Transcript Moment of a Force

Centre of Gravity
& Moments
Stability
• Two factors determine the stability of an
object
– Centre of Gravity
– Base
• If the Centre of Gravity of an object acts
through its base an object will remain stable
– For stability a line perpendicular from the Centre
of Gravity must fall inside the base
Stability
• Which object will topple • Explain why the bottle
over?
rack doesn’t topple
over.
Equilibrium
• Stable Equilibrium
– Returns to its position after a small disturbance
• Unstable Equilibrium
– Will not return to its position after a small
disturbance
Two Laws of Equilibrium
1. The vector sum of the forces in any direction
is zero.
– forces up = forces down
– forces left = forces right
2. The sum of the moments about any point is
zero.
– Sum of the clockwise moments about a point is
equal to the sum of the anti-clockwise moments
about a point
Moments
• Force causes a body to change its velocity or
accelerate.
• A force can also cause a turning effect or a
rotation.
• The Moment of a Force is the turning effect of
a force.
Moment of a Force
Moment
=Force x Perpendicular distance
Fulcrum
Perpendicular
distance
FORCE
Moments
=Force x Perpendicular distance
= 10N x 5m = 50Nm
Perpendicular
distance=5m
FORCE
=10N
Couples
• A couple is two parallel forces acting in
opposite directions
Torque
Sample Question 1
• A cyclist applies a force of 8N on the
handlebars toward himself with his right hand
and the same size force away from himself
with his left. Each hand is 13cm from the
fulcrum. What is the torque of the couple on
the handlebars?
• Ans 2.08 Nm
Sample Question 2
• A 6m long plank of wood rests on a fulcrum 2m
from the left. A boy of weight 500N stands on the
left edge of the plank, while a girl weighing 400N
stand 2m from the right end.
– If the system is in equilibrium, calculate the weight of
the metre stick.
– What is the reaction on the fulcrum?
– If the boy was replaced by a man of weight 800N
standing on the same point, where would the girl
need to position herself for the system to be in
equilibrium again?
Experiment: Investigation of the laws
of equilibrium for a set of co-planar
forces