists share the goal of “hitting harder”. This is usually expressed in colloquial terms as hitting with “more force” or “more power”. But even a basic knowledge of physics will tell you that “force” and “power” are not the same thing. Which is it that makes you hit “harder” – force, power or both? And is it more helpful to talk in terms of something else such as momentum?

Understanding force

Many people think of “force” in very nebulous terms (perhaps explaining such misappropriations as “The Force” in Star Wars). So what is “force” as understood in physics?

Force is something that enables you to cause an object with mass to accelerate.

In other words, it is Newton’s famous formula: f = m x a.

Using this formula people commonly argue that in order to hit “harder” they need to –

1. maximise their mass; and
2. maximise the acceleration of their attack.

For the most part this is true: if you have a big mass and accelerate that mass well, you will maximise your chance of applying a greater force to a target. But it is important to note that this argument refers to force you are applying to your own body. It does not refer to the force you are applying to the target.

Put another way, in this argument the equation f = m x a assumes that m = your body mass, not the mass of your target (as it should for the purposes of calculating the force on that target!).

So instead of looking at the force applied to your body, let’s examine the force applied by your body to a target.

For example, consider a stationary object weighing 1 kg. For simplicity’s sake, let’s say you hit/push it for one second causing it to reach a velocity of 1 m/s. We can use the following formula to determine the accleration of the object:
v = u + at
or

where a = acceleration, v = the final velocity (1 m/s), u = the initial velocity (0 m/s) and t = time.

Accordingly we can determine the acceleration of the object as 1 m/s2.

Now in order to determine how far the object will move in that one second, we can use the formula:

or

where s = distance.

From this formula we can determine that the object will move 0.5 m. Put another way, if we cause a stationary 1 kg mass to move 0.5 m in one second we’ve caused it to accelerate at 1 m/s2. By reference to the formula f = m x a you’ve applied a net force of 1 N or kg/m2 to effect this result.

Clearly the more force you apply, the farther you will move it in the same time (by accelerating it to a higher velocity). So imagine you have managed to force the 1 kg object 1 m in one second (ie. you’ve managed to accelerate the object from 0 m/s to 2 m/s). In that case you’ve applied a force of 2 N.

So far so good. At this point you might be forgiven for thinking: “If I can knock my opponent across the room with my punch, surely I must be hitting very hard?”

Well yes and no. Let me explain by asking a question: Does “hitting harder” really mean moving something farther? If it does, a strong push or shove arguably constitutes the “hardest hit”, because a strong push is likely to move something farther than a punch or kick. I’m sure you’ll agree that this is an extraordinary proposition (for more on this topic see my article "Visible force vs. applied force"). The former is designed to move, the latter to hurt (with or without moving).

It seems that “movement”, and hence the acceleration of your target, might not be the most significant indicator of “how hard you have hit” after all.

Furthermore, you might hit a brick wall with your fist as hard as you can. You’d be unlikely to move the wall at all. Does this mean you haven’t hit it “hard” (you have) or with any force (you have, except that the wall has exerted an equal force on your fist as per Newton’s Third Law)?

Theoretically if you were heavy enough you could push the brick wall over (ie. move it) – but we’re back to “pushing”. Theoretically if you punched fast enough your peak force1 would be sufficiently high to break through the brick wall without “moving” it (eg. a high velocity bullet can pass through objects without displacing them).2

So what distinguishes a strike from a push? It is your deceleration before hitting the wall. The more you decelerate before you reach your target, the slower the velocity at impact. If your velocity at the point of impact is low, you’ll effect more of a push. If your velocity at the point of impact is high, you’ll effect a blow.

By now you should be starting to see the limitations of using f = m x a as a means of determining “how hard you hit”. The equation might tell me how to increase the force applied to my body (which is necessary in order for me to apply force to someone/something else), but it doesn’t tell me how to apply my force.

Rather than talk about the acceleration of your mass or the mass of the target, it seems to be more useful to consider the velocity of your mass at the moment of impact.

This is momentum.

