Download Newton`s Third Law - Jan Roscoe Publications

Jan Roscoe Publications
AQA Examinations
AS and A Level Physical Education
AS / A year 1 (A1)
AS 7581
Section 3.1 Factors affecting participation in physical
activity and sport
3.1.5 Biomechanical movement
3.1.5.1 Biomechanical principles
INDEX
3.1.5 Biomechanical movement
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AQA AS / A1 Level Physical Education
NEWTON’s LAWS OF MOTION
NEWTON’s FIRST LAW OF MOTION
NEWTON’s FIRST LAW OF MOTION - THE EFFECT OF FORCES
NEWTON’s SECOND LAW OF MOTION
NEWTON’s THIRD LAW OF MOTION
EXAMPLES OF NEWTON’s THIRD LAW
REACTION
DISTANCE
DISTANCE - DISPLACEMENT
POSITION
SPEED - VELOCITY
CENTRE OF MASS (GRAVITY)
BALANCE and TOPPLING
BALANCE and TOPPLING - STABILITY and TOPPLING
BALANCE and TOPPLING - FACTORS AFFECTING STABILITY
BALANCE and TOPPLING
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s LAWS OF MOTION
1st LAW
zero net force
acts
- constant
velocity
2nd LAW
a net force acts
F=ma
force produces
acceleration
NEWTON's LAWS
3rd LAW
one body exerts
force on another
- reaction
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s FIRST LAW OF MOTION
NEWTON’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
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s FIRST LAW OF MOTION
NEWTON’S FIRST LAW
• with zero net force an object
– is stationary or
– moves at constant speed in
the same direction
•
for the sprinter, horizontal forces
cancel out
•
and vertical forces cancel out
•
hence he or she travels at constant
speed
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s FIRST LAW OF MOTION
NEWTON’S FIRST LAW
• examples:
– a sprinter running at constant
speed
– a cyclist going at constant
speed
– a swimmer swimming at
constant speed
– any vehicle going at constant
speed
– any sportsperson standing still
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s FIRST LAW OF MOTION
NEWTON’S FIRST LAW
THE EFFECT OF FORCES
• this law does not mean that there
are no forces
• very large forces can act
• but if the object is going at constant
speed
• these forces MUST cancel out
•
•
for the sprinter, vertical arrows are
the same size and therefore cancel
out
horizontal forces are the same size
and therefore also cancel out
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s SECOND LAW OF MOTION
NEWTON’S SECOND LAW
•
this law is used when a NET FORCE acts on an object
•
net force forwards produces acceleration - positive
•
net force backwards produces deceleration - negative
•
net force sideways produces change of direction
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s SECOND LAW OF MOTION
NEWTON’S SECOND LAW
FORMULA
• force = mass x acceleration
F
= m x a
index
•
hence the bigger the force the
bigger the acceleration
•
the bigger the mass, the smaller
the acceleration
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s SECOND LAW OF MOTION
THE SPRINTER
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•
four forces are acting
•
upwards force = downwards force
•
therefore there is no upward acceleration
•
the sprinter runs horizontally
•
net horizontal forces act backwards
•
there is a net backwards force
•
producing a negative acceleration
•
or deceleration
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s THIRD LAW OF MOTION
NEWTON’S THIRD LAW
•
this law is used when two bodies exert forces
on one another
•
action and reaction are equal and opposite
in direction
Helen Roscoe Photography
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s THIRD LAW OF MOTION
NEWTON’S THIRD LAW
•
action of jumper down on ground
(force in black)
•
= reaction of ground up on jumper
(force in red)
•
the harder you push down on the
ground, the more the ground pushes
up on you
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s THIRD LAW OF MOTION
APPLICATIONS
•
at the swim start - the swimmer pushes back on
the blocks as hard as possible
•
the blocks push forward - and provides forward
acceleration - on the swimmer
Helen Roscoe Photography
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
NEWTON’s THIRD LAW OF MOTION
APPLICATIONS
• a swimmer drives backwards on water
with hands and feet (force in black)
•
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the water pushes the swimmer forward
(force in red)
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
EXAMPLES OF NEWTON’s THIRD LAW
REACTION FORCES
•
are forces acting via Newton’s Third Law
•
when one object pushes on another, the first object experiences
a force equal but opposite in direction to the second
•
jumper pushes down on the ground,
ground pushes up on the jumper
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
EXAMPLES OF NEWTON’s THIRD LAW
REACTION FORCES
•
are forces acting via Newton’s Third Law
•
when one object pushes on another, the first object experiences
a force equal but opposite in direction to the second
•
weight lifter pulls up on
weight, weight pulls down on
lifter
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
EXAMPLES OF NEWTON’s THIRD LAW
REACTION FORCES
• swimmer pushes backwards on the
water
•
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reaction force thrusts the swimmer
forward
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
EXAMPLES OF NEWTON’s THIRD LAW
REACTION FORCES
• canoeist pushes backwards on the water
•
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reaction force thrusts the canoe forward
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
EXAMPLES OF NEWTON’s THIRD LAW
REACTION FORCES
• sprinter pushes back and down on the ground
•
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the ground pushes upwards and forwards on
the sprinter
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
EXAMPLES OF NEWTON’s THIRD LAW
REACTION FORCES
• in cycling, the tyre on the rear wheel
pushes backward on the ground
• the ground pushes forward on the rear
wheel
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
REACTION
INTERNAL FORCES
• are exerted on both origin and insertion of a muscle.
