Biu Jiu Speedy?: Outer Linear Speed vs Inner Angular Acceleration

When we say to do Biu Jee one should move speedily, it can mean different things. In one scenario of “speedy” elbowing (Common mechanism 1 in the post Kneeing Elbow in Biu Jee – Tangential Force and Sucking in Whirlpool Analogy), the arm does so by travelling at high speed throughout a very noticeable distance. Because the distance is noticeable, we can see the “fastness”. Then we perceive that this move is speedy, and since speed means power, this elbowing must be too powerful to be blocked. At this point, let’s derail to consider the difference between speed and acceleration with regard to momentum.

“Momentum measures the ‘motion content’ of an object, and is based on the product of an object’s mass and velocity…… Force, on the other hand, is the push or pull that is applied to an object to CHANGE its momentum.“ * That is to say, in order to experience a force, the speed of the object must change (thus a change in momentum), by accelerating (giving out force) or decelerating (being stopped by a force). This is commonly notated as F=ma.

In the above scenario, the elbowing arm, which is both the initiator and traveller of the movement, looks very fast when travelling a noticeable distance. Over that high-speed range of distance, the speed is high but it doesn’t equate to a high acceleration (change of speed) too. When the travelling forearm lands on the target or being blocked in the course, its change of speed is indeed high (greatly reduced) but it is a deceleration caused by the blocking, and its resulting force is actually a reactionary force to that blocking force. This is so because the directions of its speed (being the traveller) and acceleration (being the initiator) are the same. Force in direction opposite to it can directly cause deceleration, cancelling out and finally overriding its acceleration, unless the latter is overwhelmingly greater than the former.

In the whirlpool-type elbowing, the initiator (the upper centre) and the traveller (the arm) are separate, so are the initiating acceleration and the travelling speed. The change in the traveller speed is caused by the spinning initiator “angular” acceleration. It is “angular” because it is from the centre rotating the arm at its “periphery”, such that the direction of the acceleration at the periphery is always changing and not at the opposite to the blocking force, making it hard to be stopped (decelerated). A high angular acceleration in the centre can readily give out great peripheral acceleration (and thus great force there at tangent) within very short peripheral distance. The distance travelled by the arm is then not that noticeable, possibly seen as “not speedy”. Biu Jee performed in this way is better described as speedy in terms of high acceleration in contrast to high speed. Nevertheless, it is up to you whether to let the arm travel a noticeable distance, as far as the “angular” acceleration in the centre is “speedy”!

(* https://www.physlink.com/education/askexperts/ae462.cfm)

2019.06.28

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