The importance of momentum and its transfer

In physics momentum is defined by the following equation:

Momentum (p) = mass (m) x velocity (v)

Obviously a one tonne (m) car travelling at 60 km/h (v) will do some damage if it hits you. A fist, while not so destructive, obeys the same physics.

It appears that a well-thrown fist reaches its maximum velocity when the arm is about 80% extended. Accordingly if your punch covers say, 60 cm from a fully chambered position to full extension, then your punch reaches its maximum velocity at 48 cm.

This speed is generated by moving a number of body parts toward the target; eg. the whole body (by stepping or lunging), hips, leg, shoulder, upper arm and lower arm. If the full range of movement is used, and the body parts act in a staged way to transfer momentum in a whip-like sequence from larger to smaller body parts, then the fist will be accelerated to the fastest possible speed. A reverse punch is biomechanically better suited to transfer momentum than in this way than a leading arm punch.

So far so good. But doesn’t this come to more or less the same thing as previously argued – in other words to hit “hard” you want a mass travelling at a high velocity? Again, close. But there is more. You not only want your body to have momentum; you want to transfer it effectively.

Now, momentum transferred is called "impulse". The equation for impulse is as follows:

Impulse = force x time

In other words force = impulse / time

With impulse a fixed quantity, force and time are necessarily inversely proportional. In other words, one can deliver a given amount of momentum by:

1. transferring a large force for a short time; or
2. by transferring a smaller force over a longer time.

So, the longer it takes to transfer momentum, the less force is applied. If you want to maximise your force, you must ensure that as much of your momentum is transferred on impact as possible. This is true for both focused punches and follow-through punches used by boxers. The only real difference is that punching with boxing gloves increases the time it takes to transfer momentum which reduces the force compared to a similar punch with bare knuckles. In other words, when punching with gloves the momentum transferred will be the same as bare knuckles but there will be less impact force and instead the target will feel more of a push.

"Power punching" vs. traditional punching

When you throw a cross punch (see my article “Chambering punches”), the punch travels a longer distance than a straight punch – particularly if (as is inevitably the case) the cross follows a curved path to some extent. The further the punch travels from the chambered position to the target, the more time it has to accelerate and the faster its maximum speed will be (at 80% extension). This is why the cross punch carries more momentum than any other punch.

If this is so, why wouldn't one always use a boxer's cross punch? Why would one ever stop one's punches at a pre-determined point rather than use a follow through? The answer to this lies in the inherently conservative approach of traditional strikes - ie. they are part of a civilian defence strategy that puts "not being hit" ahead of "hitting with maximum force". A traditional martial artist focuses a strike so as to stop it at a pre-determined point in space in order to avoid over-committing (eg. if the target was to duck or evade) (for more on this topic see my article "Stopping strikes at a pre-determined point").

However it is not true that the traditional strike is "weak": it achieves the maximum possible force that is possible if one doesn't use a follow-through. It does so by stopping the punch as quickly as possible at an optimum predetermined point just beyond the target. This is the definition of "kime" or focus, a concept central to the study of karate and many other traditional Far Eastern fighting disciplines - see my article "Kime: soul of the karate punch".

As with striking a makiwara or pad, the strike will be stopped earlier than the predetermined focus point by the target if it does connect. But if the strike does not land, the traditional martial artist's positioning, balance and defensive capacity won't be compromised. For more on this topic see my articles "Stopping techniques at a pre-determined point" and "Karate punches vs. boxing punches".

The role of power

That a competitive boxer can apply a staggering amount of force with a gloved cross compared to, say, a suburban karateka performing a bare knuckle reverse punch, does not detract from the fundamental observation I have made above: a gloved punch has more “push” and less “hit” than any bare knuckle variant. The competitive boxer is big enough, and is moving fast enough, to compensate for any “diffusion” of his/her force resulting from the wearing of gloves. He or she might knock you across the room – and hurt you at the same time.

However this in no way invalidates a karate punch (ie. a straight punch) in which the impulse is maximised. Put another way, while the karate punch might not carry the same momentum as a boxer's cross, it can still achieve an effective result - without necessitating the same amount of work as one sees in a boxing contest.

