Equilibrium
centre of mass & stability
Objectives
• Be able to define centre of mass.
• Describe how centre of mass can be
found.
• Use centre of mass to discuss stability of
objects.
• Explain why some objects are more
stable than others.
Vocabulary
• Center of Mass
How is he
balanced?
How are
these
rocks
balanced?
Centre of Mass.
Definition: A point representing the mean
position of the matter in an object.
Or: The point at which the entire mass of a
body may be considered concentrated
Symmetrical objects.
The centre of mass of symmetrical objects can be
found where the lines of visual symmetry cross.
Non Symmetrical objects
Any object will hang so a
vertical line passes
through it’s centre of
mass.
Try it.
Rang an object from a
point and draw a vertical
line, repeat this from a
second point. Where the
lines cross is the centre of
mass.
Balance and Stability
• An object will balance around its centre of
mass, but wont always be stable.
The lower
an objects
centre of
So what makes
an object
‘Stable’?
mass the more stable is will be.
How & why do
the eggs stay
Why is the
upright?
Butterfly so
stable?
Balance and Stability
The egg is weighted with a very low centre of gravity (
).
Force due to gravity can be shown as a single force ( ) acting at
the centre of gravity.
The egg pivots around the point it is in contact with the table ( )
The egg balances when the centre of gravity is over the
pivot point so that the force due to gravity acts over the
pivot (also the base of support for the egg). When it is on
its side there is a moment (turning force) that returns it to
its balance point.
Balance and Stability
The wings are weighted so its centre of gravity (
) is below the
pivot point ( ) outside the butterfly.
Force due to gravity can be shown as a single force ( ) acting at
the centre of gravity.
If theItbutterfly
either
way then
a moment
balancestilts
so well
because
the there
centreisof
(turning
force)
that returns
it to the balance point.
gravity
is below
the pivot.
Balance and Stability
What other main factor makes an object more stable?
Which Lego tower is more stable? Why?
How is he
balanced?
How are
these
rocks
balanced?
When will they fall?
When will the bus fall over?
Why doesn’t he fall over?