1. CHAPTER 9 : Stability

2. Stability
 Stability of an object refers to the
ability to maintain its original form.
 Stable : if it returns to its original
position when tilted and released.
 Unstable : if it continues to move
further than its original position
after being tilted and released.

3. Point of Equilibrium
 An object is said to be in equilibrium
when it is stationary or at rest.
 Is also its centre of gravity.

4. What is Centre of Gravity
(CoG)?
 Definitions :
 The point where the Earth’s gravity acts
on that object.
 The point on which the whole weight of
an object appears to act.

5. Centre of Gravity

6. CoG for regular and uniform
objects
 The geometrical centre of some common
shapes-
 This is also where the centre of gravity and
the object’s weight, W, can be considered
to act

7. CoG for a triangle
 The centre of gravity (and
geometrical centre) is found by
drawing a line from the corner to the
opposite line’s midpoint

8. Irregular objects
1. Hang the string and a plumb line
from a pin as shown. Mark the
position of the plumb line with a
pencil.
1. Repeats the process again, but
this time place the pin at a
different location, and mark the
position of the plumb line string.
1. Where the two pencil lines cross
is the centre of gravity of the
shape.

9. Irregular objects
1. Hang the string and a plumb line
from a pin as shown. Mark the
position of the plumb line with a
pencil.
1. Repeats the process again, but
this time place the pin at a
different location, and mark the
position of the plumb line string.
1. Where the two pencil lines cross
is the centre of gravity of the
shape.

10. CoG of an irregular shape
 Find the CoG of an irregular shaped
lamina
Pin held by
clamp
a
b
c
b
c
a
c
a
hole
plumb line
b

11. Irregular objects
 When more than one material makes
up an object the centre of gravity will
be affected.
 The CoG can be outside the object.

12. External CoG
Stool
Ring
2 uniform
perpendicular
planks

13. Factors affecting stability
 2 factors :
 The position of its centre of gravity.
 Its base area.
 The lower the centre of gravity, the
more stable the object.
 The larger the base area of an object,
the more stable the object.

14. Position of Centre of Gravity

15. Double decker bus
1. The bus opposite has a low
centre of gravity since
there is a large weight in
the lower section of the
bus. This means that the
force of gravity will act
through the base of the bus
to keep it on the road.

16. Double decker bus
The bus opposite has a high
centre of gravity since there
is a large weight in the top
section of the bus. Due to the
high centre of gravity this bus
is unstable because the
centre of gravity is acting
outside the base of the bus.

17. Improving the stability of
objects

18. Improving the stability of
objects
 2 ways:
 Lowering its CoG.
 Reducing its height.
 Attaching a heavy weight onto its base.
 Increasing its base area.

19. Applying the principle of
stability

20. Applying the principle of
stability

21. Why does the object balance?
 Linked to Principle of Moments
Anti- Clockwise Moment = Clockwise Moment
The Humble
metre rule
 When the object is suspended from a point
it will come to rest when the clockwise
moment is equal to the anti clockwise
moment

22. Icebergs!

23. As these guys melt their CoG
changes and they roll over!

25. Learning Intentions
By the end of the lesson we will be able
to…
 Understand the term Centre of
Mass/Gravity (CoG)
 Recognise the factors that affect an
object’s CoG
 Investigate how to find the CoG of an
irregular object.

26. When an object is suspended
 When an object is suspended, so that
it can swing freely, it will come to rest
with its CoG vertically below the
point of suspension
See Page 52 and 53 for extra notes in the book

27. Balancing Act!
 Now, try to balance the object at that
point…
c
a
b
 Question- explain in terms of moments why
the object can be balanced at this point?
Answer- on one side of the point of CoG, the object’s weight is causing a
turning effect. This is counter balanced on the other side where the
weight is causing a turning effect in the opposite direction.

28. Example
 1 m long spade balances in a string loop when
the loop is 30 cm from the end of the blade.
2 kg
 A 2 kg mass is placed at the handle and the
balance point is at the centre of the spade.
 What is the mass of the spade?

29. Learning Intentions
By the end of the lesson we will be able
to…
 Recall how the stability of an object
depends on the…
 position of the centre of gravity
 size of the object’s base

30. Stability
 An object will be in STABLE
EQUILIBRIUM when it returns to its
original position after given a small
displacement

31. Page 53
 Neutral Equilibrium
 Unstable Equilibrium

32. Stability
 For an object to start rotating it
needs to have an unbalanced
moment acting on itRotates Clockwise
Rotates Anti-Clockwise
Pivot
W
Stable
W
Stable
Pivot
W
Unstable

33. • An object becomes unstable when its Centre of
Gravity falls outside of the object’s base.
Why?
All of the object’s weight can be considered as acting
through the CoG. If this acts outside the base then it
will cause a turning effect or a moment

34. The Traffic Cone
A great example of a
VERY stable object.
What features of the
cone make it so
stable?
-Low Centre of Gravity
-Wide base

35. Balance and Sport

36. Balance and Sport

37. Balance and Sport

38. The Double Decker Bus
 Each of the diagrams on the
right represent a Double
Decker bus with passengers
inside.
 Draw out and label the one
which shows all passengers
on
- the lower level
- the upper level
- both levels

39. Delete a word to complete the
sentences
 If more people sit upstairs on the bus
the CoG will rise/fall. This will make
the bus more stable/unstable and it
can be tilted by a greater/lesser angle
 The bus will fall over when the CoG
acts inside/outside of the bus’ base
 A stable object has a low/high CoG
and a narrow/wide base

40. Question Time
 Page 56
 Questions 37-39