turning effects of forces

moment

• moment of a force is the product of the force and the perpendicular distance from the pivot to the line of action of the force

• moment of force = F x d

• SI unit - N m

• can be clockwise or anti-clockwise

centre of gravity

• is the point at which the weight of the object appears to act

• regular objects have their centre of gravity at their geometrical centres

• the centre of gravity of a body can lie inside or outside of a body

pivot and weight

• if the line of action of the weight has a perpendicular distance from the pivot, a moment is generated and makes the object rotate

• an object is balanced if it is supported at its cg (in the absence of external forces)

equilibrium

• the resultant force on the object is zero

• the resultant moment about any point on the object is zero

• taking moments about the centre of the pivot, sum of clockwise moments = sum of anticlockwise moments

principle of moments

• for an object to reach equilibrium, the sum of clockwise moments about any point is equal to the sum of anti-clockwise moments about the same point

stability

• stability of an object is the measure of its ability to return to its original position after it is slightly displaced

• stability can be increased by

  • lowering centre of gravity

  • increase its base area

resting position in pivoted body

• in balancing toys, moments due to hidden weights cause cg to be directly below the pivot

• weight of the object does not produce a moment about the pivot when balanced

  • perpendicular distance from line of action of weight to pivot is zero

  • objects behaves like a pendulum