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A2 Revision

Mechanics of movement

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Biomechanics

Terms:• Displacement;• Velocity;• Acceleration

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Displacement or Distance

AB

Length of journey in meters =

Straight line from start to finish in meters =

distance

displacement

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Displacement and velocity

• Velocity • Displacement and velocity have direction

displacement divided by time

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Velocity/time graphs – 100-m

Velocity

Time

Velocity changes

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Change in velocity

• Average velocity changes

• Change in velocity over a period of time is called

Acceleration =

acceleration

final velocity – initial velocity

time taken

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Velocity

Time

On a velocity-time graph acceleration is shown by the

of the line

steepness (gradient)

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Velocity

Time

Highest acceleration?

zero acceleration?

deceleration?

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Vectors and scalars

• Displacement, velocity and acceleration have direction as well as magnitude =

• Temperature, time, speed, etc do not have direction =

vectors

scalars

Typical Question

Explain, in terms of the player moving towards a ball, the difference between velocity and acceleration. (3 marks)

Answer

1. Velocity = rate of change of displacement/displacement ÷ time;

2. How fast/quickly player moves towards ball;3. Acceleration (not speed) = rate of change of

velocity/change in velocity ÷ time (taken to change)/[Vf - Vi] ÷ time/a = v ÷ t;

4. Push off ground to accelerate (not speed)/change velocity;

5. (Both) have direction/are vectors.

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Forces

Forces cause ; A force is a

“A force is that which alters or to alter a body’s or of in a line.”

If a body changes or , a has been applied

force

movementpush or a pull

tendsstate of rest

uniform motion straight

direction velocity

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Forces affecting motion

• Action/muscular forces• Reaction forces• Gravity• Air resistance/friction

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Forces acting on a sprinter

Action force of muscular contraction

Friction

Gravity

Air resistance

Equal and opposite Ground Reaction Force

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Forces applied - runner

• forces applied to ground• Reaction force from• • between ground and foot• Air resistance

Muscularground

GravityFriction

to forward motion

Typical question

What forces act on a player when they are running during a game? (3 marks)

Answer

1. Gravity;2. Friction;3. Air resistance;4. Action force/muscular forces;5. Reaction forces/ground reaction force;

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Newton’s Laws

• 1st Law –

• A body will remain in its / until affected by a

• A body has a to change its

Law of inertia

state of motion

reluctance

rest force

state of motion

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Applying Newton’s 1st Law

• Sprinter remains in on blocks until a causes him to change his

• Ball keeps moving until a causes it to change its

force

set positionforce

state of motion

state of motion

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Momentum

• In sporting situations, remains constant; therefore inertia

• All objects in motion have mass and

• Product = • Momentum =

massstays the same

velocity

momentumquantity of motion

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Newton’s Laws

• 2nd Law –

• The rate of change of is to the force causing the change, and the change takes place in the in which the force was applied • In sport, remains constant and therefore momentum equates to

Law of acceleration

momentumdirectly proportional

directionmass

acceleration

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Applying Newton’s 2nd Law

• The (size) and of force applied by the sprinter to the blocks determines the and of received – because mass remains constant• Acceleration is to applied•

magnitude direction

magnitude directionacceleration

proportional force

F ∞ a

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Newton’s Laws

• 3rd Law – Law of Reaction

• To every force there is an and reaction force

equalopposite

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Applying Newton’s 3rd Law

• The sprinter applies a to the ground. This results in a force causing movement

• The sprinter cannot move the

• Reaction force from ground =

muscular forcereaction

earth/blocks

Ground Reaction Force

Typical question

Use Newton’s Laws of motion to explain how a sprinter leaves the starting blocks. (6 marks)

Answer1. Newton’s First law – a body remains in a state of motion until acted

upon by a force;2. Sprinter remains stationary until a force causes them to change their

state of motion/overcome their inertia; 3. Newton’s Second Law- body accelerates/changes momentum with a

magnitude that depends on magnitude of force causing change (nb not F=ma);

4. Also force gives direction;5. Sprinter applies force to ground;6. Magnitude of muscular force determines acceleration given to sprinter;7. Newton’s Third Law – to every force there is an equal and opposite

reaction force;8. Action/applied force are muscular contractions given to earth;9. Equal and opposite reaction force moves the lighter mass of the

sprinter;

