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Moment of a Force
Learning Objectives
 recall and use the relationship
between the moment of a force and
its distance from the pivot:
Moment = force x perpendicular
distance form pivot
 recall that the weight of a body acts
through the centre gravity
 Define the torque of a couple
Moments
 Forces can make
objects turn if there is
a pivot.
The see-saw
 In order to make the seesaw turn
about its pivot, forces have to be
applied on either side of the plank
The see-saw
 What happens when
one person moves
closer to the fulcrum
or pivot?
 The turning effect of
the force is also
dependent on the
distance of the force
from the pivot.
Turning effect
 the turning effect is
called the moment
of force (or simply
"moment“)
 the distance is
called the moment
arm (or lever arm)
of the force.
Moment of a Force
 To work out a
moment, we need to
know two things:
 the force or weight
applied
 the distance from the
pivot that the force or
weight is applied.
 Force and distance
must be
perpendicular to each
other
Moment of a Force
 Product of force and the
perpendicular distance
 From the pivot
Moment and Equilibrium
 Conditions for Equilibrium
 Net force is zero
 Net moment is zero at any point
Example
 Find the force F that will balance the
seesaw.
Example
 Find the force F that will balance the
seesaw.
Center of gravity
 The centre of gravity of an object is
the point where the whole weight of
the object may be considered to act.
Center of gravity
 For a regularlyshaped object, the
centre of gravity is
at its centre and,
where supported
there, it balances.
Example
 A uniform plank, 100 cm long and weighing
1.0 N is balanced at its midpoint by a
support. A weight of 1.20 N is hanged 5 cm
from the left end. It was found out that an
unknown weight W would balance the plank
if it is positioned 72 cm from the left end.
 What is the weight W of the object?
 How much force is exerted by the support on
the plank?
Couple
 Two parallel forces
equal in magnitude
 But opposite in
direction
 Has a turning
effect about a pivot
located midway
between them
Torque of a Couple
 Product of
 One of the forces
 And the distance between them
Your turn!
Answer numbers 1 to 4, page 56