BIOMECHANICAL PRINCIPLES

PROJECTILES• the motion of objects in flight

– human bodies– shot / discus / javelin / hammer– soccer / rugby / cricket tennis / golf balls

• is governed by the forces acting– weight– air resistance– Magnus effect– aerodynamic lift

• and the direction of motion

angle ofrelease

DI STAN CETR AVELLED BY

PR OJ ECTI LE

height ofrelease

speed ofrelease

Module 2562 A.2.4

FORCEFORCE• FORCE is push or pull

• the unit is the NEWTON (10 N is approx the weight of 1 kg)

• force changes the state of motion of an object• force causes acceleration or deceleration or change of

direction• the more force the bigger the acceleration

• force changes the shape of an object

• WEIGHT• FRICTION• REACTION FORCES• AIR RESISTANCE / FLUID FRICTION

– all these forces affect the sportsperson

Motion and Movement - Newton’s Laws

Module 2562 A.2.5

NEWTON’S FIRST LAW of MOTIONNEWTON’S FIRST LAW• this law is used when ZERO NET FORCE is

applied to an object• this doesn’t mean that zero force acts, but

that all forces MUST CANCEL OUT

• with zero net force an object– is STATIONARY or– moves at CONSTANT SPEED in the

SAME DIRECTION

• a sprinter in full stride has four forces acting• but they cancel out exactly• therefore he / she travels at constant speed

Motion and Movement - Newton’s Laws

Module 2562 A.2.6

NEWTON’S SECOND LAW of MOTIONNEWTON’S SECOND LAW• this law is used when a NET FORCE acts on an

object

• net force FORWARDS produces ACCELERATION

• net force BACKWARDS produces DECELERATION

• net force SIDEWAYS produces CHANGE OF DIRECTION

• the bigger the force the bigger the acceleration

• the sprinter slows down at the end of a race• there is a net force backwards• so the sprinter decelerates

Motion and Movement - Newton’s Laws

Module 2562 A.2.7

NEWTON’S THIRD LAW of MOTIONNEWTON’S THIRD LAW

• this law is used when two bodies EXERT FORCES ON ONE ANOTHER

• ACTION AND REACTION ARE EQUAL and OPPOSITE IN DIRECTION

• action of jumper down on ground= reaction of ground up on jumper

• the harder you push down on the ground, the more the ground pushes up on you

• this upward force on the jumper is the force acting to cause the take off

Motion and Movement - Newton’s Laws

Module 2562 A.2.8

The EFFECTS of FORCEEFFECTS of FORCE• force causes linear acceleration or deceleration• including change of direction

• the point of action of a force affects what happens

• friction acts at the feet of a sportsperson, not enough of it and the person’s feet slip

• if a force acts through the person’s centre of mass (CofM), then linear motion is caused

• if a force acts to one side of the CoM then rotation is caused

• like take-off in the high jump, the reaction force acts to one side of the CoM

Motion and Movement - The Effects of Force

CENTRE OF MASS

CENTRE of MASS (CoM)• this is the single point in

a body which represents all the spread out mass of a body

• the weight acts at the CoM since gravity acts on mass to produce weight

WHERE IS THE CENTRE OF MASS?

• position of centre of mass depends on shape of body

• this is how the high jumper can have his CoM pass under the bar

• but he could still clear the bar

BALANCE and TOPPLING

BALANCE• to keep on balance the

CoM must be over the base of support

TOPPLING

• the CoM must be over the base of support if a person is to be on balance

• toppling would be caused by the weight acting at the CoM creating a moment about the near edge of the base of support

• this can be used by divers or gymnasts to initiate a controlled spinning (twisting) fall and lead into somersaults or twists

CENTRE OF MASS - GENERATION OF ROTATION

FORCE ACTING AT TAKE-OFF THROUGH CoM

• the line of action of a force on a jumper before take-off determines whether or not he rotates in the air after take off

• if a force acts directly through the centre of mass of an object, then linear acceleration will occur (Newton's second law), no turning or rotating

• example : – basketballer : force acts through CoM

therefore jumper does not rotate in air

CENTRE OF MASS - GENERATION OF ROTATION

FORCE ACTING AT TAKE-OFF NOT THROUGH CoM

• a force which acts eccentrically to the centre of mass of a body will cause the body to begin to rotate (will initiate angular acceleration)

• this is because the force will have a moment about the CoM and will cause turning

• example : – high jumper : force acts to one side of

CoM therefore jumper turns in air

STABILITY AND BASE OF SUPPORT

3 key termsCentre of Gravity (COG) the theoretical point where all the body weight is concentrated or the theoretical point about which the body weight is evenly distributed.Line of Gravity (LOG)– a straight line from the COG to the floorBase of Support (BOS) – The area in contact with the floor.

Stability is defined as the ability to hold or maintain a position in space.

There are four basic principles underlying stability.

Principle #1

• “The closer the line of gravity is to the centre of the base of support the greater the probability of maintaining balance.”

Why is this body in a stable position?

• “The line of gravity from the centre of gravity passes through the centre of the base of support.”

What happens if we move the line of gravity closer to edges of the base of support?

• We become more unstable. The chance of losing balance increases.

Principle #2

“The wider the base of support, the greater the probability of maintaining balance”

Consider wrestling. Is a wrestler more stable on all fours or standing up? Why?• This is because the size of the base of support is larger

so the center of gravity has further to travel to get outside the edges of the base of support so this becomes a stable position.

Principle #3

• “The probability of maintaining balance is increased when the centre of gravity is lowered in relation to the base of support.

This is because; ;.

As the centre of gravity is lowered, the distance the line of gravity has to travel to reach the edges of the base of support is greater than when the center of gravity is higher.

