Biomechanics Yr 2 Flashcards
Displacement
Final position-initial position (M, rads, revs)
Is a vector quantity since it has a magnitude and direction
Velocity
(Final position-initial position)/time
Velocity is a vector quantity since it has magnitude and direction
Acceleration
(Final velocity-Initial velocity)/time
Acceleration is a vector since it has magnitude and direction
Examples of kinematic biomechanical variables
Time, position, displacement, velocity and acceleration
Shutter speed
How long the cameras shutter is open for
Why do we calibrate a video camera
To scale measurements
Camera set up- Perspective error/Parallax error
- are you too close to the area of interest
- are you perpendicular to the plane of motion
- is the camera at the correct height
2d motion analysis- Setting up capture volume
Pan- ensure full width of action is visible
Roll- ensure camera is level
Tilt- ensure camera is angled directly perpendicular to the movement
Camera height- should be same height as the area of interest
Zoom- eliminate useless space in capture volume
Camera settings
Sampling frequency- how many frames per second
Shutter speed- how long each frame is captured for
Iris- how wide is the aperture of the camera
Difference between kinematics and kinetics
Kinematics describes the observed motion (focuses on the geometry of motion of objects: such as displacement, velocity, acceleration…)
Kinetics explains how the observed motion was achieved. Study of the forces acting within a body and the changes they produce in the body itself.
Impulse equation
Force x Change in time
Momentum equation
Mass x Change in velocity
types of forces
Internal- acting between body parts
External- acting between the body and the environment
Gross mechanical effects- influence body movement
Local biological effects- contacting tissues
Ways to measure force
- force transducers and sensors/load cells
- capacity to be used in Vivo
- force platforms
- pressure plates
Force axes
Y- anterior-posterior
Z- vertical direction
X- medio- lateral
Ground reaction forces
- vertical force
- vertical impact and vertical force
- impulse measure of the effect of a force during the time that the force acts
Kinetics equations
Force= mass x acceleration
Moment= force x perp. distance
Impulse= force x time
Momentum= mass x change in velocity
Impulse= momentum
Work, energy and power
Work (J)- whenever force is used to move something
Energy (J)- ability to do work/apply a force
Power (W)- how quickly work is done
Mechanical work
Mechanical work is a measure of the transfer of energy from one object/body to another.
Work done by a force on an object= the magnitude of the force multiplied by the displacement of the object along the line of the force
Work= force x displacement
Positive mechanical work
The force is applied in the same direction as the resulting movement
Negative mechanical work
The force is applied in the opposite direction to the resulting movement
Often associated with eccentric exercise
Kinetic energy
Energy of motion
The energy possessed by a body because it is moving
Potential energy
Energy of position
Stored energy which can become kinetic due to relative height or deformation
Gravitational potential energy
Due to relative position above a surface
PE= weight x height
PE= mass x gravity x height
Strain potential energy
Due to deformation (strain or elastic energy)
SE= 0.5 x stiffness constant x amount of deformation squared
Conservation of energy
- mechanical energy is never lost
- when gravity is the only acting external force, mechanical energy is constant (airborne)
Mechanical power
- the rate of doing work (how much work is done in a specific amount of time)
- power= work/time
Angular momentum
Quantity of rotational motion
Angular momentum= moment of inertia x angular velocity
Constant during flight
Units are kg.m2/s
Moment of inertia
Resistance to rotational motion
If more mass is closer to an axis of rotation, it is easier to alter rotational motion of a body
If mass is distributed further from the centre of rotation, moment of inertia will be greater
Moment of inertia= mass x square of the perpendicular distance to the rotation axis
Units= kg.m2
Angular velocity
Speed of rotational motion
Units are rads/s
Aerial motion
- during flight the path of the centre of mass is fixed
- rotation is the only thing we can control
- segments can be repositioned around the centre of mass (Fosbury flop)
Resistance to rotational motion
Inertia= resilience to motion
Mass is a linear measure of inertia
When considering rotation, moment of inertia is a measure of a body’s resistance to motion
Conservation of angular momentum
- when gravity is the only external force, angular momentum is conserved
- determined at takeoff
- therefore if moment of inertia decreases, angular velocity increases
Newton’s angular laws
1- a rotating body will continue to rotate with constant angular momentum unless an external force acts on it (conservation of angular momentum)
2- rate of change of angular momentum is proportional to the torque causing it and takes place in the direction in which the torque acts
3- for every torque exerted by one body on another, there is an equal and opposite torque exerted by the second body in the first
Torque
- Torques are the rotational forces
- torque= force x distance
- torque is the ability of force to produce rotation around an axis
- amount of torque depends on:
- amount of force exerted, distance between force and the axis of motion (moment arm)
Factors affecting coefficient of restitution
Material properties of the object:
Material type, flexibility, shape/size, performers kinematics.
E.g. string tension set by tennis player
Affects length of contact time. COR increases with contact time.
Impact location of tennis racket (centre of percussion, the node, the point of maximum coefficient of restitution)
Coefficient of friction
- number that serves as an index of the interaction between 2 surfaces in contact.
- friction is the force acting perpendicular to 2 surfaces in contact
- rebound angle dependent upon coefficient of friction.
String surface properties rather than string plane
What is a projectile
A projectile is an object that is propelled by the application of an external force and moves freely under the influence of gravity and air resistance.
What are the properties of a projectile
- a projectile has both a horizontal and vertical component
- these act independently
Horizontal and vertical motion (projectiles)
- no horizontal forces act on a projectile (newton’s first law: an object will not change its motion unless a force acts on it) so there will be uniform motion
- gravity affect vertical motion. Velocity increases by 9.81m/s every second, downwards. Mass does not change acceleration when an object is in free fall.
The result of both vertical and horizontal motion is a porabolic trajectory of a projectile.
It is symmetrical on the way up and down
Displacement formula (projectiles)
1/2 x gravity x change in time squared
Streamlines
Diverging streamlines= slowing down
Converging streamlines= speeding up
Curved streamlines
A pressure gradient must exist
Push streamlines around corner
Relative local pressure difference
Total drag
- skin/surface drag or viscous drag
- form/profile drag or pressure drag
- wave drag
Turbulent boundary layer
- Roughness (trigger turbulent BL)
- if pressure gradient ok, turbulent bl remains attached
- slightly more friction drag
- can withstand larger adverse pressure gradient
Why do objects experience lift
- Streamlines not symmetrical
- areas of high pressure and low pressure cause lift
Magnus effect
Magnus force- a lift force created by the spinning of an object (perpendicular to direction of movement)
Magnus effect- deviation in trajectory of a spinning object toward the direction of spin, resulting from the Magnus force
Units of measurement for angular velocity
Radians, revolutions, degrees
Single point digitising
- follows location of single point through time.
- records coordinates of marker to give linear kinematics:
- position, displacement, velocity, acceleration
Multipoint digitising
- if 3 joint centres of connected segments are tracked, can calculate angular kinematics.
- hip, knee and ankle= knee joint
- can be used to locate the CM/CG
Kinematics
- Describe the change in motion (without regard to the forces causing the motion)
- used for describing actions and movements (how high, fast far, which direction…)
