Transcript fundamental terminology

~ Through the Looking-Glass by Lewis Carroll
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Linear Motion of a Segment
translation of the body such that its
orientation in space does not change
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Linear Motion - monitor a single point or a
series of points
(e.g., body’s center of mass while airborne, the
head in running)
the point about which
the mass of the object is
BALANCED
it represents the point
where the TOTAL effect
of GRAVITY (weight)
acts on the object
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Angular Motion
rotation
body rotates around
a fixed axis
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Leg Swing during gait (walking/running)
AXIS
AXIS PASSES THROUGH JOINT CENTER
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General Motion
combines both linear and angular motion
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Mass
Volume
Scalars
Density
quantities which possesses magnitude
but has no direction associated with it
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Mass
• the quantity of matter composing a body
• units = kg (metric) or slug (English)
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Density
• mass per unit volume
captures effect of size and mass
r (“rho”) = m/V
• units = kg/m3
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Volume
• amount of three-dimensional space occupied
by a body
• for a box - the 3 dimensions are height, width,
and depth
Vbox = h*w*d
• units = m*m*m = m3
also measured in liters (l): 1 liter = 1000 cm3
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VECTORS
have both magnitude and direction
This can be shown pictorially by using an arrow.
displacement
length of arrow = magnitude of force
direction arrow points = direction of force
velocity
kinematics
kinetics
acceleration
Force
Torque
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Force (F)
• force - a push or pull acting on a body
• SI Units (“newtons,” N)
• 1 N = (1 kg) (1 m/s2)
• magnitude (size)
• English units (lb)
• direction (line of action)
• 1 lb = (1 slug) (1 ft/s2)
F = ma
• point of application
line of action
BOX
F
pt of application
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Forces are best described by VECTORS
because a vector describes both the SIZE
(magnitude) and DIRECTION of the force.
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Weight
amount of gravitational
force exerted on a
body
CM
W
W = mg
(g = -9.8 m/s2 on earth)
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Pressure
• amount of force per unit area
P = F/A
• units = N/m2 = Pa (Pascal)
• high heel example (p. 67)
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Torque (T)
aka “Moment of Force”
• angular analog of Force
Moment Arm
• causes rotation
T = Fd
• units = N m
AXIS OF ROTATION
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parallel line
moment arm
line of
action
NOTE: d is the
perpendicular distance
between a force’s line of
action and a parallel line
passing through the
axis of rotation
d is known as a
“moment arm”
Axis of
Rotation
Why is a door knob on the
outer edge of a door?
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