Transcript File

BIOMECHANICS EXAM
QUESTIONS AND MARK
SCHEMES
When learning the shot putt, young performers are often given a light implement, such as a
tennis ball, to practise the action. They often end up throwing the shot, rather than putting
with a straight-arm push.
Why would this be a bad idea when using the correct weight implement? Ignore the laws
of the event; your answers should refer to the principles of moments and inertia (4marks)
• If the shot is thrown, the length of the resistance arm is increased.
• When a shot is used instead of a tennis ball the increased weight will
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mean the moment of resistance will be huge and require a huge effort
force to overcome it (principle of moments).
This means that a greater force will need to be applied to release the
shot at the same velocity.
Angular momentum = angular velocity x moment of inertia
Moment of inertia = mass x distance from axis of rotation
If arm is brought out to ‘throw’ the object then moment of inertia will
increase (due to increase in distance from axis)
Increase in moment of inertia will cause a decrease in angular velocity
(due to conservation of angular momentum)
This will mean slower release of shot.
Also risk of injury to elbow joint due to size of resistance force.
Why do shot-putters use either the glide or a spin technique prior
to the release of the shot (3marks)
• Angular momentum= angular velocity x moment of inertia
• If the mass moves closer to the axis of rotation, the
moment of inertia decreases. If the moment of inertia
decreases, then the angular velocity must increase,
because angular momentum is conserved.
• Angular velocity and the moment of inertia have an
inverse relationship.
• Therefore by spinning in a tucked position the shot putter
will decrease their moment of inertia and thus increase
the angular velocity applied to the shot.
• Both spin and slide techniques increase the impulse
applied to the shot as the force is applied to it over a
greater period of time.
A number of factors affect the distance travelled by a
shot when thrown.
What are the three most importance factors that affect
distance travelled by a shot? (3marks)
• Height of release
• Speed of release
• Angle of release
With reference to the diagram, explain the
variations in the horizontal and vertical
components. (4 marks)
• As the shot starts its flight it has both vertical and horizontal
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components as it moves up and away from shot putter.
Gravity continually acts upon the shot.
Gravity transforms the positive vertical component into a negative
vertical component.
As gravity will always pull the shot down to Earth it is best for the
shot putter to release the shot at the highest point possible.
The horizontal component remains the same throughout (although
is eventually stopped by the negative vertical component).
Use Newton’s laws to explain how the ball moves once
the player makes contact with it in football. (7 marks)
• 1st law- law of inertia. Force is needed to change body’s state
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of motion.
The ball will remain stationary until an external force is applied
to it- kick from footballer.
Net forces will be zero until external force is applied.
2nd law- magnitude and direction of force applied is
proportional to acceleration of a body.
Ball will accelerate in the direction that the player kicks it.
The rate of acceleration of the ball is proportional to the force
applied (how hard it is kicked).
Newton’s third law- action/reaction.
When a force is given to football from the players foot, equal
and opposite force is given back to foot.
Ball moves because the player has greater inertia than the ball.
Explain how the impulse changes
throughout a 400 meter sprint. (5 marks)
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Impulse- force acting over a period of time.
Impulse= force x time
Impulse is also a change in momentum.
Positive impulse- impulse that moves body and negative impulse is a
force generated when absorbing body motion.
Leaving the blocks- positive impulse is much larger than negative as
large force is applied to blocks.
This makes the sprinter accelerate.
Middle of the race- positive and negative impulses are equal as the
force of absorbing body motion when landing is equal to the ground
reaction force pushing the body forward.
Net impulse is zero and velocity is constant.
End of the race- negative impulse is larger than positive (force when
landing is larger than that when pushing forward.
This causes deceleration.
The graph shows a velocity/time graph for an elite 100m runner. Use the graph
to determine the velocity of the sprinter after 3 seconds, and identify the period of
time when the sprinters acceleration was the greatest. (2 marks)
• Velocity after 3 seconds = 8 m/s-1
• Greatest acceleration between 0-2 seconds (when graph is at
its steepest).
