Study Midterm 1 Flashcards
Parts of a lever system
A fulcrum (pivot point or axis of rotation)
A load moment arm (with a length of dL)
An effort moment arm (with a length of dE)
1st class lever
dL >, < or= dE
load and effort arm on each side of fulcrum
ex: crowbar or scissors
2nd class lever
dL < dE
ex: wheelbarow o bottle opener
3rd class lever
dL > dE
with fulcrum on one end
DA >1
MA <1
ex: most levers in the musculoskeletal system
Mechanical advantage
M.A.
(force advantage)
- the amplification (or reduction) in force due to the relative lengths of the effort and load arm
M.A. = FL/FE = dE/dL
Distance advantage
D.A.
(Speed advantage)
- the amplification (or reduction) in distance moved (and the speed) due to the relative lengths of the effort and load arm
D.A = FE/FL = dL/dE
Torque
tau, moment of force
- the force that causes an object to rotate about the axis
- distance between the axis of rotation and the applied force is called the moment arm
T= F * d , SI units: N m (newton meters)
counter clockwise (CCW) = +ve, CW= -ve
What do you need to know to calculate how much torque is acting on an elbow with the arm held horizontally when all forces are acting perpendicular (ie 90 degrees) o the forearm?
need to know the force acting on the forearm (m * g)
need to know the length of the moment arm (dL= distance from fulcrum to center of gravity of the arm)
Scalar
a physical quantity that has a magnitude
ex: mass, length, area, volume, speed, density, pressure, energy, work
Vecor
has both magnitude and direction
ex: acceleration, velocity, direction, momentum, force, displacement, lift, drag, thrust, weight
Newtons 1st law
a body stays at rest or in uniform motion in a straight line unless a force is applied to it
Newtons 2nd law
accelertation is proportional to the applied force and is in the same direction as the force
Newtons 3rd law
when one body exerts a force on another, the second always exerts a force on the first; the two forces are equal in magnitude, opposite in direction and act along the same line. (action/ reaction)
What is Force?
- an influence that causes an object to undergo change in movement, direction of geometrical construction
- has magnitude and direction (a vector)
- measured in newtons (N) represented by the symbol F
- F= m (mass (kg)) *a (acceleration (m/s^2))
What happens when the forces acting on a stationary object are balanced?
there is no movement (i.e. no acceleration)
What happens when forces acting on an object in motion are balanced?
there is constant velocity (i.e. no acceleration)
What happens when forces acting on a rigid object that is stationary are unbalanced?
the object will move (acceleration) in the direction of the net force
What happens when forces acting on a moving object are unbalanced?
there will be either acceleration (positive or negative if the forces are in line) or a change in direction (if the force is perpendicular to the direction of motion)
gravity
is acceleration acted on a mass due to earths graviational field (9.81 m/s^2)
F= m*g
Work
- is done on a body when a force applied ot the body causes a displacement in the direction of the force
- work= force aplied to an object (N) * displacement (d, meters) of the object, in the direction of the force
- Work (J)= force (N) * displacement (m)
Using SOH CAH TOA what happens when the angle between two vectors of interest is: 0 degrees, 180 degrees, 90 degrees or 270 degrees?
0 degrees, then Fx = F
180 degreed then Fx= -F
90 degrees then Fx=0
270 degrees then Fx= 0
Power
-is in units of watts
-is the rate at which work is done
-the rate at which energy is generated or consumed
-Power(W)= work(J)/ time (s)
For an object surrounded by a fluid (gas or liquid) which direction is pressure exerted?
90 degrees (‘normal’) to the surface of the object.
AKA pressure in fluids is omnidirectional- at any given point within a fluid the molecules are pressing equally in all directions
Atmospheric pressure: what is the diff between sea-level and Eversest in atmospheric pressure?
at sea level= 101.3kPa
the difference between sea level and Mt Everest(30kPa) is 3-fold
Hydrostatic pressure: How does it change?
pressure increases by ~1atm with every 10m of depth
101.3kPa at surface to ~110,000 kPa at bottom (>10000 fold)
Relation between pressure and volume in a container
inversely proportional
if P doubles V is halved
P1V1 = P2V2
If dE > dL then what happens to a 1st class lever
force is amplified by lever i.e. MA >1
If dE < dL then what happens to a 1st class lever?
the force is reduced by the lever i.e. MA <1
and there is a distance advantage/ speed advantage DA > 1 inversely proportional to MA
Why is MA the reciprocal of DA and vise versa?
