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58 Cards in this Set
- Front
- Back
Why can't a heavy crate be pushed across a floor
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The force applied is less than the maximal frictional force
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What are the two types of frictional forces
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Dry friction: between two dry surfaces and they need 2 contacting forces
Fluid friction: resistance generated between layers of fluid in motion |
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What is a normal-tangential axis
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axis along a slope to simplify the problem
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What is the magnitude of the frictional force dependent on
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if the forces are in motion and the magnitude of the force pushing them together - the normal force
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What are the types of frictional coefficients
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Static and dynamic
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What are the required and utilized coefficient of friction
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minimal coefficient required to avoid slips and during the movement
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What is the general calculation of COFU
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ratio between the horizontal and vertical components of the GRF generated by a person walking on a dry, uncontaminated surface
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What is dynamics
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Describing motion and explaining its movement
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What is kinematics and what's it based on
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description of the temporal and spatial components of movements
based on relationships between position, velocity and acceleration |
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Describe linear motion
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everything moves the same distance at the same speed and deals with forces
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Describe angular motion
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if the body moves through the same angle at the same time and deals with moments
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What is general motion
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combination of linear and angular motion
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What is the integral of a velocity-time graph
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the change in position: displacement
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What do the projectile motion equations allow us to predict
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trajectory of the motion as well as its histories
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When is the optimal throwing angle above 45 degrees
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When the task emphasizes displacement, or a mix of vertical displacement and speed
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When is the optimal throwing angle at or below 45 degrees
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When a mix of maximum horizontal displacement and speed are required - during throwing and striking events
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What happens to the force required when release angle is increased
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you need more force
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Where is the velocity vector of a point on a body rotating about a fixed axis
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it is tangent to the path of motion
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What is relative motion
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description of a motion depending on the observer's frame of reference
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What is inertia
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Object in motion remains in motion at a constant speed unless acted upon by an external force
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Usefulness of inverse dynamics
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Calculating the resultant force acting on an object when the acceleration is known
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Usefulness of forward dynamics
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Calculating how an object will move knowing the forces acting on it
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Moment of force vs. moment of inertia
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moment of force: rotating effect of a force and the product of the force and moment arm
moment of inertia: resistance to angular accelerations |
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What is the mass moment of inertia
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tendency to resist angular acceleration when a rotation-causing force is applied
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is mass moment of inertia scalar or vector
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it is a scalar quantity: kg*m^2
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Which axis can rotational inertia be calculated around
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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 |
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what does the mass moment of inertia depend on
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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 |
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What is point mass and when is it used
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it is the point, equal to the mass of the entire body, located at a distance from the axis of rotation
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What does radius of gyration measure
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it measures the average spread of mass about an axis of rotation which is NOT the same as the CG
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How does the radius of gyration compare to that of the radius of rotation
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the radius of gyration is always slightly larger
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How can the mass moment of inertia be calculated
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The cadaver based ratio method
Through geometric models Mass scanning-based ratio method Mechanical perturbations |
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What are some characteristics for exceptional ground force reactions
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Strength
Muscle fiber type Coordination Mechanical technique |
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what is the point of inverse dynamics
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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 |
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What are the assumptions when doing inverse dynamics
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joints are frictionless, segments are rigid with mass concentrated at centre of mass, no co-contraction of agonist and antagonist muscles, air friction minimal
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What does the moment from inverse dynamics tell us
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which muscle is active (flexor/extensor) and how much torque that muscle is exerting
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How are the flexors and extensors represented
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flexor moments are positive and extensor moments are negative
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What are some limitations to inverse dynamics
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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
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What are centripetal forces and which direction does it point
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the force required to keep a body moving on a curved path, and it always acts towards the centre of the circle
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What are pseudo-forces
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Not true forces, but are effects that appear when observing motion from a moving frame of reference
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What are examples of pseudo-forces
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Centrifugal force
Coriolis force g-Force |
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What is the centrifugal force
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force that appear to cause the objects to flee the centre of the circular path
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What is the coriolis force
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force that appears when body is observed from a rotating frame of reference
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What is the g-Force
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appears whenever a body undergoes rapid acceleration/inertia of the body to the acceleration imposed on it
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Relationship between force and movement
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Force*time=change in (momentum*velocity)
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What are deformations and how do they occur
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they form during collisions and there two types: elastic which is recoverable, and plastic which is permanent
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What is a one-dimensional collision
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perfectly inelastic: ballistic pendulum
perfectly elastic: billiard problems Elasto-plastic problem: basketball problem e=relative velocity of separation/relative velocity of approach |
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What is a two-dimensional collision
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same concepts but use vectorial properties of the parameters involved in the system
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What is the law of conservation of momentum
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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 |
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What is work
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influence of a force on the movement of a body
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What is a key characteristic pertaining specifically to work
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the force must be in the same direction as the displacement
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What is the work energy theorem
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the kinetic energy of a system
the potential energy of a system the rotational kinetic energy of a system |
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What is external work
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work done by the resultant and the moment
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What is positive vs. negative work
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positive work absorbs energy and negative work releases energy
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What do positive and negative mechanical work values mean
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positive: work was done on the body
negative: body did work on some other body |
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What is power
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rate of doing work
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What is a conservative force
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a force acting on a body that does the same amount of work independent of the path of the motion of the body
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What is the best link between exercise physiology and biomechanics
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mechanical efficiency
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What is the internal work
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difference between total work and external work
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