KINETICS Flashcards by Tara Track

Newton’s 1st law

-law of intertia
-an object at rest will remain at rest unless acted upon by an external force
-an object in constant motion will remain in constant motion unless acted upon by an outside force
-an objects momentum will remain constant

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Newon’s 2nd law

-law of acceleration
-acceleration is proportional to force applied + inversely the mass of the body
-F = ma

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force

the action of one body on another

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center of pressure

-the theoretical point of application of a force/load transmitted through 2 contact surfaces
-weighted average of the location of all forces acting between 2 surfaces

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linear intertia

mass

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angular interia

moment of inertia

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pseudoforces

-inertial forces such as g-forces, centrifugal force, Coriolis force, etc.
-felt as the result of another force accelerating the mass

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intertial force

(-ma in [F - ma = 0])
-accounts for the tendency of a body to continue in its current state

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net force

-to predict the acceleration of a body we cannot look at a single force but rather must account for all applied forces
-the sum of all applied forces is the net force which will determine the acceleration of an object from the equation Fnet - ma = 0

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Newton’s 3rd law

-law of reaction
-for any action, there’s an equal + opposite reaction
-when 1 body exerts a force on a 2nd body, the 2nd body exerts a reaction force that is equal in magnitude + opposite in direction on the 1st body

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if 2 football players collide (1 with a mass of 100kg + the other with a mass of 75kg), who feels a greater force at impact

-both players feel the same amount of force due to Newton’s 3rd law
-the difference is that the acceleration is going to be different for the 100kg vs the 75kg individual

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if you jump off a cliff…describe forces

-you will splat but the earth will be relatively unaffected
-doesn’t mean that the force the earth felt was any different in magnitude than the force that you felt

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2 general forms of contact forces between 2 surfaces

-normal force
-frictional forces

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normal force orientation to surface

along axis perpendicular to surface

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frictional forces orientation to surface

parallel to surface

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ground reaction forces GRF

the force which the ground applies a body in contact with it

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friction

force acting at an area of contact between 2 surfaces in the direction OPPOSITE of that of motion or motion tendency

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friction ____ motion between contact surfaces

opposes

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what direction is a sprinter’s GRF out of the blocks

anteriorly directed
-counterintuitive because sprinting out of the blocks you are pushing back + down into the ground
-the point is that the ground is pushing on YOU in the forward direction, causing you to move forward (otherwise you would move backward)

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static friction

-also called maximum friction
-maximum amount of friction that can be generated between 2 static surfaces
-that means these surfaces aren’t moving relative to each other

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object not moving = static/kinetic friction

static

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object moving = static/kinetic friction

kinetic

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what is the cutting movement

static friction
-no kinetic friction

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when do you have kinetic friction

when you are sliding

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kinetic friction

-constant magnitude friction generated between 2 surfaces in contact during motion -these surfaces are moving relative to each other

which force is almost ALWAYS less: kinetic friction or static friction

kinetic friction

based on what does the coefficient of friction change

depending on whether the object is stationary or moving

which coefficient of friction will always be less: kinetic or static

kinetic

how are forces measured in lab

force plates

6 main outcome variables of force plates in lab

Fx: X component of GRF Fy: Y component of GRF Fz: Z component of GRF Mz: free moment about Z-axis Cx: X location of COP Cy: Y location of COP

strain gauges

-measure force applications -mounted to all 4 corners of a force plate -you can resolve where the COP is if you have this spatial distribution of what forces are being applied to the force plate

free moment

-the twisting effect you may apply to a force plate about the z-axis -force plate measures this twisting effect

any time you apply a force that is off axis of its COM, what happens

you cause rotational or angular acceleration alongside linear acceleration

if you have another force that is equal + opposite + applied at a different point, you can only have what type of acceleration

rotational (angular) -forces cancel out in the x direction because their line of rotation is removed but parallel -only causes turning effect on that object -that is what a free moment is

vicon

type of motion capture software

what should vertical GRF look like for quiet standing

static (basically motionless) -around BW -force plate is like a scale in this case -fluctuations corresponse to noise in the signal, small postural adjustments you may make while quietly standing

what should horizontal GRF look like for quiet standing

again, static (small postural adjustments) -BELOW BW, horizontal GRF = 0 -this is a frictional force, so no potential for relative motion when just standing -no external applied force other than gravity SO you should not have a frictional force if quietly standing

what should vertical GRF look like for gait

-in single leg support we have fluctuations of GRF above + below BW -above BW = upward acceleration of COM -below BW = downward acceleration of COM

what should horizontal GRF look like for gait

significantly smaller than vertical GRF because any nonzero force causes acceleration

what part of shoe is typically worn down

lateral side of heel

in the absence of outside force, what is conserved

momentum

elastic collision

-conservation of energy + momentum -2 balls colide + bounce off each other perfectly elastically

inelastic collision

-energy + momentum are conserved -2 balls collide + continue as 1 combined mass

real world collisions where objects don't stick together tend to be elastic/inelastic

partly elastic, partily inelastic

coefficient of restitution

indicative way of denoting how elastic/inelastic a collision is

impulse can give us some idea about what

likelihood of injury

when right leg is braking, is horizontal (anterior-posterior) GRF negative/positive

negative- meaning posterior

when right leg is in propulsion, is horizontal (anterior-posterior) GRF negative/positive

positive- meaning anterior

anterior = propulsion = positive horizontal GRF

posterior = braking = negative horizontal GRF

consider a continuous gradual increase of an applied horizontal force to this sled. once the applied horizontal force FA on a sled shown overcomes static friction, the dynamic friction will increase linearly as the applied force continues to increase

false

during quiet standing the horizontal ground reaction force is approximately zero

true

if 2 hockey players, one with a mass of 80kg and a velocity of 3 m/s and the other with a mass of 90kg and velocity of -2.8 m/s collide elastically (bounce off). what is true of their respective accelerations during the collision?

the smaller player will have greater acceleration

2 hockey players, one with a mass of 80kg and a velocity of 3 m/s and the other with a mass of 90kg and velocity of -2.8 m/s collide. what is true of the forces felt by the hockey players during the collision?

they both feel the same magnitude of force

if the magnitude of a person's ground reaction force is less than their mass*9.81 what is happening?

COM has a downward acceleration

during countermovement phase (descent into the loaded position) of the countermovement jump, the vertical ground reaction force does which of the following? -first rises above BW then drops below BW -first drops below BW then rises above BW -it drops below bodyweight and stay below body weight until the COM reaches its lowest position -it rises above bodyweight and stay below body weight until the COM reaches its lowest position

first drops below BW then rises above BW

true/false: during the launch and landing of a countermovement jump, the maximum vertical ground reaction force measured will always be the same

false

to make a cutting movement in soccer effectively and efficiently it is best to have ________________ between your foot and the ground -kinetic friction -no friction -static friction

static friction

true/false: an increase in the mass of a person will increase the maximum static friction between the person's feet and the ground

true

the maximum vertical ground reaction force occurs during what phase of the countermovement jump?

landing

sliding = kinetic/static

kinetic

cutting = static/kinetic

static

running is static/kinetic

static