Use Newton’s laws of motion to explain
how a sprinter leaves the starting blocks.
(7 marks)
• The sprinter on the starting blocks applies a force that
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provides them with acceleration (Newton’s third law)
Magnitude of acceleration given to the sprinter is directionally
proportional to the magnitude of force exerted (Newton’s
second law).
Therefore the amount of force the sprinter applies to the
blocks determines the amount of force that the sprinter is
pushed forward with and by pushing backwards the sprinter
will drive forwards.
Force that sprinter applies is an internal force, the blocks
provide a ground reaction force.
The sprinter will remain on the blocks in a stationary position
until a force is applied (Newtons first law).
a) In relation to a sprinter describe what is meant by the term impulse and how it
can be shown on a force time graph. (4 marks)
b) Draw and label a force – time graph to show the impulse of an accelerating
sprinter. (3 marks)
a)
• An impulse (force x time) is a force acting over a period of time.
• Impulse is also a change in momentum.
• The longer that the sprinters foot is in contact with the ground when pushing forward
the greater the impulse (positive impulse)
• Acceleration is a positive impulse.
• positive impulse is a force that moves the body
• a negative impulse is a force generated when absorbing body motion e.g. footfall.
• To accelerate the sprinter must produce more force when pushing forward (positive
impulse) than the force that is used when landing (negative impulse).
b)
The performer is completing a somersault. Describe what is
happening to the performer in terms of their rate of rotation and
why. (7 marks)
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Angular momentum = angular velocity x moment of inertia
Moment of inertia is the resistance to change the state of motion when rotating.
Moment of inertia = mass x distance from axis of rotation
As mass remains constant the athlete can decrease their moment of inertia by
moving their body parts closer to the axis of rotation (centre of mass).
• The conservation of angular momentum states that angular momentum remains
constant unless an external force acts upon a body.
• Therefore, if moment of inertia decreases then angular velocity must increase (as
angular momentum remains constant).
• The athlete can increase the speed of rotation by tucking in and then slow down
the speed of rotation (for a safe landing) by opening out and increasing moment
of inertia.
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’? (2marks)
• Impulse: the effect of a force acting over a period of time.
• Impulse = force x time
• Impulse is the same as a change in momentum
• Momentum = mass x velocity
• Force x Time = mass x velocity
Identify the forces A-E in the image below, that
act on the sprinter during the race (5marks)
The image below shows a high jumper at take-off. Using Newton’s three laws of
motion to explain how the high jumper takes off from the ground. (7marks)
• Newton's first law – the law of Inertia – a force is needed to change a body’s state
of motion.
• The athlete must push down into the ground in order to change the state of motion
from horizontal to vertical.
• Newton's third law – for every action there is an equal and opposite reaction.
• To drive the body upwards the athlete must produce an action force into the
ground.
• This produces a ground reaction force.
• Newton's second law – the magnitude and direction of applied force determines
the magnitude and direction of acceleration.
• The more force that the athlete pushes down with, the more force with which they
are pushed upwards.
• The athlete pushes down into the ground in order to move upwards over the bar.
Using ‘Newton’s First’ and ‘Second Laws of Motion’, explain how
the swimmer dives off the starting blocks. (4 marks)
A. Force is applied by the muscles
Newton’s First Law of Motion/Law of inertia
B. Performer will remain on the blocks unless a force is
applied
C. Performer continues to move forwards with constant
velocity until another force is applied
D. Water slows the swimmer
Newton’s Second Law of Motion/Law of Acceleration
E. Mass of swimmer is constant
F. Greater the force exerted on the blocks, the greater the
acceleration/momentum
G. Force governs direction
Explain how a gymnast can alter the speed of
rotation during flight. (7 marks)
A. Changing the shape of the body causes a change in speed
B. Change in moment of inertia leads to a change of angular
velocity/speed/spin of rotation/ angular moment;
C. Angular momentum remains constant (during rotation)
D. Angular momentum = moment of inertia x angular velocity
E. Angular momentum - quantity of rotation/motion
F. Angular velocity - speed of rotation
G. Moment of inertia - spread/distribution of mass around
axis/reluctance of the body to move
H. To slow down (rotation) gymnast increases moment of inertia
I. Achieved by extending body/opening out/or equivalent
J. To increase speed (of rotation) gymnast decreases moment of
inertia
K. Achieved by tucking body/bringing arms towards rotational axis
The final stage of an endurance race often involves a sprint finish.
