304 Exam 3

kinetics

study of the action of forces
ex. inertia, mass, force, weight

Newton's first law

a body will maintain a state of rest or constant velocity unless acted on by an external force

Inertia

tendency for a body to resist a change in its current state of motion

inertia and mass

proportional relationship. increase in mass= better maintenance of current state of motion

Newton's 2nd Law

a force applied to a body causes an acceleration of that body of a magnitude proportional to the force, in the direction of the force, and inversely proportional to the body's mass. F=ma

Newton's 3rd law

for every action there is an equal and opposite reaction

friction

force acting over the area of contact between two surfaces in the direction opposite of motion or motion tendency

force and friction

proportional relationship

To move an object….

F must exceed maximum static friction in order to induce motion

Kinetic Friction

acts upon box during motion. is constant regardless of applied force. always less than max static point

coefficient of friction

(mew) unites number serving as an index for the interaction between two surfaces in contact. higher=more friction. static>kinetic

normal reaction force (R)

force acting perpendicular to two surfaces in contact. R increases with weight.

Manipulating R

-increasing/decreasing weight
-pulling vs pushing
(direction of force) pulling tends to decrease our reaction force and friction

Manipulating Coefficient of Friction

can make surfaces more/less sticky
-using gloves in sports
-adding wax on skis

linear momentum

quantity of motion that an object possesses. with no external forces, M stays constant. M=mv

change in momentum

means either a change in mass or a change in velocity. In a collision the objects will move in direction of greatest M

impulse

the product of a force and the time interval over which the force acts.
Impulse= change in M
Impulse=Ft

Perfectly Plastic Impact

impact resulting in the total loss of system velocity. at least 1 body deforms and doesn't regain its shape. bodies don't separate

Perfectly Elastic Impact

impact during which the velocity of the system is conserved. relative velocities of two bodies after impact are same as relative velocities before impact.

coefficient of restitution

number that serves as an index of elasticity for colliding bodies. unites number between 0-1. (e)

e = 0

more plastic

e = 1

more elastic

work

force applied against a resistance, multiplied by displacement of the resistance. W= Fd.
no movement= no work

positive work

when both the net muscle torque and direction of angular motion at a joint are in same direction
(concentric contraction)

negative work

when the net muscle torque and the direction of angular motion at a joint are in opposite directions
(eccentric contraction)

Power

rate of work production
P= W/t
P=Fd/t
P= Fv

Energy

capacity to do work

Kinetic Energy

energy of motion
KE= 1/2 mv2

Potential Energy

stored energy or energy of position
PE= m(9.81)h

Strain energy

elastic energy. capacity to do work by virtue of a deformed body's return to original shape
SE= 1/2 kx2

k= constant factor
x= distance of deformation

Conservation of Mechanical Energy

when gravity is the only acting external force a body's mechanical energy remains constant

Equilibrium

state characterized by balanced forces and torques. either motionless or constant state of motion

mechanical stability

is dependent on a body's ability to resist linear and angular motion

torque

rotary effect of a force about an axis of rotation.produces movement of the body segment
T=Fd
Counterclockwise (+)
Clockwise (-)

moment arm

perpendicular distance between a forces line of action and an axis of rotation. largest + strongest moment arm at 90 degrees

moment arm and torque

more moment arm= easier torque= less force needed

Agonist

prime mover

antagonist

controls movement velocity. opposite of movement. increases stability of joint

net torque

forces being created on both sides of the joint

torque in real world applications

quantifying joint torques provides general estimates of muscle group contribution during activities.
increased movement speed increases joint torque
increased speed requires a reduction in muscle tension of antagonist which decreases stability of movement.

lever

a simple machine consisting of a relatively rigid bar-like body that may be made to rotate about an axis

fulcrum

the point of support or axis which a lever may be made to rotate.

first class lever

lever positioned with the applied force and the resistance on opposite sides of the axis
ex. seesaw, scissors

second class lever

lever positioned with the resistance between the applied force and the axis
ex. wheelbarrow, wrench, eccentric contractions

third class lever

lever positioned with the applied force between the fulcrum and resistance
ex. shovel, broom, most concentric contractions

most levers in human body

third class levers

mechanical advantage

mechanical effectives quantified as the ratio of the force arm to resistance arm for a given lever. can be affected by angle at which muscle pulls on bone.
force arm/ resistance arm

force advantage

moment arm of force> moment arm of resistance
-the magnitude of force can be less than the magnitude of resistance.

resistance advantage

moment arm of force < moment arm of resistance
-most common in the body

advantages of third class levers in body

better range of motion and angular speed

disadvantages of third class levers in body

causes mechanical disadvantages in many situations.

static equilibrium

a motionless state characterized by three conditions
-all vertical forces=0
-all horizontal forces= 0
-all torque=0

dynamic equilibrium

concept indicating a balance between applied forces and inertial forces for a body in motion.
-all acting forces result in equal and oppositely directed inertial forces

center of gravity

point around which the mass and weight of a body is balanced no matter how the body is positioned.
symmetrical object= exact center
non-symmetrical= towards greater mass
can be outside of body

locating center of gravity

varies slightly depending on body position. we locate it with reaction board and segmental method

stability

resistance to disruption of equilibrium

balance

a person's ability to control equilbrium

base of support

area bound by the outermost region of contact between a body and support surfaces

factors that affect stability

mass, friction, CG height

• CG height= less stable
• friction= more stable
• mass= more stable
• base of support= more stable
  CG outside of body= less stable
  lower CG= more stable
  shift CG toward oncoming impact= more stable

Levers- deciding between F v R

F= causing motion

moment of inertia

inertial property for rotating bodies representing resistance to angular acceleration.
-based on mass and the distance the mass is distributed from axis of rotation
I=mr2

inertia and axis of rotation

inverse relationship.
mass distribution closer to axis of rotation= less inertia=easier to start/stop motion

moment of inertia of entire body

is the sum of movements of inertia of all particles the object contains.
I=mk2
k=radius of gyration

radius of gyration

the distance from the axis of rotation to a point where the body's mass could be concentrated without altering its rotational characteristics

angular momentum

quantity of angular motion possessed by a body. remains constant in absence of external factors

change in angular momentum

means a change in mass, radius, radius of gyration, angular velocity.
radius of gyration is most influential
no angular velocity= no angular momentum

angular impulse

change in angular momentum
Angular Impulse = Tt

centripetal force

center seeking force. force directed toward the center of rotation for a body in rotational motion