Sports Mechanics Principles

Forces Centre of Gravity Newton’s Laws Impulse Reaction Momentum Acceleration Linear Motion

Forces Internal forces – produced by the muscles External forces – gravity, air resistance etc.

Forces “something that causes an object to be deformed or moved.” (Roberts & Falkenburg, 1992) Force can: Get objects moving Stop objects moving Change the direction of a moving object Change the speed of a moving object Balance another force to keep an object still

External Forces Worksheet Get objects moving Stop objects moving Change the direction of a moving object Change the speed of a moving object Balance another force to keep an object still Gravity Friction Air resistance Ground reaction force Point of application Direction Resultant action

Summation of Joint Forces In whole body sports skills: Using (recruiting) joints in the order, big to small, will make objects move faster. (acceleration). Using (recruiting) joints in the order, small to big, will usually result in deceleration.

Locating Centre of Gravity Single segmental objects have their Centre of Gravity directly above the base of support. Sometimes the Centre of Gravity can be found outside the body. (Arc)

Gravity: A Constant Force The Earth’s rotation produces a force called gravity. Gravity provides a constant force on matter, which is commonly understood as weight. Weight is mass multiplied by gravity. Gravity causes objects to travel toward the earth at a constant rate of acceleration. 10m/sec 2

Human Body Centre of Gravity Standing still – centre of gravity is located in the abdominal cavity, about 6 inches above the pubis symphysis. As your position changes – so does your centre of gravity. The position of the centre of gravity will determine whether the body is in balance.

Definition The centre of gravity is defined as the point around which a body’s weight is equally balanced in all directions. (Hall, 1999)

Centre of Gravity, Stability & Balance Stability and balance will be easier if: The mass is large The base is large The centre of gravity is low The centre of gravity is located over the base Headstand Handstand

Centre of Gravity When the force of gravity acts on a body, it acts through the centre of gravity and always moves towards the centre of the earth. Symmetrical objects like balls and cubes have their CoG in the exact centre of the object. Objects are 3 dimensional, so the CoG will be at the point where the axes of all 3 planes meet.

Inertia - Newton’s First Law Newton’s first law of motion states that: “a body will continue in a state of rest or in a straight line of uniform motion unless acted on by an external force.” (Roberts & Falkenburg, 1992) Inertia is the resistance of an object to change. An object at rest will remain at rest unless acted upon by a outside force.

Impulse When a force is applied to an object, the product of the force (F) and the length of time (t) that the force is applied, is called the impulse of the force. Impulse = Ft Impulse is equal to Force x time, measured in Newton Seconds.

Impulse Depends on: The time for which the force acts The size of the force applied

Reaction - Newton’s Third Law Newton’s third law of motion states that: “for every action there is an equal and opposite reaction.” (Roberts & Falkenburg, 1992)

Momentum Refers to the quantity or amount of motion Momentum = Mass x Velocity The runner has a mass of 75 kg and is running at 5 m.s -1. What momentum does he have? 375 kg.m.s -1

Acceleration - Newton’s Second Law Newton’s second law of motion states that: “When a force acts on an object, the object accelerates in the direction in which the force is acting.” Acceleration is the rate of change of velocity and is determined by force. Acceleration = The final velocity minus the initial velocity divided by time. A = v – u t

Linear Motion Movement in a straight line of a body or body part. (Translatory motion) The sprinter runs from A to B.