CH. 5 Linear motion of the system
what happens to momentum in collisions?
conservation of momentum -momentum before collision=momentum after collision
rate of motion is NOT always
constant
without external forces, the spatial and temporal characteristics of motion (kinematics) remain
constant
without forces (kinetics), kinematics remain
constant
uniform motion
constant motion
acceleration = 0 at
constant motion OR at rest -equilibrium
projectile motion follows a ___ path
curvilinear
ground reaction force is
decelerating
decelerating means
decrease in momentum
increasing the time over which you allow the earth to change your momentum...
decrease the force on your body
if something is speeding up and going right, direction= ___ acceleration= ___
direction + acceleration +
if something is slowing down and going right: direction= ___ acceleration= ___
direction + acceleration -
if something is going at a constant speed (no change) and going right: direction=___ acceleration = __
direction + acceleration 0
if something is slowing down and going left: direction=___ acceleration=___
direction - acceleration +
if something is speeding up and going left, direction=___ acceleration=___
direction - acceleration -
if something is going at a constant speed (no change) and going left: direction=___ acceleration=___
direction - acceleration 0
drag force
dynamic fluid force acting in opposite direction of motion
work is an
energy transfer
newton's law of gravitation
every body attracts every body =force is product of masses/ square of separation
translation/linear motion (3 qualifications)
every point moves at the -same time -same direction -same distance
if an object is already in motion and traveling a given velocity, there is no change in that velocity unless
external force applied
law of acceleration
f=ma -acceleration directly proportional to force magnitude -acceleration inversely proportional to mass
law of action/reaction forces
for every action there is an equal and opposite reaction -forces exist in pairs
motion does not require a
force
kinematic values are measurements of the system exhibited by the system as a result of
force application Kinetics
kinetic measurement tool: *primary measurement tool in biomechanics
force plate
linear impulse is the product of
force x interval of time of force application
kinetics are
forces
___ is required to generate horizontal (anteroposterior and mediolateral) forces
friction
greater force =
greater change in linear motion
acceleration is directly proportional to force means
greater force= greater acceleration
acceleration is inversely proportional to mass means
greater mass= less acceleration (heavy, can't move quickly)
more force required to stop motion=
greater momentum
force plate measures
ground reaction force
the time over which you apply a force makes a difference...this is called
impulse
accelerating means
increase in momentum
in order to increase the impulse and momentum,
increase time to which you can apply a force
any movement made while airborne is caused by ___ forces
internal
linear impulse (J)
interval during which force is applied
SI unit of work
joules
example of this:
jump down from something you don't lock your knees. bend knees and slow down, increase time in which you allow earth to change your momentum and decrease the force put on your body.
units for momentum
kg m/s
units for momentum:
kg x m/s
the forces involved are the study of
kinetics
newton's second law:
law of acceleration
newton's third law:
law of action/reaction forces
newton's first law:
law of inertia
what is the vector version of linear distance traveled?
linear displacement
center of pressure (COP)
location of the resultant GRF Ax, Ay
because of varying ____, equal and opposite forces can produce unequal reactions
mass
momentum can be gained by increasing (2 things)
mass or velocity
peak rate of motion:
max velocity acheived
weight (w)
measures gravitational force upon mass -mg
free moment (T2) -what type of motion
moment about vertical axis through Center of pressure -twisting motion
average net force x time interval of force application=
momentum (m(Vf-Vi)
greater momentum=
more difficult to stop motion
projectile motion
once in motion, nothing can change path except external forces
when an object is slowing down, the acceleration is in the ___ direction of velocity
opposite
example of linear impulse:
pitcher wind up
center of pressure is the average
point on force plate where pressure is equally distributed around
Newton's first law (law of inertia) can also be written as
principle of conservation of linear momentum
peak velocity is not that important, in many activities it is more important how
quickly you can change your velocity (accelerate)
average acceleration:
rate of change in velocity/entire interval
instantaneous acceleration:
rate of change of velocity at one instant in time
instantaneous velocity
rate of motion at one given instant in time
newton's first law helps us understand:
relationship between kinematics and kinetics
equation for speed (s)
s= l/DT -speed= linear distance traveled/change in time
when an object is speeding up, the acceleration is in the ___ direction of velocity
same
linear distance traveled is a
scalar
internal forces change the
shape of the system
Ax
shifts forward or back
Ay
shifts side to side
kinematics are
spatial and temporal descriptions of movement
inelastic collision
stick together -share a common final velocity
plumbob:
string with weight
if linear velocity does not change then
system does not accelerate -acceleration=0
momentum (M)
system's quantity of motion
if you stand on one foot, center of pressure is within
that one foot (below it)
for mechanical energy,
the object has to move in order for work to be done
speed (s)
the scalar rate of motion
kinetics
the study of forces that inhibit, cause, facilitate, or modify motion
displacement will NEVER be greater than
total distance traveled
linear distance traveled (l)
total length of the path traveled by the system
unless a force acts upon the system,
total momentum remains constant
linear motion is also called
translation
if acceleration is greater than 0, positive
v final > v initial (positive change in velocity)
velocity (v) equation:
v=d/Dt -velocity=displacement/change in time
linear displacement is a
vector
momentum is a ____
vector
What is the vector rate of motion?
