Biomechanical Principles and levers Newton's Laws of Motion

Biomechanical Principles and levers This sections will develop your understanding of the mechanics of movement, looking at how motion (how we move) and forces (cause movement to change) can be applied to performance in physical activity and sport.

Newton's laws of linear motion Linear motion is motion in a straight or curved line, with all body parts moving the same distance at the same speed in the same direction. A 100- metre athlete will travel with linear motion in a curved line when running the bend. Newton's laws are physical laws which you may have encountered in physics lessons. They describe the relationship between a body and the forces acting upon it, and the body’s motion in response to these forces to help us understand human movement.

Key Terms You Should Know Before we start this section Linear Motion Motion in a straight or curved line, with all body parts moving in the same direction. Inertia The resistance an object has to a change in its state of motion.

Newton's First Law of Motion (The law of inertia) Inertia is the resistance an object has to a change in its state of motion. If an object is at rest, it will remain still. Similarly, if it is moving in one direction it will continue to do so at the same velocity until another force is exerted upon it. The bigger the mass, the larger the inertia of a body or object. This means that more force will be needed to change its state of motion. Consider two rugby league players running towards you, one weighs 75kg and the other 100kg. Which would be easier to stop? The player weighing 75kg, as they have less inertia

Newton's First Law of Motion (The law of inertia) Continued Newton's first law states ‘A body continues in its state of rest or motion in a straight line, unless compelled to change that state by external forces exerted upon it’ In simple terms, a force is required to change the state of motion. If a body changes its state of motion, it starts, stops, accelerates, decelerates or changes direction. For example in high jump, the athlete changes their state of motion at take-off when they travel vertically to clear the bar.

Newton's Second Law of Motion (The law of acceleration) ‘The rate of momentum of a body (or the acceleration for a body of constant mass) is proportional to the force causing it and the change that takes place in the direction in which the force acts.’ In simple terms, this law means that the magnitude (size) and direction of the force applied to a body determine the magnitude and direction of the acceleration given to a body. The rate of acceleration is directly proportional to the force causing the change and the following equation is often used to calculate the size of a force. Force = Mass X Acceleration (F=ma)

Newton's Second Law of Motion (The law of acceleration) Continued This suggests that if the mass involved remain constant, then acceleration is equal to the size of the force causing it. To provide the acceleration at the start of a sprint race, an athlete will have to apply a large force internally with their gluteal, quadriceps and gastrocnemius as they drive forward. Similarly a tennis player will impart a large force on the ball so that it accelerates over the net in the direction in which the force has been applied.

Newton's Third Law of Motion (The law of action/reaction) To every action (force), there is an equal and opposite reaction (force) Newton's third law of motion describes what happens when two bodies (or objects) exert forces on one another. Action and reaction are equal and opposite and always occur in pairs. Action acts on one of the bodies, and the reaction to this action acts on the body. At a sprint start, the athlete pushes back on the blocks as hard as possible (this is the ‘action’) and the blocks push forward on the athlete (this is the reaction) providing forward acceleration.

Newton's Third Law of Motion (The law of action/reaction) Continued Most instances with newton's third law involve ground reaction force (GRF). This is the force exerted on the ground by the body in contact with it. An individual standing still on the ground exerts a contact force (weight of the individual) and at the same time, an equal and opposite ground reaction force is exerted by the ground on the individual. For example a swimmer pushing backwards on the water with their hands and feet. At the same time, the water thrusts the swimmer forward

Key Terms You Should Know Before we start this section Newton's first law of inertia ‘A body continues in its state of rest or motion in a straight line, unless compelled to change that state by external forces exerted upon it’ Newton's second law of acceleration ‘The rate of momentum of a body (or the acceleration for a body of constant mass) is proportional to the force causing it and the change that takes place in the direction in which the force acts.’ Newton's third law of action/reaction To every action (force), there is an equal and opposite reaction (force)

Apply it to football Newtons Law Application Inertia In a penalty, the ball (body) will remain on the spot (state of rest) unless it is kicked by a player (an external force is exerted upon it) Acceleration When the player kicks (force) during a game, the acceleration of the ball (rate of change of momentum) is proportional to the size of the force. Eg the harder it is kicked, the further it will go in the direction it is kicked Action/reaction When a footballer jumps up (action) to header a ball, a force is exerted on the ground in order to gain height. At the same time, the ground exerts an upward force (equal and opposite reaction) upon the player