EXAM 2: FORM & FUNCTION

force

how hard it is to move an object

stress

how intensely a force is applied

strain

change in length/original length

pressure

when force is inside pushing outward

equilibrium

when a body exerts a force on another, the second exerts an equal and opposite reaction force

moment

tendency to cause rotation around a joint (force acting at a distance from an object)

allometry

changes in size as well as size

geometric similarity/isometry

changes in all size dimensions occur in the same proportion

fulcrum

fixed pivot point around which a lever rotates/balance (I.e. joint)

mechanical advantage

get more force out than you put in

There are five different kinds of loads that skeletal structures can experience. The third type of load is compression, which is when the ends along a long axis are “squashed” together. Another type of load is tension, which is when the ends along a long axis are “stretched” away from each other (pulled away). A third load is torsion, which is when there is twisting in opposite directions. Within torsion, we can see another type of load called shear. This is a sliding load, as the structural layers slide past each other in opposite directions. The last type of load is bending, which is when a structure is faced with compression (squashing) on one side and tension (stretching) on the other side.

describe the different loads that skeletal structures can experience

These loads can be resisted through different mechanisms and depend on the loading regime, as things like material and shape can help determine the resistance of a structure. For example, some bones can better resist certain loads because of how the material is distributed, like hollow limb bones resist bending because it is further from the neutral axis. The same can be said about support structures, like “I-beams” for building, because the flanges are not as close to bending load (and neutral axis). 

how are the different load that skeletal structures can experience resisted?

A biological safety factor is the degree to which a structure is “overdesigned”. This is found through dividing the failure value by the typical value. For example, bones break at 200MPa and only experience 50MPa when running. This gives a safety factor of 4 (200/50), as bones can withstand more force than what is expected in a standard activity like running.

what is a biological safety factor?

An advantage of a high safety factor is that it means there is more ability to withstand unexpected high loads, so it can limit the probability of severe injuries or even death. However, the disadvantage of this is that building and carrying extra material costs a lot more energy and limits what else this energy could be used for. 

what are the benefits and costs of a high safety factor?

The same can be said for low values, as the advantage in this circumstance would be that there is less energy required which can be used for other processes (i.e. growth or reproduction). While it can also mean an improvement in performance (i.e. speed/agility) because of less “weight” of a higher safety factor, the disadvantage is that the likelihood of more severe injury or death will be significantly higher than one with high safety factors. This is because it cannot withstand unexpected environmental changes that exceed what it typically expected. 

what are the benefits and costs of a low safety factor?

1. size

2. material (including geometry)

3. environment

4. ancestry/phylogeny

what are the four overarching factors for function?

translation

change in position

time

how long does it take?

velocity

how fast it the object moving?

work

force x distance

power

rate of doing work

forces (& mass) on bones increase faster than areas (ability to resist forces)

as size increases among geometrically similar organisms, ....

volume

as animals get bigger in length or height, body mass (& forces it puts on bones) increase like a(n) _________

area

as animals get bigger in length or height, bone cross-sectional areas (that resist force) increase like a(n) _______

extensibility (failure strain)

maximum amount an object can be deformed

stiffness

slope of initial linear stress/strain

organismal function

functional morphology or biomechanics

rotation

change in orientation

length

how far (& in what direction) does it go?

mass

how big is it (how hard to move)?

acceleration

how fast is velocity changing?

momentum

how much motion has occurred?

object doesn’t move

forces are still exerted even if _____ _____ _____

greater

forces applied over smaller area act with ______ intensity

1. a body at rest or in motion stays that way unless force is applied

2. F = mass x acceleration

3. equilibrium: when a body exerts force on another, the second exerts an equal and opposite reaction force

newton’s three laws of motion

vectors (magnitude and direction)

what are forces critical to motion analyzed as?

strength

maximum stress an object can withstand before failing

fracture

where the object breaks; obvious failure

yield

irreversible deformation; also considered to indicate failure

Young's modulus of elasticity

what is stiffness also known as?

toughness

amount of work an object will absorb before breaking (area under stress/strain curve gives index)

• bones are stiff to transmit force

• collagen tendons allow elastic energy storage and recoil

give some examples of how different materials in organisms have different properties and perform corresponding functions

compression

squashing along structure axis (long axis)

tension

pulling along structure axis (long axis)

torsion

twisting load (opposite directions)

shear

within torsion; sliding load as structural layers slide past each other in opposite directions

bending

load with compression on one side and tension on the other

loading regime

what does a structure's ability to resist a load strongly depend on?

bending stress; more frequent cause of damage in support structures

is axial or bending stress typically higher?

material and shape

what does a structure's resistance to load depend on (in the context of bending)?

bending in a specific direction

what is I calculated for?

I increases

what does I do as material is placed further from the neutral axis?

force(in) x lever(in) = force(out) x lever(out)

equation for equilibrium

increase L(in) relative to L(out)

how do you get mechanical advantage?

low velocity

what is the trade-off for high force?

velocity advantage

higher speed by inputting low force

decreased L(in)

how do you get velocity advantage?

locations of different variables (I.e. fulcrum and lever arms) can vary with different configurations

what influences the definition of a lever system?

lever arms are always perpendicular distances from the line of action of a force to the fulcrum

what is necessary to know about examining different lever systems?

mechanical; armadillos dig and that requires more force than speed

an armadillo has a long L(in), what type of advantage is this? why might this be more beneficial for this type of animal?

velocity; cheetahs are runners that require more speed than force

a cheetah has a shorter L(in), what type of advantage is this? why might this be more beneficial for this type of animal?

as organisms get bigger, forces on bones increase faster than ability to resist forces, unless proportions change

why can’t humans be 50 ft tall?