KINETICS Flashcards
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
Newon’s 2nd law
-law of acceleration
-acceleration is proportional to force applied + inversely the mass of the body
-F = ma
force
the action of one body on another
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
linear intertia
mass
angular interia
moment of inertia
pseudoforces
-inertial forces such as g-forces, centrifugal force, Coriolis force, etc.
-felt as the result of another force accelerating the mass
intertial force
(-ma in [F - ma = 0])
-accounts for the tendency of a body to continue in its current state
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
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
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
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
2 general forms of contact forces between 2 surfaces
-normal force
-frictional forces
normal force orientation to surface
along axis perpendicular to surface
frictional forces orientation to surface
parallel to surface
ground reaction forces GRF
the force which the ground applies a body in contact with it
friction
force acting at an area of contact between 2 surfaces in the direction OPPOSITE of that of motion or motion tendency
friction ____ motion between contact surfaces
opposes
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)
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
object not moving = static/kinetic friction
static
object moving = static/kinetic friction
kinetic
what is the cutting movement
static friction
-no kinetic friction
when do you have kinetic friction
when you are sliding
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
