Download CHAPTER 19 – STABILITY and LEVER SYSTEMS

BIOMECHANICS
CHAPTER 19 – STABILITY and LEVER SYSTEMS
Text between pages 117 and 120 answers to questions on page 121 of the text book.
figure 19.12 – a long
jumper in flight
1) Explain with diagrams what is meant by the centre of mass of a body.
Explain with the aid of pin-man diagrams how the centre of mass of a long jumper changes
from the take-off position to the flight phase shown in figure 19.12. 5 marks
Answer
• The centre of mass of a body is the point (which can lie inside or outside the body) at which the
weight (force) of the body acts for the body as a whole.
• See figure Q19.1 for an idea of where the centre of mass is in a body depending on shape.
For the long jumper:
• Three marks for 3 diagrams showing the approximate position of the centre of mass - see figure
Q19.2.
• The idea that this red dot represents the position of the overall mass of the body.
figure Q19.1 – centre of mass position
in the body
figure Q19.2 – centre of mass position
for long jumper
CofM
centre of mass
2) Figure 19.13 shows a swimmer holding a balance just before the start of a race.
Explain how the position of the centre of mass can affect the swimmer’s
balance. Describe how the swimmer in figure 42 can use his knowledge of
balance to achieve his most effective block start.
5 marks
figure 19.13 – swimmer
starting a race
Answer
• The swimmer’s centre of mass must lie directly above the base of support – in this
case his feet.
• The bigger the area of support (the further apart his feet) the more stable (able to
keep on balance) he will be.
• The lower the centre of mass the easier it will be to maintain balance.
• If he moves so that the centre of mass moves away from a position directly above
his feet, he will begin to topple (see figure Q19.3).
• Since his line of action of his weight (the force exerted by gravity on his centre of
mass) will not pass through his feet.
• Which causes a turning (moment) of the swimmers body – he topples.
To make a block start:
• The swimmer will lean forward so that the centre of mass of his body moves
forward.
• The centre of mass of the swimmer then lies forwards of the block.
• And outside his base of support (his feet) - see figure Q19.4..
• So he will topple forward into the pool - he then pushes hard with his legs to drive forwards in the direction of his swim.
figure Q19.3– centre of mass position
affects stability
figure Q19.4 – swim start and
toppling
toppling
stable
Chapter 19
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SECTION B –
OPTION B2
CHAPTER 19
ANSWERS TO QUESTIONS
3) Sketch the lever system which would represent the action of the biceps muscle in flexing the arm. Show on your diagram
the resistance arm of the lever. 3 marks
Answer
figure Q19.5 – the elbow and
• See figure Q19.5.
forearm lever
• E is the effort force in the biceps muscle.
• L is the load force applied at the hand.
E = effort
• The triangle is the pivot or fulcrum of the lever.
• The resistance arm is the structure (forearm) between hand (load) and elbow (fulcrum).
4) In figure 19.14 of a jumper taking off, name, sketch and label the lever system
operating at knee B during this action.
3 marks
Answer
• See figures Q19.6 and Q19.7.
• This is a class 3 lever (effort between pivot and load).
• Note that the effort (figure Q19.6) is transmitted to the tibia via the patella tendon,
which passes over the knee and inserts below the joint (figure Q19.7).
figure 19.14 – long jumper
taking off
fulcrum (pivot)
figure Q19.6 – jumper’s
knee 1
L = load
figure Q19.7 – jumper’s
knee 2
effort force
in quadriceps
muscle
B
fulcrum (pivot)
at knee joint
B
load force
on foot
5) In softball, what order (class) of lever is shown in the hitting action in figure 19.15?
State one disadvantage and one advantage of reducing the bat length for a beginner.
3 marks
Answer
• This is a class 1 lever.
Advantage of shortening bat length is:
• Learning the skill involved may be easier - since the strike point is nearer the hand.
Disadvantage of shortening the bat would be:
• Less force can be applied as a load for a given effort, hence the strike on the ball would
impart less speed to the ball.
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fulcrum (pivot)
at knee joint
load force
on foot
effort force in
quadriceps muscle
via patella tendon
figure 19.15 – softball bat
bat
fulcrum
(pivot)
weight
effort
BIOMECHANICS
6) Name, sketch and label the lever system which is operating at the ankle of leg C when doing the sprint set action illustrated
in figure 19.16.
3 marks
Answer
figure 19.16 –ankle
• See figure Q19.8.
lever system
• This is a class 2 lever.
• Note that the load force is a combination of the
weight of the athlete acting downwards through the
tibia/fibula on the ankle joint, and the reaction to
the accelerating force driving the sprinter forwards.
figure Q19.8 – ankle foot lever
C
C
effort force in
calf muscles
figure 19.17 – a press-up
load force
on foot
A
fulcrum (pivot)
under ball of foot
7) a) Figure 19.17 shows an elbow joint A of a person performing an exercise.
