At the outset, please note that the projectile (in this case, your body) is made to be rigid.\u00a0 Since your body shape does not change, (at least during the roll itself, unless something has gone terribly amiss), and further, since no mass is lost, you possess, according to mechanical theory, a rigid body<\/a>.\u00a0 In fact, the rigidity of your body is augmented by your muscles and joint cartilage, which take the brunt of stress during a fall, thereby relieving at least some structural strain.\u00a0 In mechanical terms, your joints and muscles act as a spring, which are compressed by the impact of falling.<\/p>\n Knowing this, the first step is to define the issue.\u00a0 In our case, the issue is the interplay between a rigid body and an exchange of energy, or more plainly stated, a forward throw.<\/p>\n Sensei teaches that rolls are meant to be taken close to the ground, to minimize injury.\u00a0 The principle behind her instruction is sound: the less distance an object travels from its original position, the less potential energy the object contains.\u00a0 The less energy the object contains, the less severe the jolt your body will receive at the moment of impact.\u00a0 (This is a good thing).\u00a0 Since energy can neither be created nor destroyed, the energy escapes from your body into the ground, partly in the form of heat, but also as stress on your legs, since you lowered yourself to take the fall.<\/p>\n The question, however, naturally arises: Why roll at all?\u00a0 Why not, in fact, fall straight down, if all we’re talking about is transferring energy from one system to another?\u00a0 Consider figure 1:<\/p>\n Here, the rigid circular body drops straight down.\u00a0 When force is applied in only one direction, or vector, there is only one point of impact, at the bottom of the circle.\u00a0 Assume the circle represents uke.\u00a0 Assume further that the point of impact represents, say, uke’s shoulder.\u00a0 In the situation described, therefore, the entirety<\/em> of force represented by the arrow acts upon uke’s shoulder, causing maximum shear stress at that point.\u00a0 This is not a good thing, and may in fact cause injury.<\/p>\n We roll to transfer energy forward<\/em>, so that the energy we receive is not just transferred downward<\/em>, into the mat.\u00a0 Consider figure 2:<\/p>\n Here, we add a force, in the forward direction.\u00a0 This forward force creates a moment arm<\/a>, which causes the body to rotate.\u00a0<\/p>\n This rotation causes you to roll, which again is a good thing, since the energy now has a path that doesn’t lead straight down.\u00a0 (Ouch!)\u00a0 Essentially, the energy is defused into the forward roll.<\/p>\n This<\/em> is why we roll.\u00a0 We use our rotation to transfer energy from an unwanted result, as described in figure 1, to one which allows us to practice another day.\u00a0 Both scenarios expend the same amount of energy.\u00a0 The difference is that in the latter, the structural integrity of uke is not tested, potentially to a breaking point.<\/p>\n Standing Tall<\/strong><\/p>\n We have one more wrinkle to discuss: Returning to our feet.\u00a0 Remember, a rigid body requires the same amount of energy to fall as it does to return to its former position.\u00a0 (That pesky law of conservation<\/a> again.)\u00a0 The entire process, therefore, is balanced.\u00a0 Ideally, we as Aikidoka should use that energy, rather than oppose it, to maintain our own balance, on and off the mat.<\/p>\n Before I wrap up, I want to address a misconception I sometimes hear about forward rolls.\u00a0 It is a fallacy to state that the forward and downward motions described in figure 2 negate each other.\u00a0 In reality, the energy transferred into the roll is still very much present.\u00a0 If the vertical force was in fact negated by the horizontal force, then even a perfect sphere, (or the world’s roundest Aikidoka), would not roll.\u00a0 Rather, the startled fellow would merely slide, on his shoulder in our example.\u00a0 And yes, this would be a bad thing, since the unlucky Aikidoka could not make use of that rolling motion to transfer his energy into a graceful upright stance, ready to train again.<\/p>\n Editors Note: This article was originally published in the January 2009<\/a> edition of our dojo newsletter.<\/a><\/span><\/p>\n![\"Figure](\"http:\/\/www.aikidocenterofjacksonville.com\/newsletters\/2009\/01\/figure-1.jpg\")
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