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Projectile motion

•Flightpath = parabola

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Components

• Motion of projectile has components• component• component

two

VerticalHorizontal

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Projectile motion

Vertical motion is affected by gravity –

Horizontal motion is affected by friction –

Vertical component

Horizontal component

continually decreasing

usually negligible

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Variations in vertical and horizontal components

This causes the observed parabolic flight and affects the motion components as follows:

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Projectile motion

Factors affecting distance travelled:• • •

Speed of release Height of release Angle of release

Typical question

Elite golfers use their clubs to overcome the forces acting on the golf ball so that it travels long distances.(i)Describe how the impact of the golf club, gravity

and air resistance affect the velocity and acceleration of a golf ball. (4 marks)

(ii)The flight of a golf ball is said to be parabolic. Explain the term parabolic and the main factors that limit the distance that a golf ball will travel in flight. (4 marks)

Answer(i)1. Golf club - Applies force2. Gives acceleration/changes momentum3. Gravity - Only affects vertical component of flight4. Reduces/negative effect on velocity/ decelerates5. Air resistance - Affects both vertical and horizontal components6. Reduces/negative effect on velocity/ decelerates(ii)7. Left and right sides match/mirror each other8. Height of release – higher height above landing more distance9. Angle of release – closer to 45° the better10. Speed of release – more force more speed more distance

Factors affecting the fight of a parabola

• Velocity of Release- increase in speed of release = increase in displacement of projectile.

• Height of Release- increase in release height= increase in displacement of projectile.

Angle of Release

1) Release height= landing heightLofted pass in footy= 45 degrees2) Release height> landing heightShot put= < 45 degrees3) Release height< landing heightBunker shot in golf >45 degrees

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Impulse

• • Measured during – force

platforms on track• Displayed as

Force x time

sprinting

Force.time graph

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timeforce

negative

positive

• Impulse = force x time• shown as area under graph

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• Impulse is mainly concerned with

• Involves • Two parts

single events – a footfallhorizontal forces only

to a footfalllanding – negative horizontal forces

Push-off – positive horizontal forces

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Typically

timeforce

neg

pos

Landing - negative impulse

Push-off - positive impulse

Net impulse is difference between positive and negative impulses

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Typically

timeforce

neg

pos

Small negative impulse

Large positive impulse

Net impulse is positive – performer is accelerating

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Typically

timeforce

neg

pos

Large negative impulse

Small positive impulse

Net impulse is negative – performer is decelerating

Typical question

(i)As a sprinter accelerates along the track at the beginning of a race, they generate a large impulse. What do you understand by the term impulse? (2 marks)

(ii)Sketch and label a graph to show the typical impulse generated by the sprinter at this stage of a race. (6 marks)

Answer(i)1. Impulse is force x time/force applied in unit of time;2. Equates to change in momentum;3. If mass constant equates to change in acceleration;(ii)4. Positive clearly larger than negative;5. x axis – time;6. y axis - force;7. Units of force shown as Newtons;8. Units of time shown as milliseconds/less than 1 second9. Time intersecting at zero on force axes;10. Positive and negative force axes labelled;11. Shape of graph - negative and positive components of force shown with

negative first;12. Negative and positive components of force labelled;

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Angular motion

• During free rotation – remains constant

angular momentum

• This situation occurs when and during flight and during ice-skating (ignore friction)

somersaulting spinning

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• Angular momentum is the

• Angular momentum =

• Angular velocity =

• Moment of inertia =

Angular Momentum

quantity of rotation

angular velocity x moment of inertia

rate of rotation or spin

spread of body’s mass

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Constant angular momentum

• If you change the moment of inertia; then the also changes

• Tuck body up small to moment of inertia and angular velocity

• Seen during and

angular velocity

lowerincrease

somersaultsice-skating spins

Typical question

Explain the mechanical principles that allow spinning ice-skaters to adjust their rate of spin.

(6 marks)

Answer1. Ice may be regarded as a friction free surface/friction is

negligible;2. During spins angular momentum remains constant;3. Angular momentum is the quantity of rotation;4. Angular momentum = angular velocity x moment of inertia;5. Angular velocity = rate of spin/how fast skater spins;6. Moment of inertia = distribution/spread of mass around axis;7. Changing/reducing moment of inertia affects/increases

angular velocity;8. Skater brings arms into body allowing rate of spin to increase;