moments Flashcards
they act in
opposite directions
find the ….. before findign the moment
perpendicular distance
find the moment about each point
remember: moment = force times perpendicular distance
B= 0
d=3 *12= 36nm
c=3 * 12= 36nm
A= 0
do you have to find the perpendicular distance or perpendicular force before finding the moment ?
PERPENDICULAR DISTANCE
Notation: lamina
2d object in which its thickness can be ignored
notation : coplanar
forces that act on the same plane
notation: resultant moment
sum of all moments actig on a body
find the resultant moment !
NOTE: dont overcomplicate the clockwise and anticlockwise directions ,TAKE THEM AS THEY ARE !
note: extend the line ( dashed line ) out so that you get perpendicular distance
DONT FORGET to also multiply the 400n force at point B by the perpendicular distance of 30 metres ( since B is the midpoint )
what did i do wrong here?
mixed up the vertical and horizontal components of clockwise and anticlockwise movements. eg i did 6000cos (theta) *d instead of the correct one: 6000sin(theta) *d for the clockwise moment
note: always draw out the components instead of doing them in your head !
notation :”uniform rod”
the weight of the rod is at its midpoint
on the back of the card : the reason why T isnt included in the clockwise side of hte equation ( left hadn side) is explained in “common mistakes
- realise that the weight of the rod is also included (20 newtons ) , not just the particle
- take momentsa round each point to find the tensiosn
- realise that this is all about equillibriums there moments on each side ARE THE SAME ]
.figure out T
find the reaction force
important note: FIRST state the pivot you are calculating about
get moments about that pivot, and then calculate !
the problem i had with this question : finding
its so easy, just equate vertical components ( up and down) and get R (this is especially true if the reaction forces are factors of one and other or if they are equal )
and then solve to find the distance “x” by equating moments!
find moments about one of the pivot SO YOU CAN IGNORE ABOUT ONE OF THE REACTION FORCES AND FIGURE OUT THE OTHER
then resubstitue into the vertical component equation
R will tilt because the moment around the reaction around the other pivot is NEGATIVE therefore causing R to tilt
NOTE: when trying to prove something tilts/ tilting questions, TAKE THE MOMENTS AROUND THE PIVOTS OF INTEREST
if a question asks for “fidn hte least and most value”
eg
consider both pivots
NOTE
ALWAYS ADD A “g” NEXT TO “m” IF MASSES ARE GIVEN IN “m”
