Angular Kinetics Flashcards
Newtons first law for angular kinetics
a rotating object will continue to do so at constant angular momentum unless acted on by an external force
Newtons second law for angular kinetics
an external moment produces an angular acceleration that is - proportional and in the direction of the moment
Newtons third law for angular kinetics
for every applied moment there is an equal and opposite reaction moment
Moment (or torque) definition
The tendency of a force to cause rotation about a specific axis
“measurement of turning effect”
How is a Moments value calculated
product of magnitude of a force and the perpendicular distance from the line of action of the force to the axis of rotation (mass x distance)
What do eccentric forces do?
cause rotation as well as translation
line of action does not pass through centre of gravity
How can rotation be acheivied without translation
applying two equal and opposite parallel forces (2 corners of square)
- rotation occurs around centre of gravity (Force Couple)
How is the turning effect of a force couple calculated?
magnitude of one of the forces x perpendicular distance between lines of action = moment
Moment calculation and explanation around a pivot
Moment (N.m) = force (N) x distance (m)
the bigger the distance from pivot (moment arm) the greater the resultant force (moment)
Resultant moment definition
the sum of all moments about a defined point
what are clockwise moments denoted as?
negative direction
thumb down - fingers go clockwise
what are anti-clockwise moments denoted as?
positive direction
thumbs up - fingers go anti clockwise
Equilibrium definition
when all parts of a body at rest or are moving with the same constant velocity
Centre of Gravity definition
point through which resultant force of the gravity acts
where body weight theoretically acts
How can an objects centre of gravity location be estimated
Suspension method
Reaction board method
Segmental method
How does the suspension method work?
suspend an object and use intersecting plum lines to find the centre of gravity
How the does the reaction board method work
person lies on the balanced reaction board and scales
allows centre of gravity to be estimated as a length from feet
Reaction board equation
Length x (R2 - R1) / W (mass x gravity)
R2 = scales without person R1 = scales with person laying still
How does the segmental method work
uses sum of moments = moment of the resultant rule
each body segment has its moment calculated (distance from x & y axis when graph is superimposed)
Segmental method calculation (X, Y coordinates)
X = sum of segment mass x segment CoG coordinates / mass
How is segment data for humans acquired? (3)
Cadaver data (weighed on balancing table)
Mathematical modelling
Scanning (MRI)
How does the mathematical model work?
mathematical model divides body into segments (accounts for gender & varied densities) and estimates
