moments Flashcards
what is a rigid body?
an object that does not bend or stretch
what does uniform mean?
mass is distributed evenly so weight acts in the centre of the body
what does at point of turning mean?
some force at a point of support has just become 0
what is the moment of a force?
strength of its turning effect about a given point (all moments in our spec will be in equilibrium)
what is the formula for a moment?
force X perpendicular distance from pivot
how do you represent clockwise and anticlockwise in calculations?
clockwise is a positive value for moment while anticlockwise is a negative moment
what is a resultant moment?
sum of moments acting about a point on a body
when is a rigid body in equilibrium?
- the resultant force in any direction is 0
- the resultant moment about any point is 0
when asked to find the reaction force at 2 supports on a rod, how is this done when given only the weight and lengths?
for a uniform rod, label the weight force straight down in the centre choose one of the supports and take the moment af the weight from this point.
then find an expression in terms of RB (reaction force at the other support) and the distance from the initial point.
these should sum to 0 and hence find the value of RB.
subtract RB from total weight to give RA
when given a rod on 2 supports and asked to find the point of tilting for a mass acting on a side of one of the pivots, what occurs on the other support?
the other end of the rod will be lifted off the other support so normal reaction force will be 0.
hence, the moment of weight to the position of the mass must be the same magnitude and equal 0 and the mass can be deduced
when is a rod at point of rotating/ tilting?
when reaction force/tension of one of the supports/cables is 0.
what are the notations for moments?
moment of A:
m (A)
A⤸
how do you find the moments of forces on angle to a pivot?
find the perpendicular distance between the line of action of force and the pivot using trig. multiply this by the force to find moment’
