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58 Cards in this Set

  • Front
  • Back
Why can't a heavy crate be pushed across a floor
The force applied is less than the maximal frictional force
What are the two types of frictional forces
Dry friction: between two dry surfaces and they need 2 contacting forces

Fluid friction: resistance generated between layers of fluid in motion

What is a normal-tangential axis
axis along a slope to simplify the problem
What is the magnitude of the frictional force dependent on
if the forces are in motion and the magnitude of the force pushing them together - the normal force
What are the types of frictional coefficients
Static and dynamic
What are the required and utilized coefficient of friction
minimal coefficient required to avoid slips and during the movement
What is the general calculation of COFU
ratio between the horizontal and vertical components of the GRF generated by a person walking on a dry, uncontaminated surface
What is dynamics
Describing motion and explaining its movement
What is kinematics and what's it based on
description of the temporal and spatial components of movements

based on relationships between position, velocity and acceleration

Describe linear motion
everything moves the same distance at the same speed and deals with forces
Describe angular motion
if the body moves through the same angle at the same time and deals with moments
What is general motion
combination of linear and angular motion
What is the integral of a velocity-time graph
the change in position: displacement
What do the projectile motion equations allow us to predict
trajectory of the motion as well as its histories
When is the optimal throwing angle above 45 degrees
When the task emphasizes displacement, or a mix of vertical displacement and speed
When is the optimal throwing angle at or below 45 degrees
When a mix of maximum horizontal displacement and speed are required - during throwing and striking events
What happens to the force required when release angle is increased
you need more force
Where is the velocity vector of a point on a body rotating about a fixed axis
it is tangent to the path of motion
What is relative motion
description of a motion depending on the observer's frame of reference
What is inertia
Object in motion remains in motion at a constant speed unless acted upon by an external force
Usefulness of inverse dynamics
Calculating the resultant force acting on an object when the acceleration is known
Usefulness of forward dynamics
Calculating how an object will move knowing the forces acting on it
Moment of force vs. moment of inertia
moment of force: rotating effect of a force and the product of the force and moment arm

moment of inertia: resistance to angular accelerations

What is the mass moment of inertia
tendency to resist angular acceleration when a rotation-causing force is applied
is mass moment of inertia scalar or vector
it is a scalar quantity: kg*m^2
Which axis can rotational inertia be calculated around
it can be calculated around any axis of interest: the equation is I=sum of ((M-initial)(r^2-initial)

I - rotational inertia


m - mass


r - distance to axis/point of interest


r^2 has more of an effect than the mass

what does the mass moment of inertia depend on
Material and geometric properties: mass of object and how it's distributed

Location and orientation of axis of interest: Varies with the axis of rotation

What is point mass and when is it used
it is the point, equal to the mass of the entire body, located at a distance from the axis of rotation
What does radius of gyration measure
it measures the average spread of mass about an axis of rotation which is NOT the same as the CG
How does the radius of gyration compare to that of the radius of rotation
the radius of gyration is always slightly larger
How can the mass moment of inertia be calculated
The cadaver based ratio method

Through geometric models


Mass scanning-based ratio method


Mechanical perturbations

What are some characteristics for exceptional ground force reactions
Strength

Muscle fiber type


Coordination


Mechanical technique

what is the point of inverse dynamics
the aim is to know what the muscles are doing: timing of contractions, amount of force generated, power of contraction.

so we work backwards from the kinematics to derive the kinetics responsible for the motion

What are the assumptions when doing inverse dynamics
joints are frictionless, segments are rigid with mass concentrated at centre of mass, no co-contraction of agonist and antagonist muscles, air friction minimal
What does the moment from inverse dynamics tell us
which muscle is active (flexor/extensor) and how much torque that muscle is exerting
How are the flexors and extensors represented
flexor moments are positive and extensor moments are negative
What are some limitations to inverse dynamics
Relies on assumptions that aren't always valid, estimating joint centre is prone to error, anthropometry are approximates and generalizations, it can only determine net moments and power, and can't differentiate between different muscles
What are centripetal forces and which direction does it point
the force required to keep a body moving on a curved path, and it always acts towards the centre of the circle
What are pseudo-forces
Not true forces, but are effects that appear when observing motion from a moving frame of reference
What are examples of pseudo-forces
Centrifugal force

Coriolis force


g-Force

What is the centrifugal force
force that appear to cause the objects to flee the centre of the circular path
What is the coriolis force
force that appears when body is observed from a rotating frame of reference
What is the g-Force
appears whenever a body undergoes rapid acceleration/inertia of the body to the acceleration imposed on it
Relationship between force and movement
Force*time=change in (momentum*velocity)
What are deformations and how do they occur
they form during collisions and there two types: elastic which is recoverable, and plastic which is permanent
What is a one-dimensional collision
perfectly inelastic: ballistic pendulum

perfectly elastic: billiard problems


Elasto-plastic problem: basketball problem


e=relative velocity of separation/relative velocity of approach

What is a two-dimensional collision
same concepts but use vectorial properties of the parameters involved in the system
What is the law of conservation of momentum
linear momentum: when resultant force acting on body is 0, body's linear momentum is conserved

Angular momentum: this stays the same unless acted on by another force

What is work
influence of a force on the movement of a body


What is a key characteristic pertaining specifically to work
the force must be in the same direction as the displacement
What is the work energy theorem
the kinetic energy of a system

the potential energy of a system


the rotational kinetic energy of a system

What is external work
work done by the resultant and the moment
What is positive vs. negative work
positive work absorbs energy and negative work releases energy
What do positive and negative mechanical work values mean
positive: work was done on the body

negative: body did work on some other body

What is power
rate of doing work
What is a conservative force
a force acting on a body that does the same amount of work independent of the path of the motion of the body
What is the best link between exercise physiology and biomechanics
mechanical efficiency
What is the internal work
difference between total work and external work