2Mechanics, Motion and
Movement
Motion
Motion is movement and is divided into three main categories:
Linear Motion
Angular Motion General Motion (bola)
Linear Motion
One example of Linear Motion in a straight line could be a tobogganist. All
parts of the body and the toboggan are moving in a straight line, the same
distance, in the same direction at the same speed.
Linear Motion in a curved line is very rare in human motion but can be
seen in a shot put (if no spin is impacted at release). All parts of the shot are
moving in a curved line, the same distance, in the same direction at the same
speed.
Angular Motion
The definition of Angular Motion is perhaps more difficult to understand.
To produce angular motion, the movement must occur around a fixed point or
axis, for example, like a bicycle wheel turning about its axle or a door opening on
its hinges. When we apply this concept to the human body we often talk of
athletes spinning, circling, turning and somersaulting, which implies that the
athlete or part of the athlete is moving through a circle or part of a circle about a
particular point. However, the most obvious examples of angular motion are the
limbs in our own bodies because they move around our joints, which are fixed
points. Consider flexion and extension of your elbow joint. You will notice that
the lower arm is moving through part of a circle about a particular point – your
elbow joint, which in this example is the axis of rotation.
General Motion
Most movements in sport are combinations of linear and angular motion
and therefore this third type of motion is the easiest to exemplify. The approach
run of a javelin thrower shows general motion. During the approach the javelin
and the torso of the athlete are showing linear motion by moving in a straight
line with all parts moving the same distance, in the same direction at the same
speed. However, the arms and legs of the athlete are showing angular motion as
the non-throwing arm rotates around part of a circle about the shoulder joint,
the upper legs about the hip joints, the lower legs about the knee joints and the
feet about the ankle joints.
, A second example of general motion is a wheelchair athlete. The body of
the athlete and their chair are displaying linear motion as they move along the
track, but the swinging action of the athlete’s arms and the turning of the chair’s
wheels exhibit angular motion.
Force
A force can perform the following actions:
Cause a body at rest to move, for example when we take a penalty flick in
hockey, the force we apply with the stick on the ball causes it to start
moving towards the goal.
Cause a moving body to:
Change direction, for example when we return a tennis serve, the force we apply
with the racket causes the ball to return to the server’s side of the net
Accelerate, for example at the end of the 1500m race, the force we apply to the
track enables us to accelerate toward the finish line faster than our opponents
Decelerate, for example at the bottom of a ski run, the force we apply to the
snow enables us to slow down
Change an object’s shape, for example when we jump on a trampoline,
the force of our body weight causes a distortion on the bed.
The extent to which the forces mentioned achieve the examples above
depends on where the force is applied and the size and direction of the applied
force. The size and direction of the force will obviously affect the change of
motion. The line of application of the force will also affect the subsequent
motion.
Centre of Mass
The mass of a body is the amount of material of which it is made, so it is
quite easy to say the a shot put will have a greater mass than a football, as it is
solid and therefore made from more as well as heavier material. The centre of
mass of a body is the point where all of its mass could be considered to be
concentrated.
In uniform symmetrical objects in which the mass is evenly distributed,
the centre of mass is found at the geometrical centre of that object, for example
a shot put, a discuss or a tennis ball. At this point, half of the mass is above and
half is below, half the mass is in front and half is behind, half the mass is to the
left and half is to the right of the centre of mass. However, centre of mass is not
always as simple to locate, as it is an imaginary point that can lie outside the
body. Consider a ring doughnut – its centre of mass will be at its centre which is
in the middle if the hole!
In the human body, the centre of mass is not a fixed point located in a
specific part of the body. Its location will vary depending on body position and,
as we will see, it can also be a point outside the local body.
,Locating an Athletes Centre of Mass
Athletes’ bodies are not uniform symmetrical shapes with mass evenly
distributed from head to foot. They are made up of bone, muscles, fat and
tissue, all of which vary in mass. To complicate things further, athletes
demonstrate a considerable range of body positions. In the simplest situation, for
an athlete standing upright with their arms by their sides, the centre of mass for
a male is about two to three centimetres above the naval and for a female it is
slightly lower. This is because, in general, males tend to have more body mass
concentrated in their shoulders and upper body whereas females tend to have
more concentrated at their hips.
As soon as the athlete moves from its symmetrical position, their centre of
mass also shifts, meaning that an athlete’s centre of mass rarely stays in the
same position for very long. If the athlete raises their arms, the centre of mass
will be higher to ensure that the body remains balanced in all directions form it.
If the athlete raises their arms while holding a barbell the centre of mass will be
raised even further as a majority of the mass is now concentrated at the top of
the body.
In extreme body shapes such as a good pike jump in tampolining or a
bridge in gymnastics, the centre of mass is a point that lies outside the body. In
the case of the trampolinist, the athlete’s arms and legs have moved so far
forward that the centre of mass has also had to move forward so much that it
has temporarily moved outside the body.
Stability
An athlete who is kneeling on all fours is more stable than an athlete who
is standing on one foot because the first will require a greater force to tip them
over. Stability is important in sport as a stable body position will enable an
athlete to resist motion, whereas an unstable one will enable an athlete go into
action.
The stability of an athlete is determined by a number of mechanical
principles that depend on the following:
Position of the athlete’s centre of mass
Athlete’s base of support
Position of the athlete’s line of gravity
Mass of the athlete
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