Levers conserve work!
Work (J) = Force (N) x displacement (m)
example: lifting a 1kg mass 10 m requires the same amount of energy as lifting 10kg mass 1 m
with an arm held at 90 degrees and holding still what is the moment arm for the muscle
distance between muscle insertion point and elbow
this is where the force exerted by he muscle will act
dE
If the force is applied to the lever arm at any angle other than 90 degrees, how do you calculate the force that is not contributing to the torque around he axis of rotation
- calculate the component of the applied effort FE that is perpendicular to the moment arm L using cos(angle)
L= dE for perpendicular component of FEperp
(T= FEperp x L) - or calculate the length of the moment arm (dE) which is perpendicular to the line of action and , therefore, FE!
Muscles
the biological actuators that drive the stiff levers of the musculoskeletal system
Why do muscles attach so close to the fulcrum?
Muscles are good at generating force, but not very good at getting shorter (also keeps them out of the way!)
For a muscle to contract a short distance but produce a long movement at he end of a limb requires a small dE and a large dL (i.e. a D.A. >1)
what is a myofibril
its the basic unit of the muscle that contracts to shorten the muscle and generate force
Sarcomere
the functional unit of the muscle
between two z-lines
fibers shorten in the direction of the contracting muscle
muscle shortens by only ~20- 25% of relaxed length
contraction force of a muscle (ie force of a muscle is determined by)
the number of sarcomeres in parallel
cross-sectional area of muscle is proportional to number of fibers
therefore cross-sectional area of muscle is proportional to the force it can exert
Work a muscle can do proportional to
its volume
Work= Force x displacement
displacement(contraction distance) is proportional to muscle length
muscle volume = CS area (force) x length (proportional muscle shortening proportional to displacement)
A sarcomere can contract ____ and thus speed is determent by ____
~20% of relaxed length
speed determined by number of sarcomeres in series
What is Youngs modulus of elasticity, what relationship does it describe and what’s its equation
The stiffness of elastic modulus of a material
E (young’s modulus) = tensile stress/strain= change in sigma/change in E
steeper slope= stiffer material
What structural property of a biological material leads to a J shaped stress/strain curve
long elastic fibers may show a J-shaped curve (like collagen)
This is because when no stress is applied the fibers are coiled and crumpled up. Thus small increase in stress causes a large increase in stain (extension) as coil unwinds. Once fiber is stretched tight there it requires much greater increase in stress to further strain (stretch)material
Stress
force/ CS area measured in Pa
Strain
Dimensionless ratio of length change due to stretching
(DL) to initial un-stretched length Lo (i.e., DL/Lo)
Where on a stress/ strain curve is stiffness high? where is it low?
Stiffness (E)= change stress (sigma)/ chance in strain (e), units Pa
high is where slope is steep
low is where slope is low
What is toughness? How is it calculated? what are its units?
Work required to strain a unit value of material to failure
units are J/m^3
calculated by integrating area under the stress/ strain curve.
Shear Stress
= shear force (force applied parallel to surface)/ area force is aplied
Shear Strain
=displacement (change x)/ height
Shear modulus
G= shear stress (T)/ shear strain (y “gamma”)
relationship indicated degree an object will deform for a given amount of shear stress
units Pascals
Dynamic viscosity
u= shear stress/ shear strain rate (y “gamma” dot)
describes fluids ability to resist a continuously applied shearing stress by flowing (straining) at a certain rate
units Pascals
Shear strain rate
“gamma” dot = change velocity/ l (distance)
Brigham plastic
flows once stress exceeds yield stress
straight line on shear stress/ shear strain rate graph with a y intercept