Using Newton’s Second Law of Motion, explain how an athlete is
able to accelerate towards the finish line. (3 marks)
A. Mass of runner is constant
B. Force = Mass x Acceleration
C. Greater the force exerted on the floor, the greater the
acceleration / momentum / proportional
D. Force governs direction
E. Force provided by muscular contraction
F. Ground reaction force
The diagram shows a diver performing a tucked backward one-and-one-half
somersault. Use the diagram to explain why performing this dive in a tucked
position is easier than performing it in an extended position. (5 marks)
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Angular momentum = angular velocity x moment of inertia
Moment of inertia is the resistance to change the state of motion when rotating.
Moment of inertia = mass x distance from axis of rotation
As mass remains constant the athlete can decrease their moment of inertia by moving
their body parts closer to the axis of rotation (centre of mass).
• The conservation of angular momentum states that angular momentum remains
constant unless an external force acts upon a body.
• Therefore, if moment of inertia decreases then angular velocity must increase (as
angular momentum remains constant).
• The athlete can increase the speed of rotation by tucking in and then slow down the
speed of rotation (for a safe landing) by opening out and increasing moment of inertia.
The following force–time graphs were
obtained during the various stages of
a runner’s 100-metre sprint.
Using the above figure, identify which
graph is associated with each of the
following phases of a 100-metre
sprint, giving reasons for your
answers:
(i) Early in the sprint;
(ii) During the middle part of the
sprint; and
(iii) Towards the end of the sprint.
(6 marks)
• Bottom graph associated with start of race. Negative impulse
of footfall is smaller than positive impulse on push phase of
stride. Results in positive momentum – sprinter is
accelerating.
• Top graph associated with middle of race. Negative impulse
of footfall is equal to positive impulse. Net impulse is zero
therefore no change in momentum and sprinter runs at a
constant velocity.
• Middle graph associated with end of race. Negative impulse
of footfall is greater than positive impulse of push phase. Net
impulse is negative and sprinter is decelerating.
Sketch a diagram to show the flight path of a shot from the moment it is released to
the moment immediately prior to landing, and include on your diagram vectors to
represent the vertical and horizontal components of the velocity of the shot at:
(i) The point of release;
(ii) The highest point of the flight;
(iii) A point immediately before landing. (4 marks)
• At point of release the shot exhibits both a positive vertical component and a
horizontal component in its flight.
• At highest point the shot exhibits only a horizontal component (due to effects
of gravity).
• Immediately before landing the shot exhibits a negative vertical component
and a horizontal component to its flight.
State and briefly explain the three factors
that affect the distance travelled by a shot
putt. (4 marks)
• Angle of release – optimal angle found to be 34 degrees.
• Speed of release – the steeper the angle of release the slower
the release velocity. Speed of release is the most important
factor.
• Height of release – as gravity is constantly pulling the shot
downwards it is advantageous to release at the highest
possible point above the ground.
1. What is the difference between a vector and a scalar? (2 marks)
2. State whether the following variables are vector or scalar
quantities:
Force
Speed
Velocity
Acceleration (2 marks)
1. A vector is a quantity that has two dimensions
(magnitude and direction), a scalar has only one
dimension (magnitude).
2.
Force – vector
Speed – scalar
Velocity – vector
Acceleration - vector