week 4 Flashcards
Frequency (f) -
number of repetitions of a complete oscillation per unit time. Measured in cycles per second, or hertz (Hz).
Period (T) -
time interval between the successive occurrences of a particular phase of an oscillation
Simple harmonic motion, SHM -
type of periodic motion in which the restoring force is proportional to the displacement from the equilibrium position
Hooke’s Law:
Within the elastic limits of a material, the strain is proportional to the stress, i.e., the amount of deformation is proportional to the distorting force.
mechanical properties of different body components begins by
investigating their stress-strain relationships
What we model in stress-strain relationship and what the models are used for?
We model:
• body components’ nonlinear time-dependent properties and
• their time-dependent, viscoelastic properties.
These models are used to:
• understand how bones can bend and
• the occurrence of fractures.
simplest type of passive response -
harmonic or Hookean behavior
Body components can be passive or active:
• Passive components (bones & tendons): respond to outside forces.
• Active elements (muscles), generate forces.
But this division is NOT IDEAL. Muscles are active elements and passive components depending on the situation.
deformation depends
nonlinearly on force or stress
plastic deformation -
for large stresses - irreversible => material never returns to the same size or shape when the stress is removed
Extensive properties change when?
when the size of the object changes
Elasticity -
property by which a body returns to its original size and shape when the forces that deform it are removed
Strain (ε) -
fractional deformation resulting from a stress
elastic limit -
point up to which the object returns to its initial length when the stress is removed (no permanent deformation)
FOR STRESSES BEYOND THE ELASTIC LIMIT
THERE IS PERMANENT OR PLASTIC DEFORMATION
where is yield point and its significance
at a stress higher than the elastic limit, above it much elongation can occur without much increase in the load.
For tension, the material remains intact for larger stresses until
the ultimate tensile stress (UTS)
Fracture is where?
if application of larger stress continues, fracture at point F, at a strain called the ultimate strain or the ultimate percent elongation (UPE)
is ultimate compressive stress (UCS) different from ultimate tensile stress (UTS)?
yes
for ligaments and tendons, there is resistance to
resistance to tension, but not to compression
energy needed to break long bones -
fraction of that available from the kinetic energy in common collisions
why do our bones not regularly break? (2)
because most of the energy is absorbed by:
• muscle contractions and
• the deformation of soft tissues
Why bones fracture more easily in elderly ppl?
• their bones are weaker
• their tissues are less suited to absorb energy
• they may fall more awkwardly and with less body breaking action
Bone fractures are determined by: (2)
• the mode of the applied loads and
• their orientations
Bones in regards to various stimuli:
They are:
• strongest in compression,
• less strong in tension, and
• weakest in shear.
Bones usually break:
by shear stresses or under tension, but NOT UNDER COMPRESSION
Fracture can be due to direct blows which break the bone:
• in two (non comminuted) at low energy (transverse fracture), or
• into many pieces (comminuted) at high energy.
Indirect blows, as in skiing, can lead to fractures that are:
• spiral,
• oblique,
• transverse with a butterfly fragment and so on.
occurrence of fractures depends on
ultimate strength, defects, and specifically how loads are applied
Kinds of fracture in terms of time:
- Bones can fracture when the stress on them suddenly exceeds a given failure limit
- Bones can also fracture more gradually (due to damage from: prolonged continuous stress (sitting) or prolonged cyclic stress (fatigue due to walking or running).)
- Stress fracture: rate of damage exceeds the rate of repair by the body and the bone fails
metabolism -
processes involved by the body in energy intake, storage;
More generally, metabolism is any energy usage by the body.
thermodynamics -
study and application of the thermal energy of systems
Heat (ΔQ)
thermal energy that flows from one body or system to another (in contact with it), due to their temperature difference
direction of heat flowing -
HEAT ALWAYS FLOWS FROM HOT TO COLD
zeroth law of thermodynamics:
“If bodies A and B are each in thermal equilibrium with a third body T, then A and B are in thermal equilibrium with each other.”
The First Law of Thermodynamics -
- statement of the law of conservation of energy
“If an amount of heat Q flows into a system => this energy must appear as increased internal energy ΔU for the system and/or work W done by the system on its surroundings.”
specific heat capacity c -
quantity of heat required to change the temperature of unit mass of a substance by one degree
Thermal conductivity K describes
how temperature varies spatially due to the heat flow between different regions that are separated by a distance Δx.
Ιt also describes how much heat flows due to this spatial variation in temperature.
efficiency of a heat engine describes
how efficiently it turns heat into work
humans ~5.8%
usually much less than 20%, rarely exceeds this number
four modes of heat loss:
- radiation loss (54-60% heat loss)
- convection and conduction of air from the body (≈25% heat loss)
- evaporation of sweat (≈7% heat loss)
- evaporation of water through breathing (≈14% heat loss).
magnitude and importance of heat loss modes depend on (3):
clothing
environment
surroundings
thermal radiation:
Energy transferred as heat is via electromagnetic waves
black body is:
- A body that absorbs all the thermal radiation falling onto it.
- At thermal equilibrium it emits as much energy as it absorbs.
- A good absorber of radiation and a good emitter of radiation.
ε, emissivity:
fraction of energy incident on the object that is absorbed
All objects whose temperature is above absolute zero, (do what?)
radiate energy
Conduction:
Thermal energy moves through a material as a result of collisions between free electrons, ions, atoms and molecules of the material.
When a temperature difference exists between materials in contact, the higher energy atoms in the warmer substance transfer energy to the lower energy atoms in the cooler substance.
Heat flows from hot to cold.
ΔΤ/Δx -
temperature gradient - rate of change of temperature with distance
kT, thermal conductivity
depends on the material of the body
thermal energy being transported upward by convection when?
Such energy transfer occurs when a fluid (air), comes in contact with an object (match) whose temperature is higher than that of the fluid: temperature of the part of the fluid that is in contact with the hot object INCREASES => that fluid expands and becomes less dense => expanded fluid is now lighter than the surrounding cooler fluid, buoyant forces
cause it to rise
hc, convective heat transfer coefficient per unit area
decreases with heavier clothing and increases with less and lighter clothing: When you are nude you get cold faster because of increases in the above coefficient.
convection within the body (flow of blood) in regards to average T of the body:
IT DOES NOT CHANGE THE AVERAGE TEMPERATURE OF THE BODY, BUT THE DISTRIBUTION OF TEMPERATURE WITHIN THE BODY
Tc and Th
Tc - lower critical T
Th - upper critical T
Heat production is high at low temperatures, then decreases with increasing temperature until
Tc, becomes constant, and then at Th it increases again.
Below Tc, the heat loss increases due to
radiation and convection
Above Th the heat loss is dominated by
evaporation
Thermoregulation -
maintenance of a constant core body temperature in an organism
physiological responses to T:
-
vasoconstriction (narrowing of the blood vessels resulting from contraction of the
muscular wall of the vessels, in particular the large arteries and small arterioles) to cold T - vasodilation (the widening of blood vessels) to hot T
- piloerection - fine body hairs stand on end in an attempt to reduce convective heat loss from the skin (to cold T)
- shivering to cold T
