CHAPTER 6

1. CHAPTER 6 BIOMECHANICAL PRINCIPLES OF FORCE PRODUCTION Coefficient of restitution Summation of momentum

2. KEY KNOWLEDGE • Newton’s laws of motion, including an understanding of force, mass and weight, acceleration and inertia applied to sport and physical activities. • The principle of conservation and transfer to momentum, impulse and sequential and/or simultaneous force summation applied to sport and physical activities. • Angular motion including torque, angular velocity, momentum and moment of inertia and their application to sport and physical activities. • Elasticity and the coefficient of restitution of sport equipment and the effect of rebound velocity on performance.

4. INERTIA • The tendency of an object to resist change in its state of motion. • An object will remain in a state of rest or constant motion unless acted upon by • an external force. Inertia is a tendency for a body to resist change in its state of motion, whether that be at rest or moving with a constant velocity. • It is much harder to move or change the state of motion of an object that has a greater amount of inertia. • The amount of inertia an object has is directly related to its mass. • The more massive an object, the greater the force needed to change its state of motion. • The amount of force needed to move a 100 kg barbell is much greater than the amount needed to move a 5 kg barbell. The heavier barbell has much greater inertia than the lighter one. • A wet football requires greater force to move it a set distance than a dry football because as the ball becomes waterlogged the mass increases.

5. MASS AND WEIGHT • Mass- the amount of matter that makes up an object. • Weight- the measure of gravitational force acting on a body • Mass and weight are two different quantities that are often used interchangeably. • Mass is a measure of the amount of matter an object is made up of. The units of mass • are kilograms, and because body ‘weight’ is often given in kilograms the two terms • are often used to mean the same thing. However, weight is a force that is exerted on • the body by gravity. Weight is directly proportional to the mass of an object. It can • be calculated by multiplying mass by the acceleration due to gravity (9.8 m/s2). As • weight is a force, the units are newtons.

6. FORCE • The force of gravity will pull a ball • back to the ground after it has been thrown, hit or kicked. • The force generated in a muscle when it contracts pulls on the bone to produce movement. • A force is defined as a push or pull. • Forces can produce movement or change the motion of an object. • The force a cyclist applies to the pedals produces movement of the bike, which is a change in the motion of the bike from stationary to moving. • The forces absorbed by a baseball glove when a ball is caught correctly prevent the player getting hurt by the impact of the ball. • The force changes the motion of the ball by stopping • the flight of the ball. Even though all forces produce or alter • movement, it is not always obvious.

7. MOMENTUM • Momentum is a measure of the amount of motion that an object has. The amount of momentum • an object has is directly related to its mass and its velocity. Momentum is equal to the mass of the object multiplied by the velocity of the object and has the units kg m/s: momentum = mass × velocity

8. IMPACT • A collision between two objects can be considered to be an impact. • In sport, the impact occurs between a ball and another contact surface such as a bat, body part or ground surface. • The type of collision that occurs determines the motion of the ball after impact.