Kinetics and forces
The branch of mechanics concerned with the study of forces and torques and their effect on motion, as opposed to kinematics, which studies the motion of objects without reference to its causes.
Dynamics/Kinetics, in general, is the branch of mechanics concerned with the study of forces and their effect on motion. With rugby being a sport of hitting and smashing, there is no better place to test and see Newton's laws, than on the rugby field. Concepts behind Newton's laws can be spotted everywhere, and are used to calculate the physics behind some of rugby's biggest events, such as a bone crushing tackle, or a kick keeping the play alive. Since contact is inevitable in every play in rugby, Kinetics can be argued to be the biggest physics concept when it comes to this sport
Dynamics/Kinetics, in general, is the branch of mechanics concerned with the study of forces and their effect on motion. With rugby being a sport of hitting and smashing, there is no better place to test and see Newton's laws, than on the rugby field. Concepts behind Newton's laws can be spotted everywhere, and are used to calculate the physics behind some of rugby's biggest events, such as a bone crushing tackle, or a kick keeping the play alive. Since contact is inevitable in every play in rugby, Kinetics can be argued to be the biggest physics concept when it comes to this sport
Newton's first law
Newton's First Law, the Law of Inertia states that all objects will stay at rest or at a constant velocity until acted upon by an unbalanced force. In rugby this law can be displayed through a kick. Before somebody makes contact with the ball, the ball will be at rest and will stay that way forever unless a force is acted upon it. But once it is kicked, the law still acts upon it due to the fact that the ball would continue to go in the direction it was kicked but it doesn't because of gravity, an unbalanced force, which forces the ball to come back to the ground. The horizontal speed stays constant because there is no force acting upon it but gravity is constantly acting upon it thus forcing the ball to follow its trajectory and bringing it back to earth. Once the ball stops again though, it will stay at rest until once again a force acts upon it making it leave its rest position. A player catching the ball would apply a force equal to its horizontal and vertical forces but in the opposite directions, completely stopping the ball from moving.
kicking forces
To begin the kickoff, the ball is at rest remains at rest until it is struck by the unbalanced force from the kickers kick. The kick is a huge unbalance in power, as the average force of an rugby players foot during a kick is about 450 pounds of force (2002N), even causing the ball to deform upon contact.
Newton's second law
Newton's Second Law is constantly displayed in the sport of rugby through tackles. According to Newton's 2nd Law of motion, force equals the mass, multiplied by the acceleration of which it's moving, the force of a player hitting another player can be found by multiplying his mass by his acceleration (F=ma). This means the larger the mass, the larger the force, so the bigger the player, the harder the hit. But on the other hand the faster the acceleration, the larger the force, so the faster the player, the more powerful the hit. This explains why a player running can be tackled easily by a defender waiting for him, because the acceleration of the defender from the quick burst of energy is much more than the runners. It depends on how much force the defender can generate prior to the hit by exploding from his spot as fast as he can.
I have 2 scenarios below, the first being 2 players with the average masses of a rugby wing (80 kg), the one on the left is the ball carrier and the right is the defender. The defenders acceleration is faster due to the fact that he is waiting for the offense to come and he explodes from his standing spot to tackle the ball carrier, therefore making his tackle attempt successful. The second displays the same type of play but we bring in a superstar, Jonah Lomu, who is also a wing but he stands at 130 kg (50 kg more than average!), he is a total powerhouse and we are going to see why he is so dominant with his size. He is on the left at 130 kg and the defender again is an average sized wing at 80 kg with a faster acceleration because of the quick burst of energy.
The calculations show that Jonah produces 390N of force compared to the 320N from the defender and that's why he is so difficult to tackle, below is a highlight reel of his showing many of his plays where he trampled his defenders.
Newton's third law
Newton's Third Law, states that for every force, there is an equal and opposite force being exerted. People rarely think of it, but how does a person run? Most people would say a person pushes their feet off the ground and propels themselves forward, but in reality, the earth pushes back up on your feet with the exact same amount of force, the only difference is that the earths mass is much larger than yours so it doesn't move and you do. What propels the person is the equal force being acted on you from the earth, the earth pushes back on the player, which in turn pushes the person forward. Although you can only see the person move, the earth exerts the exact same amount of force on the person, the mass of the earth is just too heavy for a person to move. In rugby players are constantly running and so the physics behind Newton's Third Law apply to rugby as well.