Describing the System [1] Flashcards
what is the law of inertia?
a body stays in its state of rest OR in motion, unless it is compelled to change that state by forces impressed upon it
- a body in motion stays in motion ect..
What is the law of acceleration?
change in motion is directly proportional to the magnitude of the applied force
F = MA
What is acceleration?
Observed change in motion
- even deceleration falls under this category
What is the law of action-reaction?
To every action there is always an opposed and equal reaction
- these forces occur in pairs
(Think of a race car; gravity pulls down, road pushes up; & engine propels forwards, drag/friction pulls backwards)
What are JRF’s?
Joint Reaction Forces: they keep our joints stable while our muscles move
- ex: shoulder abduction; humeral head stays in socket while deltoid muscles move
What is considered a force?
Something with the capability to cause a change in motion of a system
- ex: push or pull
What are the different properties of a force?
- magnitude: size of applied force
- orientation: alignmnet of the vector in relation to cardinal directions
- direction: the way the force is applied
- line of action: imaginary line extending indefinitely along the vector through tip and tail
- point of application: point at which the system receives the applied force
What types of forces are there?
- Non-contact (field) forces
- Contact forces
- External and Internal forces
- Action and reaction forces
Explain non-contact (field) forces
Law of universal gravitation explains the ineraction of objects even when not in contact with each other
- ex: magnets
- field: any object possessing mass creates an invisible field of influence stretching throughout the space
What type of force is the electromagnetic force?
Non-contact field force
This force it is between electric charges
- gravitational forces exists between bodies of mass
- our body expeireces both electromagnetic and gravitational forces
What are contact forces?
the result of phsyical contact between 2 bodies
ex: hitting a ball with a bat
What are external forces?
those that intercat with the system from the outside
- change the motion of the system
What are internal forces?
they act within the defined system
- they change the shape of the system
What is an action force?
The initially applied force
What is a reaction force?
the simultaneous equal counterforce acting in the opposite direction to the action force
- locomotion
How does Newton’s 3rd law relate to action & reaction forces?
Essentially the force the body places on the earth, earth places back on the body
- intenal forces on the system are the muscles
- the exertrnal forces are the equal and opposite force applied by the earth; which creates motion
What is ground reaction force?
keeps us stable as we walk
- on sand GRF is not quite the same and can’t give us back the force we put on it
How does gravity work in relation to weight?
Gravity pulls on an objects mass with a ceratin amount of force (it gives us weight in a downward direction)
- Weight: the measure of the force with which gravity pulls upon an objects mass
-
weight changes as we get closer to and darther from the center of the ea
What are the 2 types of resultant forces?
- Centripetal force: any force that causes a system to exhibit circular motion (center seeking)
- Centrifugal force: the force that is equal to and opposite the centripetal force (center fleeing/fugitive)
What is friction?
A force that resists the sliding of 2 objects in contact
- exists when 2 objects in contact have the potential to slide across each other
- vectors that represent frinction force has a direction that is opposite of the potential sliding direction and parallel to the 2 surafces in contact
friction is resistance to sliding
What is static friction?
occurs when 2 contacting surfaces are not sliding/moving relative to each other BUT do posses the potential for movement
- to move a box the static friction < applied force
- thus once it moves it goes from static friction to kinectic friction
desk has static friction against the floot
What is kinetic friction?
friction in which 2 surfaces are already sliding relative to each other
- easier to move a box with kinetic frinction than static friction bc coefficient is lower
What is rolling friction?
occurs whenever one surface is rolling over another but not sliding
- coefficient of rolling friction is < than static and kinetic which is why this is the easiest to do
How does pressure interact with force?
Pressure is the magnitude of applied force acting over a certain area
- Pressure = Force/Area
Ex: laying on a bed of nails manipulates pressure to avoid puncture
- weight of a person = applied force (F)
- area over which it is applied = total area of the tips of the nails (A)
What is stress?
The external force acting to deform a material
- (similar to pressure it is calculated as an external force)
What is strain?
The resulting magnitude of deformation as a result of the applied stress
- the percentage of change in length is due to an applied stress
- Strain = Change in length/initial length
Its all about HOW it deforms
Describe Poisson’s Ratio
tendency of a metrial to exhibit transverse strain simultaneously with an axial strain
[Basically it narrows as it lengthens]
- poisson’s ration has values between 0.00 (very elastic) and 0.50 (perfectly incompressible)
Describe the stress strain curve and draw and label it
- A= high strength & low ductility not malleable (ex: bone; once it deforms its done)
- B = medium strength & medium ductility (ex: ligaments and tendons; they can deform and still recover)
- C = low strength & high ductility (ex: cartilage; it is easy to deform and it handles it easily)
What are the stress/strain values
- strength= max stress or strain a material can withstand without permanent deformation
- ductility: =force p/unit area required to deform a material; represented by steepness of slope
- toughness: = total energy required to cuase material failure
What is the elastic region?
the linear portion of the stress/strain curve
- the material will return to its original shape if the tensile stress is removed within this range
What is the yield point in the stress/strain curve?
the point at which the applied stress can lead to permanent deformation
What is the plastic region of the stress/strain curve?
non-linear response of the material after the yield point
- some degree of deformation will perist after removal of stress
- occurs after yield point and before ultimate strength (which is deformation without repair)
What is tension stress?
two forces applied to a system in opposite directions, away from each other
What is compression stress?
result of 2 forces being applied to the system in opposite directions, towards each other
What is Shear Stress?
2 parallel forces that tend to simultaneously displace one part of a system in a direction opposite another part of the system
What is bending stress?
2 off-axis forces are applied such that tension stress is cuased on one side of the system and compression stress occurs on the other side
- combines tension and compression stress
What is torsion stress?
two forces being applied in such a way that part of the sytem is rotated around its longitudinal axis in a direction opposite roattion of another part of the system
twisting motion
What is the coefficicnet of restitution?
Materials vary in their ability to reform after deformation
- the coefficient is the paramter observed after reformation that indicates the aility of an object to return to its original shape after deformation
- measured by rebound
- E= sq root of (Height of drop divided by Height of rebound)
ranges from 0.00 (inelastic) - 1.00 (perfectly elastic)
What is Viscoelasticity?
A material whose deformation is affected by both the rate of loading and the length of time that it is subjected to a constant load (starts to involve fluids)
- think of the spine
What is stress relaxation?
the corresonding eventual decrease in stress that will occur as fluid is no longer exuded/compressed
this can occur while we sleep
