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Sport studies
biomechanics
Some definitions…
• Velocity – speed in a given direction
• Acceleration – rate of change of velocity
• Force – a push or pull that changes the
state of motion of an object
• Inertia – resistance to action or change
• Momentum – the quantity of motion that
occurs
Newton’s first law of motion
Law of inertia
• A body at rest or moving with constant
velocity in a straight line, will continue in that
state until a force of sufficient magnitude
occurs to overcome its inertia
• Basically, an object will not move until you apply
a force that is large enough to move it
• If it is moving, it will remain on that path, unless
a force great enough is applied in a different
direction
Newton’s second law of motion
Law of acceleration
• This law states that the acceleration of a
body is proportional to the force acting on it
and that the acceleration takes place in the
direction in which the force acts
• Basically.. the greater the applied force, the
greater it’s acceleration will be
• This law also takes into account the fact that the
accelerating effects will vary depending on the
mass of the body which experiences the force
• Therefore.. For a greater mass, a lager amount
of force is needed to provide the same
acceleration
Newton’s third law of motion
Law of action/reaction
• When on object exerts a force on a second
object there is force equal in magnitude, but
opposite in direction exerted by the second
object on the first
• So.. For every action there is an equal, but
opposite reaction
• Eg. A sprinter on the blocks experiences a force
propelling him forward. As the athlete pushes
backwards and downwards on the blocks, the
blocks push the athlete upwards and forwards
out of the blocks – ground reaction force
forces
• Internal forces – produced by the
muscles
• External forces – gravity, air resistance
etc.
• Force can…
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•
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Get objects moving
Stop objects moving
Change the direction of a moving object
Change the speed of a moving object
Balance another force to keep an object still
Gravity
• Gravity is a constant force
• Gravity provides a constant force on
matter, which is commonly known as
weight
• Weight is mass multiplied by gravity
• Gravity causes objects to travel towards
the earth at a constant rate of acceleration
(10m/sec²)
Centre of gravity
• The centre of gravity is defined as the
point around which a body’s weight is
equally balanced in al directions
• Single segmental objects have their own
centre of gravity directly above the base of
support.
• Sometimes the centre of gravity can be
outside the body (eg.Arc)
Human body centre of gravity
• Standing still – the centre of gravity is located in
the abdominal cavity, about 6 inches above the
pubis symphysis
• As your position changes – so does your centre
of gravity
• The position of the centre of gravity will
determine whether the body is in balance
• Stability and balance will be easier if:
•
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•
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The mass is large
The base is large
The centre of gravity is low
The centre of gravity is located over the base
Impulse
• Impulse is the product of the force that is
applied to an object and the length of time
the force is applied
• When a force is applied to an object, the product
of the force (F) and the length of time (t) that the
force is applied, is called the impulse of the force
• Impulse = Ft
• Impulse is equal to force X time, measured in
Newton seconds
• it depends on:
• The time for which the force acts
• The size of the force applied
Momentum
• Refers to the quantity or amount of
motion
• Momentum = mass X velocity
Impact
• The size/force applied between objects
during collision
• The greater the momentum, the greater
the impact
Frictional force
• Acts on the interface of surfaces which
are in contact and acts in the opposite
direction of motion
• The co-efficient of friction indicates the
ease of momentum i.e. the larger the coefficient of friction, the harder the
movement is
Projectiles
• Height of release
• Shotputters release the shot well above shoulder height, with
the shot landing at ground level
• If the shotputter could release the shot from ground level the
optimum angle of release would be at 45°
• Angle of release
• The shape of the objects flight path is dependant on the
angle it is released at
• If ball is thrown straight up it will go directly upward and
pulled straight back down by gravity
• If ball is thrown at an angle above 45° it will have a trajectory
in which height dominates over distance
• Speed of release
• If you throw with a faster arm action, it will go
further
• Air resistance doesn’t really have an affect on the
shotput, because it is heavy
Net force
• All force acting upon a body or object
• Ice puck example,
• Net force is 0 when puck is resting on surface of ice
because vertical forces acting on puck cancel each other
out
• When the hockey stick strikes the puck, an additional
force is introduced, causing the puck to accelerate
during the period of time the stick and puck are in
contact
• Once contact between the puck and stick is broken,
there is a zero net force situation again. Therefore,
according to Newton's 1st law of motion the puck will
continue to travel at a constant velocity