What other applications does this principle have in sports?

• In some sports we need to lower ourselves to improve stability e,g. a rugby scrum packs in low to improve stability

• What are the applications of this principle in sport.

• Where balance is important e.g sports such as tennis, gymnastics, hockey, and netball.The body often has to make adjustments to maintain balance.

Principle #4

“ The further one body part moves away from the line of gravity, the probability of maintaining balance decreases unless another body part moves to compensate for it.”

Consider the person doing the shot put.

What have they done to maintain balance in the throw?

As the shot putt and trunk moves back the right leg has moved out in the opposite direction to compensate for this to keep balance.

This is because;

The line of gravity from the centre of gravity is kept above the base of support (foot on ground is base of support).

QUESTIONS

• Explain the relationship between the 3 components – BOS, COG and LOG

• What is the effect of moving the COG outside the body?

• What will happen if the LOG falls outside the BOS?

class 1E-P-L

class 3L-E-P

class 2E-L-P

J O I NTS ASLEVER S

eff ort inm uscle

load is forceapplied

pivot atjoint

LEVERS levers have an pivot (fulcrum), effort and load

• and are a means of applying forces at a distance from the source of the force

CLASSIFICATION OF LEVERS

• class 1 lever : pivot between effort and load

• see-saw lever found rarely in the body

• example : triceps / elbow

• class 2 lever : load between pivot and effort

• wheelbarrow lever, load bigger than effort

• example : calf muscle / ankle

• class 3 lever : effort between pivot and load

• mechanical disadvantage, effort bigger than load, most common system found in body

• example : quads / knee and biceps / elbow

MOMENT OF INERTIA (MI)• the equivalent of mass for rotating systems• rotational inertia

• MI depends on the spread of mass away from the axis of spin, hence body shape

• the more spread out the mass, the bigger the MI

• bodies with arms held out wide have large MI• the further the mass is away from the axis of rotation increases the MI

dramatically• sportspeople use this to control all spinning or turning

movements• pikes and tucks are good examples of use of MI, both reduce MI

CONSERVATION OF ANGULAR MOMENTUM

ANGULAR MOMENTUM (H)angular momentum = moment of inertia x angular velocity

= rotational inertia x rate of spin H = I x w

CONSERVATION of ANGULAR MOMENTUMthis is a law of the universe which says that angular momentum of a spinning body remains the same (provided no external forces act)a body which is spinning / twisting / tumbling will keep its value of H once the movement has started

therefore if MI (I) changes by changing body shapethen w must also change to keep angular momentum (H) the sameif MI (I) increases (body spread out more) then w must decrease (rate of spin gets less)

• strictly, this is only exactly true if the body has no contact with its surroundings, as for example a high diver doing piked or tucked somersaults in the air

• but it is almost true for the spinning skater !

CONSERVATION OF ANGULAR MOMENTUM - EXAMPLES

THE SPINNING SKATERarms wide - MI large - spin slowlyarms narrow - MI small - spin quickly

THE TUMBLING GYMNAST• body position open - MI large - spin

slowly• body position tucked - MI small -

spin quickly

Force SummationSTUDENTS WILL HAVE AN

UNDERSTANDING OF THE 5 PRINCIPALS OF FORCE SUMMATION

FORCE SUMMATION

• When we are trying to generate as much momentum as possible in activities such as throwing, kicking and striking the amount of momentum we can give to an object is determined by the sum of all forces generated by the different body parts. This is called force summation.

• There are 5 basic guidelines for giving an object as much momentum as possible.

#1 Using Body Segments • We should look to use as many body segments

as possible when trying to give an object maximum momentum. Why?

• Because we can maximise the muscular force that each muscle group associated with each segment can generate

#2 Stretch out

• Before we begin the sequence of movements, such as the hitting, throwing or kicking action we should stretch our muscles out to their optimal length. Why?

• It allows the muscle to be contracted with optimal force. It gets the blood flowing through the muscle and helps warm the muscle and therefore decreases the risk of injury.

#3 Sequencing of body segments

• Generally to give maximum momentum to an object in kicking, throwing and hitting we move larger muscle groups first and the smaller muscle groups closer to the object last.

• In effect we use the body like a giant whip. • What are the benefits of this?

• The momentum generated by larger segments can be passed on to smaller ones until we make contact/release etc.

• In a shot putt illustration, how do we see this principle being applied?

• Look at the order of execution sequence• i.e. legstrunkshoulderarmswristhandsfingers shot

#4 Timing of Body Segments• Generally to give maximum momentum to an object in

kicking, throwing and hitting we need to make sure that the right body segment is adding to the overall momentum at the right time.

• What could happen if the timing of the body segments is “out of order”?

• Not only does it lack coordination but maximum force generation is lost or lessened

• How does correct timing ensure maximum momentum?

• It means we use those larger muscle groups first and the smaller muscle groups last.

#5 Full range of motion

• Generally, to give maximum momentum to an object in throwing, kicking or striking, we need to move the segments through the greatest range of motion as we possibly can.

• What are the benefits of this? • The greater the range of motion the higher the

speed of the extremities on release/contact.

• How does correct timing ensure maximum momentum?

• No speed or force is lost throughout the entire movement.

• We can apply all this information to an example

of the javelin throw.• Using your knowledge of generating momentum,

explain how the athlete generates maximum momentum to the javelin upon release.

ANSWER

• They use the large muscle groups of the legs and trunk to initiate the force. This force passes onto the shoulder, arms and finally hand at release. Forces are getting increasingly larger up to release.

• The arm is fully extended at the shoulder prior to the throwing

action • Timing is legstrunkshoulderarmshand • The arm moves through its full range of movement to maximise

lever length and force summation