By “work” I mean, of course, the concept in physics defined by the following equation:

Work (w) = force (f) x displacement (d)

In other words, work is done when a force acts upon an object to cause a displacement. Because of the diffusion of force caused by gloves, a boxer's gloved cross punch will cause more displacement than a bare knuckle punch thrown with the same force. This means that the gloved boxer has necessarily done more “work” than he or she might have had to had he or she been fighting without gloves.

Work can also be described as the amount of energy transferred by a force. In this case the gloved cross punch, using the same force, transfers less energy than it would were the boxer not wearing gloves.

As I have said above, the boxer will usually more than compensate for any diffusion caused by gloves. He or she will do so by adding more “power”. However this also means that a great deal of boxer training is concerned with power, given it's central role in the art of fighting with gloves.

In physics the equation for power is as follows:

Power = work / time

To compensate for the diffusion of his/her force because of gloves, the boxer has to work more in a shorter time. This means he/she needs more power and will train accordingly.

Don’t all martial artists need power? Of course they do. Everyone will be subject to the same laws of physics. However, remember the purpose of this article: to determine how one can “hit harder”. Power and displacement are directly proportional. Yet “hitting harder” doesn’t require greater displacement - rather, greater displacement is (at least to some extent) a foreseeable by-product of gloved fighting.

For more on the issue of displacement, see my article "Visible force vs. applied force".

Accordingly, while boxing training and methodology is eminently applicable in self defence, it is a mistake to assume that it is perfectly suited to a civilian defence system - or that other strategies designed with civilian defence specifically in mind are less effective for their purpose just because they don't develop strikes that are as "powerful" as those in combat sports.

Conclusion

If you want to “hit harder” you should look to maximising your momentum by increasing your velocity and/or your mass. But you also need to look closely at how you transfer that momentum.

Transferring momentum effectively is a question of maximising your impulse; ie. applying a large force over a short time. The alternative (a small force for a longer time) produces a push – not a “hard hit”.

The smaller your impulse (ie. the longer you take to apply a force), the more power you will need to compensate.

It is my view that a martial system that focuses on maximising power is what is classified in China as “external” or "hard" (waijia). On the other hand, a system that focuses on impulse is what is classified in China as “internal” or "soft" (neijia) – see my articles “Internal vs. external martial arts” and “Understanding the internal arts”. As I have noted in that article, no art is purely "external" or "internal" – just as no strike can be effected without some impulse and no technique can be effected without power.

A good “civilian defence system” will attempt to find a pragmatic mix of both – or ideally adopt a sequential system of teaching progressively more “impulse-oriented” techniques and less “power-oriented” techniques – something I call “sequential relativism” in martial arts training (see my article “My quest for the martial ‘holy grail’”).

Footnotes

1. It is important to note that when I refer to maximising “force” I am of course referring to the peak force – force is never applied evenly but is applied in a curve. It is the peak force that is in issue when it comes to “hitting hard” – not the average or median force etc.

2. In this article I have not attempted to address factors such as stress and pressure. In the case of the former we are all familiar with the idea that we can increase damage by targeting softer areas. In the case of the latter we also know that we can increase damage by applying force over a smaller surface area – ie. increasing pressure. These are topics unto themselves which I hope to address at another time.

Copyright © 2008 Dejan Djurdjevic

Comments

  1. Dan

    Good post, and decent coverage of power generation.

    I find however that too often instructors would tend towards creating force with effort and muscle. This is rather than using basic technique and physics and applying simple power generation principles.

    I think an important side note to power generation is the various ways power is generated by martial artists. At the basic level, I point out to my students that we generate the necessary power by the following movements: 1) linear acceleration - foot work and gap closing strategies, 2) hip rotation, 3) shoulder rotation, and 4) whipping movements. Those are mainly for upper body techniques. For lower body intermediate-level techniques there is the expansion of the body for long range penetrative/thrusting kicks, and contraction for some short range kicks.

