Biomechanics
Biomechanics is a field of sport science that combines the disciplines of biology and engineering mechanics and uses physics, mathematics, and engineering to describe the properties of biological materials.
Important relationships in explaining motion:
Speed (ms-1) = distance travelled (m)/ time taken(s)
Momentum (kgm/s) = mass (kg) x velocity (ms-1)
Change in GPE (j) = weight (kg) x vertical height difference (m)
Kinetic energy (j) = ½ x mass (kg) x [velocity (ms-1)]2
Acceleration of free fall (in UK) g = 9.81ms-2
Gravitational field strength (in UK) g =9.81Nkg-1
Work = force x distance moved in the direction of the force
Power = energy used ÷ time taken, or (force x distance) ÷ time taken, or force x velocity
Key terms
Kinetic energy -The mechanical energy that a body has by virtue of its motion.
Angular Momentum - is defined as: angular velocity x moment of inertia.
Inertia - the resistance to acceleration - reluctance of a body to change whatever it is doing.
Mass - the quantity of matter of which a body is composed of, not affected by gravity , measured in kilograms (kg).
Weight - force due to gravity - is mass x gravity (9.81ms-2)
Acceleration (Kinematics)
Acceleration is defined as the rate at which velocity changes with respect to time.
Average acceleration = (final velocity - initial velocity) ÷ elapsed time
Gravity causes objects to accelerate downwards. This acceleration continues until the downwards force matches the upwards force from air resistance (drag force), this speed is called the terminal velocity.
Distance and displacement
Distance and displacement are separate quantities. Distance is the length of the path a body follows and displacement is the length of a straight line joining the start and finish points. An example of this is a 400m race in lance one of a standard synthetic track. The distance from the star to the finish is 400m but their displacement is zero metres, as they finish where they have started.
Components of velocity
In this picture a skater is performing a complex jump. The skater’s mass is 60kg and jumps off the ice at a velocity of 10m/s-1 with an angle of 25° to the horizontal. Using this information it is possible to calculate the vertical component of the jump. Calculation: sinθ = 10sin25 = 4.2ms−1
Centre of mass/gravity
The centre of mass or mass centre is the mean location of all the mass in a system. The photograph opposite shows champion Stefan dramatically arching his back so he can make his centre of gravity pass underneath the bar as far as possible without him touching the bar. This skill applies to other disciplines in athletics such as pole vault.
Projectile motion
In this picture opposite, one arrow represents the y-velocity, one the x-velocity, and one the general direction of the object (v). The length of these arrows is related to how large a value they have. So a very short arrow means a very small velocity.
Newton’s laws of motion
• First Law - everybody continues in its state of rest or motion in a straight line unless compelled to change that state by external forces exerted upon it.
• Second Law - The rate of change of momentum of a body is proportional to the force causing it and the change takes place in the direction in which the force acts. F = ma (force = mass x acceleration).
• Third Law - To every action there is an equal and opposite reaction OR for every force that is exerted by one body on another there is an equal and opposite force exerted by the second body on the first.
