Biomechanics

In general, all motion may be described as translation or rotation or some combination of these two.

Takes place when a body moves so that all parts of it travel exactly the same distance, in the same direction, in the same time. The line taken can be a straight line (rectilinear translation) or a curved one (curvilinear translation).

Rectilinear: straight path

Curvilinear: curved path

Takes place when a body moves along a circular path about some line in space so that all parts of the body travel through the same angle, in the same direction, in the same time. This line, which may or may not pass through the body itself, is known as the axis of rotation.

rotation around an axis of rotation

General motion is a combination of translation and rotation.

combination of linear motion + angular motion

Force is measured in units called newtons (N). No movement can be started without the application of a force.

When a runner wants to leave the blocks, a force has to be exerted against the blocks. When a footballer wants to push an opponent out of the way, a force has to be exerted against the opponent.

Force can therefore be defined as a pushing/pulling or hitting/throwing action which is applied to an object to start movement, stop movement or cause changes in movement.

An Englishman, Isaac Newton (1642-1727) formulated three laws of motion:

◦ NEWTON’S FIRST LAW – The Law of Inertia

◦ NEWTON’S SECOND LAW – The Law of Acceleration

◦ NEWTON’S THIRD LAW – The Law of Reaction

Newton’sLaws

Law of ………………

Definition Examples

1st

2nd

3rd

Complete this table in your work book!!

What would happen if you threw a ball into space where there is no gravity or air resistance? In the absence of any forces, the ball would remain a constant velocity, that is it would continue to travel in a straight line with the same speed.

INERTIA – is the reluctance of a body to change what it is doing.

“A body will continue in its state of rest or uniform motion unless acted upon by an applied force”

For example, a ball rolling across the ground will continue in that state until external forces (eg. friction from the ground) cause it to come to a stop.

Inertia refers to the degree of difficulty in getting an object to move, due to the magnitude of the object’s mass.

In simple terms, the heavier the object, the more difficult it is to get that object moving and the lighter the object the easier it is to get that object moving.

If the motion is rotational i.e. Turning around an axis, we have to consider not only the object’s mass, but the distance of the mass from the axis of rotation. The rotational inertia of an object is determined by its mass and its radius of rotation.

It can be calculated using the following formula: I = mr2 (Inertia = mass x radius of rotation)

In the human body, most actions involve rotation of a mass around a turning or axis point therefore in rotational inertia the axis point becomes an additional consideration.

In this case, resistance to movement is not only concerned with the magnitude of the objects mass but also the distance this concentration of mass is from the axis of rotation (radius of rotation)

An object’s rotational inertia can be therefore increased or decreased by varying the size of the mass, and varying its distance from the axis of rotation (radius of rotation)

Rotational inertia increases when◦ The mass is heavier (if the distance of the mass

from the axis is kept the same)◦ The mass is further from the axis of rotation (if

the mass is kept the same)

Example Ice Skating The ice skater begins to spin with arms spread

apart then suddenly brings them closer to the body. The end result of tightening up is that the skater’s spin (angular velocity) increases.

Analyse your videos taken earlier in the unit to see where you are using summation of forces, and Newton’s law of Inertia in your throw or jump.

How could you apply these two concepts to create more success in your technique?