Unit 1.3.a - biomechanical principles, levers and the use of technology

Question 1

Explain four factors affecting the stability of a sports performer. (4)

Answer 1

accept opposites

• (mass) the greater the mass the more stable the performer
• (height of CoM) the lower the centre of mass/ shorter the line of gravity the more stable the performer
• (line of gravity) the closer to the centre of the base of support the line of gravity falls the more stable the performer or if the line of gravity falls inside the base of support compared to outside the stability is increased
• (base of support) the larger the area of the base of support the
  more stable the performer

Question 2

Identify the two factors affecting the size of the moment of inertia of a rotating body. (2)

Answer 2

• Mass of the body/ body part
• Distribution/ distance of the mass from the axis of rotation

Question 3

Use an example of a sports performer with angular motion to describe the relationship between moment of inertia and angular velocity. (2)

Answer 3

• They have an inverse relationship
• (tucked e.g.) low moment of inertia and high angular velocity in gymnastic tucked somersault/ recovery leg of a runner/ tucked position of an ice skater in a spin
  Or
  a gymnast tucks their body position to decrease moment of inertia which increases their angular velocity
• (straight e.g.) high moment of inertia and low angular velocity in gymnastic straight somersault/ drive leg of a runner/ when an ice skater opens arms out away from the body during a spin
  Or
  a gymnast opens their body position to increase moment of inertia which decreases their angular velocity

Question 4

The air flow diagram shows a side view of a tennis ball with topspin.

Use the airflow diagram to explain how the use of topspin in tennis causes the flight path of the ball to deviate. (5)

Answer 4

• (Direction of flow top) airflow over the top of the ball is in the opposite direction to the surface of the ball/ direction of spin
• (Direction of flow bottom) airflow underneath the ball is in the same direction as the surface of the ball/ direction of spin
• (Speed of flow) airflow over the top of the ball travels slower/ at a lower velocity (compared to below the ball or opp.)
• (Pressure top) The pressure above the ball is higher (compared to below the ball or opp.) or there is a pressure
  gradient / differential
• (Magnus force) Magnus force acting downwards (from the CoM of the ball)
• (Flight path deviation) Flightpath dips/ shortens/ ball spends less time in the air (compared to no spin) or drawing of dipped/ shortened flight path compared to no spin

Question 5

Describe one benefit of using topspin in a tennis match (1)

Answer 5

• (Hit harder) can hit the ball harder and ball still lands in the court
• (Hit higher/lob) hit ball higher and ball still lands in court
• (Deceive) ‘deceive’ their opponent into leaving a ball that appears to be going out(or equivalent)
• (Bounce) decreased angle/ accelerates on bounce (pushes opponent back/ into defence/ less time to react)

Question 6

Describe how the use of a wind tunnel could help an elite track cyclist to enhance their performance. (2)

Answer 6

• (Measure) Collect data on/measure/see the effect of air resistance on the cyclist/bike/equipment
• (Bike) To optimise the aerodynamics/streamline/airflow
  around the bike / bike parts/
  or improve bike design
• (Clothing/equipment) To optimise the aerodynamics/streamline/airflow
  around cyclist’s clothing/equipment (eg
  helmet/clothing/shoe covers)
• (Cyclist) To optimise the aerodynamics/streamline/airflow
  around body position/technique

Question 7

Sport performers will manipulate factors that affect the size of the friction force acting on
them.

Why do performers want to increase friction? (1)

Answer 7

• allows greater acceleration/deceleration/change of
  direction/speed/velocity/drive force
• to improve grip/decrease the chance of their foot/feet slipping/increase stability

Question 8

Using examples from sport, explain how performers increase friction. (3)

Answer 8

• Increased roughness of footwear/tyres
  –> athletes wear spikes
  –> hockey players wear astro shoes
  –> rugby players wear boots/studs
  –> tyres of mountain bikes have deep tread
  –> gymnasts chalking/taping hands
• Increased softness of
  contact surface
  –> rubber soles on training shoes
  –> adjust tyre pressure on bike
• Increased roughness of ground surface
  –> cross country runner choosing to run a line on rougher ground/rubber/tartan track
• Increased temperature
  –> heating tyres /warm up laps in motor sports
• Increased reaction/normal force generated
  –> spoiler on F1 car generates downforce high/long/triple jumpers dip at take off
  –> heavy rugby players/shot putters

Question 9

Define ‘centre of mass’. Explain how a rugby player can apply knowledge of centre of mass to increase their stability. (4)

Answer 9

• the point at which a body is balanced (in all directions) / the point from
  which weight appears to act
• to maintain stability centre of mass must be over base of support
• (to increase stability) rugby player lowers centre of mass by bending knees
• (to increase stability) player increases area of base by widening stance
• stability is increased if line of gravity is in centre of base of support