KINS 4200 Exam 3 Linear Kinetics I

force definition

mechanical disturbance or load that may deform a body or change its state of motion

force is often defined by

what it does

ex. push, pull, friction, impact

force unit

Newton (N), pound (lb)

force = vector qualities

magnitude, direction, point of application, line of application/action

point of application

where the force is acting

line of application/action

vector that results from action

contact forces

direct contact

ground reaction forces, joint reaction force, friction force, fluid resistance, muscle force

non-contact forces

no direct contact

gravity, centripetal force

1st Law: law of inertia

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

inertia definition

an object or body’s resistance to motion

if ΣF = 0

then ΔV = 0

inertia of a body depends on

mass of object

larger mass requires larger force to start/shop movement

2nd law: law of acceleration

acceleration of an object is

• directly proportional to net force

• inversely proportional to mass

• in the same direction as net force

law of acceleration formula

F = ma

• F = force

• m = mass

• a = acceleration

3rd law: law of reaction

for any action, there is an equal and opposite reaction

ΣF_{a on b} = -ΣF_{b on a}

law of gravitation

all bodies are attracted to one another with a force proportional to the product of their masses and inversely proportional to the distance between them

weight definition

attractive force that the earth exerts on a body/object

W = mg

weight must be calculated in

newtons

mass x (-9.81 m/s²)

ground reaction force

the equal and opposite reaction force the ground exerts on the body during stance

measuring GRF

force plate used

standing: vertical GRF = persons weight

movement: vGRF changes

GRF vectors

vertical (up-down)

anterior-posterior (forward-backward)

medial-lateral (side-side)

GRF assumed to act on the

center of mass of individual

center of mass definition

objects mass is equally distributed in all directions

vertical GRF formula

vGRF = m ( a_z + g )

when acceleration is positive

vGRF is > than persons bodyweight

due to ↑ acceleration = ↑ net force

joint reaction force definition

equal and opposite force between two articulating bones caused by weight and inertial forces on the bones

muscle force

resultant of all forces generated by muscle fibers

compressive force =

vertical joint reaction force

joint reaction force =

resultant joint reaction force

shear force =

horizontal joint reaction force