Biomechanical principles

Question 1

What are Newton’s laws of motion, how are they applied?

Answer 1

1. First law, law of inertia:
   - A body continues in a state of rest unless acted upon by an external force
   - E.g. a golf ball will remain still unless the force applied by a golf club makes it move
   - E.g. a golf ball will will continue to move at a constant velocity unless a force acts on it to slow it down, such as wind resistance or gravity
2. Second law, law of acceleration:
   - A body’s rate of change of momentum is proportional to the size of the force applied and acts in the same direction as force applied
   - E.g. when a golf ball is struck by a golf club, the velocity of the ball is proportional to the size of the force acted on it by the club
3. Third law, law of reaction:
   - For every action there is an equal and opposite reaction
   - E.g. if a tennis players hits a ball, the racquet exerts a force on the ball and the ball exerts a am equal and opposite force in the racquet. The racquet exerts the action force and the ball exerts reaction force.
   - E.g. if the tennis ball then hits the floor, it exerts a force on the floor and the floor exerts an equal and opposite force on the ball.

Question 2

What is velocity, equation and unit?

Answer 2

1. Velocity is the rate of change of displacement
2. Velocity = displacement/time taken
3. m/s

Question 3

What is momentum, equation and unit?

Answer 3

1. Momentum is the quantity of motion possessed by a moving body
2. Momentum = mass x velocity
3. kg m/s

Question 4

What is acceleration, equation and unit?

Answer 4

1. Acceleration is the rate of change of velocity
2. Acceleration = (final velocity - initial velocity)/time taken
3. m/s/s

Question 5

What is force, equation and unit?

Answer 5

1. Force is a push or pull that alters the state of motion of a body
2. Force = mass x acceleration
3. N

Question 6

What are the 2 types of force?

Answer 6

1. External force, which comes from outside of the body

2. Internal force, which is generated by skeletal muscle.

Question 7

What are the effects of force?

Answer 7

1. Create motion
2. Accelerate a body
3. Decelerate a body
4. Change the direction of a body
5. Change the shape of the body.

Question 8

What is net force?

Answer 8

1. Net force is the sum of all forces acting on the body
2. If net force = 0, there is no change in motion as forces are balanced
3. If net force is present, there is a change in motion as the forces are unbalanced. This occurs when two forces are unequal in size and opposite in direction.

Question 9

What are the external forces acting on the body, equation and unit?

Answer 9

Vertical forces:

1. Weight,
   - The gravitational pull that the earth exerts on a body
   - Weight = mass x acceleration
   - N
2. Reaction,
   - The equal and opposite force exerted by a body in response to the action force placed upon it

Horizontal forces:

3. Friction,
   - The force that opposes the motion of two surfaces in contact
   - N
4. Air resistance,
   - The force that opposes motion through the air
   - N.

Question 10

What are the factors affecting friction, examples?

Answer 10

1. Roughness of the ground surface e.g. athletes run on rough, rubberised tracks
2. Roughness of the contact surface e.g. athletes wear spiked shoes
3. Temperature e.g. F1 drivers have a warm-up lap, as friction is increased with higher tyre temperatures
4. Mass e.g. shot putters have a high mass, so due to Newton’s third law, this creates an equal and opposite high reaction force, allowing greater friction in throwing circle and preventing over-rotation.

Question 11

What are the factors affecting air resistance, examples?

Answer 11

1. Velocity e.g. the greater the velocity of a cyclist, the greater the force of air resistance opposing their motion
2. Shape e.g. most cyclists wear a tear-drop shaped helmet to minimise air resistance, known as streamlining
3. Front cross-sectional area e.g. the low crouched position of a downward skier reduces air resistance
4. Smoothness of surface e.g. lycra suits reduce air resistance.

Question 12

What is the origin and direction of the arrows representing vertical forces in a free body diagram? Relationship between the forces?

Answer 12

1. Weight arrow originates from the centre of mass, extends vertically downwards
2. Reaction arrow originates from the point of contact extending vertically upwards
3. If weight=reaction, net force is 0 so forces are balanced therefore body will remain at rest
   E.g. basketballer preparing to take a free throw, or travelling in constant vertical velocity
4. If reaction>weight, net force is positive
5. If forces are unbalanced, acceleration in an upward direction will occur
   E.g. basketballer leaving ground in the take-off phase of a layup.

Question 13

What is the origin and direction of the arrows representing horizontal forces in a free body diagram? Relationship between the forces?

Answer 13

1. Friction arrow originates from the point of contact and usually extends horizontally in the same direction as motion
2. Air resistance arrow originates from the centre of mass and extends horizontally against the direction of motion
3. If friction=air resistance, net force is 0 so forces are balanced therefore body will remain at rest
   E.g. sprint cyclist travelling in constant velocity who has reach maximum velocity is in constant velocity
4. If friction>air resistance, net force is positive, so forces are unbalanced and acceleration in a forward direction
   E.g. sprint cyclist accelerating away from starting line
5. If air resistance>friction, net force is negative, so forces are unbalanced and horizontal deceleration will occur
   E.g. sprint cyclist crossing the finish line sitting up and decelerating.

Question 14

What is centre of mass?

Answer 14

1. Centre of mass is the point in which an object is balanced in all directions
2. The point where the weight of the body tends to be concentrated.

Question 15

What is stability?

Answer 15

1. Stability is the ability to move and resist motion at rest
2. It is also the ability to withstand a force applied and return to original position without damage.

Question 16

What are the factors which affect stability?

Answer 16

1. Mass of the body: the greater the mass, the greater the inertia e.g. a sumo wrester
2. Height of centre of mass: the lower the centre of mass, the greater the stability
3. Size of base of support: the greater the base of support, the greater stability which can be increased by more points of contact
4. Line of gravity: the more central the line of gravity to the base of support, the greater the stability.

Question 17

How can maximising and minimising stability be applied to sport?

Answer 17

1. In a sprint start, a sprinter preparing in the blocks has maximum stability
2. When ‘set’ is called, the sprinter lifts their hips to raise centre of mass, lifts one knee reducing the points of contact, and leans forward thus minimising stability ready for movement.
3. When gun is fired, chest lifts raising centre of mass, hands come off the track, and the line of gravity falls in front of the base of support to maximise instability to aid performance
4. To prevent body falling forwards one leg should be driven forwards with great speed. This minimises movement time and gives the perfect start from which to drive forwards.