(Law of Inertia) : A net or unbalanced force is needed to change the state of motion of an object.
In a judo match, two players try to throw each other by first off-balancing the other player, then fitting their body into a position underneath their opponent for a throw, and finally throwing their opponent squarely on their back. The key to throwing the other player is the off-balance. Without first off-balancing your opponent, your chances of successfully completing a throw are slim. You're also asking to be countered if you don't off-balance first.
The first law of motion is important if you want to successfully throw your opponent in a match. For instance, suppose your opponent makes a move to his right in order to make you move to your left. In the instance that you are moving, you're temporarily off-balanced. Rather than letting your opponent keep control of the situation (in other words, he has you off-balanced so he can throw you), you can take advantage of his motion by using Newton's First Law. In this scenario, the statement "an object in motion will stay in motion" applies. As he is in motion (before he firmly plants his foot into the ground), you quickly turn into your opponent (for the "fit"), enabling you to lift him up with your hips and throw him. Essentially, what is happening is that as he steps, you turn into him and what he is really doing is stepping into your fit. Once he is there, he is off-balanced and all you have to do is pull him over your hips. Since he is in motion, he will stay in motion and fall over easily with little effort on your part to throw him.
(F = ma) : The acceleration of a body is proportional to the net force on it and inversely proportional to its mass.
If you are in a match, facing an opponent who weighs less than you and you push him with a quick acceleration of the hand, you will have exerted a larger force on him then you would have if you had only pushed him with a smaller acceleration and chances are that he will move or fall easier for you.
If, on the other hand,your opponent has more mass than you, the acceleration that you hit the smaller man with will have less force on the more massive man because acceleration is inversely proportional to his mass.
The faster you throw your opponent with, the larger the force that he falls with will hit the mat (and the cleaner your throw looks too, so that the referee will award you with an "ippon" - a full throw - thus ending the match)..
If he is larger than you, then he will not need to exert as much acceleration on you in order to achieve the same amount of force a man smaller than you would.
So, what does this tell you? That you'd be better off fighting a smaller man than a bigger one, that is, until you become a black belt or like Professor Jigoro Kano (the founder of judo).
(Action/Reaction) : For every force on an object there will always be an equal and opposite force exerted by that object.
This rule works when you're in a mat hold down. The force that you exert on your opponent's shoulder in order to hold him down is reciprocated by his shoulder. In other words, his shoulder pushes up at you with a force that is equal to the force you exert on him.