Mechanics of Movement

Learning Objective Describe Newton's Laws and apply them to movements

Inertia Reluctance of a body to change from its existing type of motion is called inertia. When a body is at rest it is reluctant to do anything Set of weights Bowling ball- heavy Can not be gained or lost Mass determines inertia- greater mass = greater inertia Harder to change speed/ direction

Newton's First Law (Law of inertia) A body continues in a state of rest or motion in a straight line, unless compelled to change that state by external forces exerted upon it.

Application Will keep travelling at a constant velocity But in real life forces act upon things- friction and gravity If an object is changing in velocity and/or direction then a net external force must be acting on it. If it is not changing then the net external force is zero. Snooker ball example.

Newton's Second Law (Law of Acceleration) The rate of change of momentum of a body is proportional to the force causing it and the change that takes place in the direction in which the force acts.

Momentum=Mass x Velocity Quantity of motion that a body possesses. Two rugby players same velocity different masses.. Two rugby players same mass, different velocity.. Downhill skier/rugby scrum have large momentums

Force is needed to change momentum of an object In most sports the mass of an object is assumed constant so can be removed from the equation- so change in momentum becomes change in velocity Rate of change in velocity is directly proportional to the force causing the change The change will happen in the same direction of the force Rate of change of velocity = Acceleration

Force x Mass x Acceleration Forces are vector- they have direction Acceleration will only happen whilst the force is being applied to the object i.e. foot on ball In sport contractions of various muscles provide the force.

Sprinter Sprinter on the starting blocks applies a force that provides them with acceleration. The magnitude of acceleration is proportional to the magnitude of force exerted. Direction of acceleration is dependent on direction of force Force – muscles through foot.

Newton's Third Law (Law of Reaction) To every action there is an equal and opposite reaction

In the second law we suggested that the sprinter applies a force in order to accelerate and this force determines the direction of acceleration. However the force and acceleration appear to be in opposite directions! This is due to the Third law

In sport normally to do with the performer and the ground. The sprinter exerts a force on the blocks, and there is an equal but opposite force driving the sprinter forwards; but the blocks are attached to the earth and the earth has such a large mass compared with the sprinter the acceleration given by the sprinter is insignificant. However the force given to the sprinter by the earth causes them to accelerate. Ground Reaction Force

Application of the Laws Newton’s Laws Application Law of inertia In a penalty kick the ball (body) will remain on the spot (in a state of rest) until it is kicked by the player (an external force is exerted upon it) Law of Acceleration When the player kicks (force applied) the ball during the game, the acceleration of the ball (rate of change of momentum) is proportional to the size of the force, so the harder the ball is kicked the further and faster it will go. Law of Reaction When a footballer jumps up (action) to win a header, a force is exerted onto the ground in order to gain height. At the same time the ground exerts an upward force (equal and opposite reaction) upon the footballer.

Can you give your own examples in your sport??