    And lastly, the concept that gels these various movements together at the end - is the support and lock up of the skeletal structure as the muscles focus the force upon impact and follow through with the target.

    I cover some power generation ideas in my blog. You can find them at: Posts on Traditional Taekwondo on Generating Power.

    Good seeing you keep up your blogging activities.

    Regards,

    Colin

    ReplyDelete
  2. Hi Colin

    I absolutely agree, and your blog article is very informative. I shall leave a comment there.

    In this article I just thought I'd concentrate on the physics basics. I haven't yet tried to deal with the material you cover (ie. how you can actually maximise your momentum transfer)!

    Don't know if I need to now with your blog article!

    Cheers!

    ReplyDelete
  3. Dan -

    Reading my response I can see I rushed through some of my sentences - to the point where I wasn't very clear. I'm glad at least you could understand what I was saying.

    And yes, I understand how you're positioning your article. I took the initiative to make a post pointing to your article so I don't have to touch on the topic.

    I think that the concept of power generation is tied to the major basic techniques, which in turn is tied to combat strategy/tactics.

    What I was getting at in the first post was that I think hard style instructors tend to think about muscle power and effort first before finesse of technique. I may be talking out of my hat given that I have seen both good instructors and the run-of-the-mill McDojo type.

    I think my blog covers some power generation issues - spefically on the lunge punch, and roundhouse punch/kick. I've not taken the leap into combat tactics and power. Maybe soon - I have to post on a few intermediate level patterns.

    Great seeing your activity online!

    Regards,

    Colin

    ReplyDelete
  4. thank you for this article. I will keep it in mind when at Bag work at home. I never thought that less displacement of the bag would be more shock to it from good kime.

    on a side note... is that black and white photo of Hong Kong actor Kent Cheng Jut See? Great actor. http://www.hkcinemagic.com/en/people.asp?id=20

    ReplyDelete
  5. Hi Vinny - thanks for the kind words.

    No, the photo is of Taiwanese internal arts master Wang Shujin - student of Chen Pan Ling (who was my teacher Chen Yun Ching's father).

    ReplyDelete
  6. Hi Dan,

    Overall this is an excellent post, made more impressive because physics and human mechanics is tough territory.

    However :

    "So, the longer your fist is in contact with the target, the less force it applies"

    This is inaccurate and leads to a false conclusion. If the fist hit a wall and was not retracted for ten seconds would you take ten seconds as the time element of the (force = impulse / time) equation? no, of course not. The time element here applies only to the time taken to transfer momentum. Certainly when the fist is not in contact with the target then no momentum transfer can be taking place, but the transfer could have ceased at any time before contact is broken, so again contact-time is no measure of tranfer-time.

    The speed of the fist and the nature of the target govern the transfer-time; a brick wall will absorb all the momentum quickly, a head will begin to move and lengthen the transfer time.

    So back to:
    'to hit “hard” you want a mass travelling at a high velocity' - thats it in a nutshell.
    Follow through and focus have one thing in common; they are both after the fact in terms of impact.

    ReplyDelete
  7. Yes Stuart you’re right. All you need is high velocity. Focus just means you keep the speed high until the last possible moment – and thus stop quickly. In other words, you focus to stop your strike at a predetermined point in space (to avoid over committing if the target was to duck or evade) with an appropriate degree of penetration to ensure maximum momentum transfer. Like a makiwara, your fist will be stopped earlier than the predetermined focus point by the target if it does connect. For more on this topic see my article Stopping techniques at a pre-determined point.

    So I’ll change the statement to: “So, the longer it takes to transfer momentum, the less force is applied. If you want to maximise your force, you must ensure that as much of your momentum is transferred on impact as possible. This is true for both focused punches and follow-through punches used by boxers. The only real difference is that punching with boxing gloves increases the time it takes to transfer momentum which reduces the force compared to a similar punch with bare knuckles.”

    Thanks for reading and for your insightful comments!