• the force on the insertion is a reaction to the force on the origin
•
•
•
force on origin pulls bone H to the right
force on insertion pulls bone U to the left
the two forces are equal in size but opposite in direction
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
DISTANCE
DISTANCE
• means the total path length
moved by a body
• example:
– a 10,000 m race is run round
and round the track
– 25 times 400 m, starting and
finishing POSITION are the
same
– distance travelled is 10,000 m
•
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unit the metre m
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
DISTANCE - DISPLACEMENT
DISPLACEMENT
• this means the vector distance
from a fixed point (starting point or
origin)
• the actual ‘as the crow flies’
distance between start and finish
(with direction included)
• example:
– the start and finish of a long
distance race (stage 5 of the
Tours de France)
– may be 190 km apart due
West, but the distance travelled
may be 250 km!
•
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unit the metre m
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
POSITION
POSITION
• a way of explaining where a point is relative to
some fixed point
index
•
position is usually expressed in terms of
coordinates (x and y) like a graph in maths
•
example:
– the centre forward takes a shot from a
position 20 m out from the goal line, and 10m
to the left of the left hand post
– the left hand post is the fixed point or origin
of measurement
– 20 m and 10 m are the coordinates of the
position of the centre forward relative to that
point.
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
SPEED - VELOCITY
SPEED
• = distance moved v = s unit ms-1
time taken
t
• = scalar (no direction)
• = distance moved in 1 second
VELOCITY
• = speed in a given direction
• = vector
DISTANCE / TIME graph
• gradient of graph is velocity
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
CENTRE OF MASS (GRAVITY)
CENTRE of MASS (CoM)
• this is the scientific term for centre of gravity since the concept is not dependent on gravity
• CoM is the single point in a body which
represents all the spread out mass of a body
•
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the weight acts at the CoM since gravity acts on
mass to produce weight
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
CENTRE OF MASS (GRAVITY)
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
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
CENTRE OF MASS (GRAVITY)
WHERE IS THE CENTRE OF MASS?
•
note the position of this person’s centre of mass (red
dot)
Helen Roscoe Photography
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
BALANCE and TOPPLING
BALANCE
• to keep on balance the CoM
must be over the base of
support
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
BALANCE and TOPPLING
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
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
BALANCE and TOPPLING
STABILITY and TOPPLING
• the CoM must be over the base of support if a person is to be on
balance
•
so a person holding a balance is said to be in equilibrium
•
like a person holding a handstand
•
this is an unstable equilibrium
because a very small force
could cause the CoM to move
enough to topple the person
Helen Roscoe Photography
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
BALANCE and TOPPLING
STABILITY and TOPPLING
• the CoM must be over the base of support if a person is to be on
balance
•
•
so a ball sitting on the floor can be said to be in equilibrium – in this
case neutral equilibrium – since a small sideways force will cause the
ball to move, but remain in equilibrium – still stable to toppling
as is a gymnast lying on the floor!
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
BALANCE and TOPPLING
FACTORS AFFECTING STABILITY
• the CoM must be over the base of
support if a person is to be stable
•
hence the bigger the area of the base of
support, the further the CoM would have to
be moved to make the situation unstable
(would topple)
•
so for a rugby player receiving a tackle, he
would be more stable if his / her feet were
as wide apart as possible
•
also, the higher the CoM, the less stable a
person would be, so the rugby player would
crouch low to lower his / her CoM to
make his / her situation more stable
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
BALANCE and TOPPLING
STABILITY and TOPPLING
• the CoM must be over the base of support if a person is to be on
balance
•
so a beam gymnast will need careful control of the position of her CoM
if she is not to fall off
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3.1.5 Biomechanical movement
AQA AS / A1 Level Physical Education
BALANCE and TOPPLING
TOPPLING
• during a swim start, the swimmer topples forward
• by adjusting the line of action of the weight acting through
the CoM so that it falls in front of his / her feet
• and then allows the toppling to tip him / her forwards into
the water
•
•
when the body is almost horizontal, he / she drives hard with
the legs on the start platform
to propel him / her forward into the race
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