Draw a simplified sketch to show the lever system, indicating the various forces A
operating.
4 marks
Answer
• Figure Q19.9 – this is a 2nd class lever.
• Normally the triceps elbow system is a 1st class lever, but here, the pivot is at the
figure Q19.9 – elbow triceps lever
hand as shown, which forces the system into 2nd class lever shape.
b) On your diagram draw and label the effort and resistance arm.
3 marks
Answer
• Resistance arm lies between pivot and weight force (the lever arm is at right angles to
the force).
• Effort arm lies between point of elbow and pivot.
effort force in
triceps muscle
effort arm
resistance arm
load force
(weight of athlete
acting downwards
via humerus
on elbow)
8) During physical activity the performer uses a combination of levers to produce
fulcrum (pivot)
under heel of hand
movement. Explain why the length of the lever will affect performance. 3 marks
Answer
• The longer the lever (usually length of limb - the resistance arm) the greater the speed at the load (end of limb).
• The longer the lever the less effort required.
• Shorter levers generate more force (compare compact small gymnasts/weight lifters with taller performers).
• 1st class levers are used to generate speed.
• 2nd class levers are more efficient because the effort is further away from the pivot (example - the ankle joint with calf muscles).
Chapter19
87
SECTION B –
OPTION B2
CHAPTER 19
ANSWERS TO QUESTIONS
9) a) Figure 19.18 shows an elbow joint of a person performing an exercise.
Work out the clockwise moment provided by the force of 200 newtons about the elbow as a fulcrum, then, assuming the arm is stationary, use the principle of moments to calculate the effort force exerted by the biceps muscle. Show your working.
6 marks
Answer
• 0.3m = elbow joint to hand distance.
• Clockwise moment = 200 x 0.3 = 60 Nm.
• Principle of moments is
‘clockwise moment = anticlockwise moment’
• This applies when the system is in equilibrium i.e. nothing is unbalanced or moving.
•
Distance from fulcrum to muscle insertion = 0.03m.
• Anticlockwise moment = force in muscle x 0.03.
• Therefore
force in muscle x 0.03 = 60 (this is the value of the clockwise moment from above).
• Therefore
force in muscle = 60
= 2000 N.
0.03
b) Diagram 19.19 shows the elbow joint and the position of the triceps muscle in relation to it when supporting a load (a shot) behind the head. Draw a simplified sketch to show the lever system, indicating the various forces operating.
4 marks
Answer
figure Q19.10 – triceps
• See figure Q19.10.
lever system
• Note the position of the fulcrum at the elbow.
• The forces at the triceps and hand act downwards.
elbow
• The effort acts at the triceps.
hand
• The load acts at the hand.
L
triceps lever
E
figure 19.18 – elbow joint and
biceps lever
effort
x
load
fulcrum (pivot)
d
x = 0.03m d = 0.3m
figure 19.19 – elbow joint and
triceps lever
d = 0.3m
x = 0.03m
d
x
fulcrum
(pivot)
effort
load
c) Using the values shown in the diagram and the fact that the mass of the shot is 8 kg, calculate the load. Neglecting the mass of the ulna, estimate the effort needed to balance the system. (Take the gravitational force on 1 kg to be 10 N).
4 marks
Answer
• The load = 8 kg x gravitational field strength (g) = 8 x 10 = 80 N.
• Principle of moments: clockwise moment = anticlockwise moment.
• Therefore load x distance of load from fulcrum = effort x distance of effort from fulcrum.
• Hence: 80 N x 0.3 m
= effort x 0.03 m (approximately).
• Therefore:
effort = 80 x 0.3
0.03
= 800 Newtons (approx.).
Student note
The approximate calculations refer to the fact that the angle between the effort and the lever arm will not be exactly 90o,
and since the measurement of the distance when working out the moment is at right angles to the line of action of the
force, the distance 0.03 m will therefore only be approximate.
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BIOMECHANICS
10) Figure 19.20 shows a diagram of a sportsperson’s foot pivoting at a point under the ball
of the foot. Use your knowledge of the principle of moments applied to this lever to
calculate force E. Show all your working.
4 marks
Answer
•
The load = the weight of the sportsperson = 800 N.
• The effort is provided by the force of contraction of the calf muscles (F) and will be calculated as
follows:
• The principle of moments says that:
clockwise moment = anticlockwise moment.
•
Anticlockwise moment = load x distance of line of action of load to fulcrum
= 800 N x 0.18 m = 144 Nm.
•
Clockwise moment = effort (E) x 0.24m.
• Therefore: E x 0.24 = 144 Nm.
• And: E
= 144 = 600 N.
0.24
figure 19.20 – ankle joint
and calf lever
effort
x
d
fulcrum
x = 0.24m
d = 0.18m
load= 800N
Chapter 19
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