    ReplyDelete
  8. That annoying kid who knows physics and wont shut up about itJune 8, 2011 at 6:22 AM

    "For simplicity’s sake, let’s say you hit/push it, causing it to move a whole metre in one second. You’ve accelerated the 1 kg mass from 0 m/s to 1 m/s; ie. you’ve caused the object to accelerate at 1 m/s2"

    This is incorrect. If you have in a second accelerated something from 0m/s to 1m/s it has not moved 1 meter. It has moved 0.5 meters.

    "So imagine you have managed to force the 1 kg object 2 m in one second (ie. you’ve managed to accelerate the object from 0 m/s to 2 m/s)"

    Same thing.

    I'm sure you know this. Easy thing to miss.

    Great article, as always.

    ReplyDelete
  9. You have a very good memory. I had forgotten the equation for impulse up until reading this post. Force times the interval of time the force is applied over, equals impulse.

    F=MxA. Or V^2.

    It gave me a useful conception tool to describe why duration or "sticking" to the target while your body's acceleration due to gravity is transfering kinetic force into the target surface. Before, I was mostly limited to the acceleration in F equals mass times acceleration. That higher the acceleration, due to gravity, the more powerful the blow. But this part explains duration and being sticky, which will improve my fine tuning of various strikes of mine.

    No wonder Torin said to extend the punch with the muscles after the impact of the body's force applies to the target area. That prevents a much easier phase diagram if I simply use my structure to initially reflect the force of my mass traveling at the target, and then use my muscles to maintain duration/contact until the additional acceleration and force defeats the target's resistance and absorption limits.

    Finding new and interesting ways to position my body so that it can resist the most force, yet also extend the most force along the greatest time interval (impulse written another way is force per second applied).

    ReplyDelete
  10. A source of frustration for me. I taught myself biomechanics in an attempt to understand and explain the biomechanics of a strike, in a meaningful way. It would appear that I did OK as I've been asked to co-author a book with a biomechanics professor at a Canadian university that has already published several very good books on biomechanics of various physical activities.

    Why do we want to transfer momentum? Work!? - that is just the most theoretical concept that does nothing for any practitioner. Power - please.

    Ask yourself, what causes an injury? Provide a definitive explanation, and then, you have something that can be of use to practitioners.

    I don't mean to be rude, it's just a very frustrating subject which I spent years resolving.

    ReplyDelete
  11. "Why do we want to transfer momentum? Work!? - that is just the most theoretical concept that does nothing for any practitioner. Power - please."

    I thought I explained that. Transfer of momentum = impulse.
    Force applied is impulse/time.
    We want to transfer force. To do that we need to transfer as much momentum in as little time as possible (this is to avoid a push).

    Power is, in my view, not very relevant to the discussion of the mechanics of a blow, such as punch or kick.

    "Ask yourself, what causes an injury? Provide a definitive explanation, and then, you have something that can be of use to practitioners."

    You can cause an injury without much momentum. An example would be a sharp needle being inserted into the spine. You don't need much force - no real momentum here. But that is hardly "civilian defence" - especially of the unarmed variety.

    I'm no physicist. So I might have things wrong here. But I can't see any issue arising out of your comments. Injury results from force that causes deformation (rather than force that causes movement). This means we need momentum transferred over a short period. The physics, as I see it, are simple. Other issues creep in, such as pressure. But these are tangential to the main issue (assuming we're comparing fist blows to fist blows, pressure exerted is more or less the same).

    Thanks again for reading and your comments.

    ReplyDelete
  12. You've got a lot of great class-room info here; and as a man of science I can certain appreciate it. However, at the end of your post you write "If you want to “hit harder” you should look to maximising your momentum by increasing your velocity and/or your mass. But you also need to look closely at how you transfer that momentum."

    Perhaps a couple training examples? Maybe a quick vid? I dunno, after reading all this I kind of felt like I wanted something more to sink my teeth into. Something I can do in plain english.

    I know this article is several years old, so you've probably have done this since then. Maybe a hyperlink to one of those articles?

    Osu!

    ReplyDelete

Post a Comment