Recently, I saw my friend, the Judo captain, easily defeated his much larger opponent with the famous "hip throw" as shown here. Upon researching, I found out that this throw can allow a weaker opponent to defeat the stronger one. I find this rather impossible. How is this possible?
I speculate that it is torque and forces at play. Based on my understanding, the person has to place his elbow at the opponent's centre of mass. I am not sure why this is so.
Furthermore, why does an incorrectly executed "hip throw", meaning that you do not pull your opponent close enough to you, fail? How does a mispositioning cause this to fail? I would appreciate if someone can explain to me the physics behind this Judo "hip throw".
Best Answer
The basic physics of the throw is momentum and the lever. You encourage your opponent to topple over, pivoting about the ground and your hip. At the same time you limit his options for evasive action. As long as your opponent is passive, unable to use his strength, it makes little difference that he is much stronger or much heavier than you.
Your opponent could be modelled as a tall column. To make a stationary column fall you pull or push high up, as far as possible from the pivot. The higher up you apply the force, and the lower down the pivot, the less force you need to use. Ideally the pivot is at the ground : you use your legs and hip to prevent your opponent moving his/her feet forward. The fact that the knee does not bend backwards means that the thigh has to move forward to get the foot forward, and your hip should obstruct his thigh.
It is easier to topple a column with a narrow base than one with a wide base. So you pull your opponent forward, where the base is one foot wide, instead of to the side, where the base is much wider if the legs are spread defensively.
If possible you time your throw when your opponent is moving towards you, to make use of his forward momentum and lack of a defensive posture. Walking can be described as continuously falling forward, pivoting about each foot in turn. With an unwary opponent, his momentum and weight do almost all the work. It is only in competition with an experienced opponent that you need to exert much force. See Problems hip-throwing a larger opponent.
The purpose of the thrown is not only to get your opponent on the floor but also onto his back where he is (presumably) more vulnerable. To do that you have to roll him over your hip. It requires the least amount of effort to do this if your hip is just below his centre of mass. Just below means that his weight and forward momentum create a torque which keeps him turning in the right direction.
If your hip is too high you will need to use more effort to lift your opponent over, and the difficulty will increase as the opponent is heavier. If your hip is too low you will not be able to prevent him from flexing his knees to bring his body low to avoid falling forward. Also, any force you exert on your opponent's arms from this position will be downwards, almost through the pivot, so it will have less turing effect (torque). See When doing a hip throw is there such a thing as too low?
A hip throw could fail if you allowed your opponent enough space to plant a leg forward to resist toppling or to flex his knees and drop towards the ground, lowering his centre of mass below your hip. Even with a passive 'opponent' which does not flex at joints, if your hip is too far forward the 'opponent' will make contact with it higher up, above its centre of mass.