velocity
friction is not required to generate ___ force
vertical (z)
which component of GRF has largest magnitude>
vertical (z) -gravity, body weight
Change in velocity calculated by
vf-vi
how does force plate work:
when you step on it, it causes structural changes in metal, the electrical current generated is analogous to force
shod running
with shoes -heel strike (shin splints)
abebe bikila
won olympic marathon in rome on cobblestone barefoot
if you attach a plum bob to center of gravity and let it come to rest,
would track where COG is above the force plate
width of plate
x (front to back)--anteroposterior
length of plate:
y (side to side)--mediolateral
vertical
z (body weight)
work can be
+ or -
for projectile motion, acceleration in y direction is
-9.8m/s2
units of impulse:
N s
does the person in the race who has the highest peak/instantaneous velocity necessarily win the race?
NO
so we have to calculate:
-average rate of motion: -average speed -average velocity
two other things to look at in force plate besides GRF:
-center of pressure -free moment
collision equations depend on: (5)
-degree of stick -elasticity -friction -air resistance -gravity
Linear kinematics (4)
-displacement -velocity -direction -acceleration
2 types of collisions
-elastic -inelastic
law of inertia
-every body at rest remains at rest or in uniform motion in a straight line unless a force acts upon it
barefoot running: difference
-land on forefoot -no heel strike (no initial peak of Vertical GRF)
linear kinetics (4)
-momentum -work -power -energy
force transducers have 2 things
-piezoelectric sensors -strain gauge
translation 2 types:
-rectilinear -curvilinear
force plate directions: (3 components of GRF)
-vertical (Z) -anteroposterior (X) front to back -mediolateral (Y) side to side
force plate data has 6 quantities of interest:
-z GRF -x GRF -y GRF -center of pressure Ax -center of pressure Ay -free moment (twist)
at equilibrium, the sum of all forces =
0
at rest, acceleration=
0
at rest, velocity=
0
constant motion (no change in velocity), acceleration =
0
without linear displacement, velocity is equal to
0 -no change in position
for projectile motion, acceleration in x direction is
0 -because velocity is constant
force plate has a ____ coordinate system
3D
center of pressure is mapping where the
Center of gravity is on the force plate (monitors balance)
average speed/velocity:
calculated over a significant time interval
change in motion...
DOES require a force
linear displacement (d or Dp)
change in linear position of the system in a straight line
law of gravitation formula:
F=Gm1xm2/r2 -r2=separation
instantaneous acceleration equation:
F=ma
work=
Fd force x distance
average acceleration equation:
Fnet=mDv/Dt
linear impulse equation
J=FnetDt
linear impulse (J) is not the same as
Joules (SI unit for work)
equation for momentum (M)
M=mv -mass x velocity
linear acceleration (a)
change in rate of motion (vector rate of motion) -change in velocity
if acceleration is less than 0, negative
V final < V initial (negative change in velocity)
kinematics:
a measurement of the motion exhibited by the system as a result of force application
equation for acceleration (a)
a=Dv/Dt -acceleration= change in velocity/change in time
the observed change in motion is called
acceleration
equilibrium
at rest or constant velocity
at equilibrium:
at rest, acceleration=0 constant motion, acceleration=0
what tells who won the race
average velocity
center of pressure assesses
balance
if you stand on both feet center of pressure is
between the two
elastic collisions
bounce off
since linear displacement is a vector, it can be
broken down in x and y components
