moments

define moment

the turning effect of a force

what happens if the force acts through the pivot?

the object will not rotate

what happens if the force acts at a distance from the pivot?

the object rotates

state the equation for moment

moment = force x perpendicular distance from pivot

(sadly no symbol equation as moment doesn’t have a symbol)

state the unit of moment

newtonmetRE (Nm)

is moment vector or scalar?

vector (clockwise and anticlockwise)

2 moments in the same direction will…

add

2 moments in opposite directions will…

subtract

state the Principle of Moments

for an object in equilibrium (balanced) the total clockwise moment = total anticlockwise moment

centre of gravity

the point in an object where all the weight is said to act

what happens if we support an object at the centre of gravity?

will be a reaction force up

what happens if the pivot is not the centre of gravity?

there is a moment = w x d and the object will rotate. It stops rotating when the CoG is directly under the pivot

where is the centre of gravity in regular objects?

the object’s centre

how do we find the centre of gravity of an irregular object?

hang the object from a point

hang a plumb line from the same point

mark the line of the plumb line on the object

repeat from two different points

CoG is where the lines meet

does the centre of gravity have to be in an object?

no

in the supported beam 2 pivot problem, what happens when there is no extra mass

F1=F2

F1+F2 = w

in the supported beam 2 pivot problem, what happens if you add extra mass at the centre?

F1=F2

F1 and F2 are both bigger

in the supported beam 2 pivot problem, what happens if you add extra mass at one side?

F1+F2 gets bigger

side closer to the mass (eg F1) is bigger than the other side

making distance x smaller makes the moment smaller

a smaller moment is needed at 2, so force F2 is smaller