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CHAPTER 9 : Stability
Stability
 Stability of an object refers to the
ability to maintain its original form.
 Stable : if it returns to its or...
Point of Equilibrium
 An object is said to be in equilibrium
when it is stationary or at rest.
 Is also its centre of gr...
What is Centre of Gravity
(CoG)?
 Definitions :
 The point where the Earth’s gravity acts
on that object.
 The point on...
Centre of Gravity
CoG for regular and uniform
objects
 The geometrical centre of some common
shapes-

 This is also where the centre of gr...
CoG for a triangle
 The centre of gravity (and
geometrical centre) is found by
drawing a line from the corner to the
oppo...
Irregular objects
1. Hang the string and a plumb line
from a pin as shown. Mark the
position of the plumb line with a
penc...
Irregular objects
1. Hang the string and a plumb line
from a pin as shown. Mark the
position of the plumb line with a
penc...
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 ...
Irregular objects
 When more than one material makes
up an object the centre of gravity will
be affected.
 The CoG can b...
External CoG
Stool
Ring

2 uniform
perpendicular
planks
Factors affecting stability
 2 factors :
 The position of its centre of gravity.
 Its base area.

 The lower the centr...
Position of Centre of Gravity
Double decker bus
1. The bus opposite has a low
centre of gravity since
there is a large weight in
the lower section of th...
Double decker bus
The bus opposite has a high
centre of gravity since there
is a large weight in the top
section of the bu...
Improving the stability of
objects
Improving the stability of
objects
 2 ways:
 Lowering its CoG.
 Reducing its height.
 Attaching a heavy weight onto it...
Applying the principle of
stability
Applying the principle of
stability
Why does the object balance?
 Linked to Principle of Moments
Anti- Clockwise Moment = Clockwise Moment
The Humble
metre r...
Icebergs!
As these guys melt their CoG
changes and they roll over!
Learning Intentions
By the end of the lesson we will be able
to…
 Understand the term Centre of
Mass/Gravity (CoG)
 Reco...
When an object is suspended
 When an object is suspended, so that
it can swing freely, it will come to rest
with its CoG ...
Balancing Act!
 Now, try to balance the object at that
point…
c
a

b

 Question- explain in terms of moments why
the obj...
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 ...
Learning Intentions
By the end of the lesson we will be able
to…
 Recall how the stability of an object
depends on the…
...
Stability
 An object will be in STABLE
EQUILIBRIUM when it returns to its
original position after given a small
displacem...
Page 53
 Neutral Equilibrium
 Unstable Equilibrium
Stability
 For an object to start rotating it
needs to have an unbalanced
moment acting on itRotates Clockwise

Rotates A...
• An object becomes unstable when its Centre of
Gravity falls outside of the object’s base.

Why?
All of the object’s weig...
The Traffic Cone
A great example of a
VERY stable object.
What features of the
cone make it so
stable?
-Low Centre of Grav...
Balance and Sport
Balance and Sport
Balance and Sport
The Double Decker Bus
 Each of the diagrams on the
right represent a Double
Decker bus with passengers
inside.
 Draw out...
Delete a word to complete the
sentences
 If more people sit upstairs on the bus
the CoG will rise/fall. This will make
th...
Question Time
 Page 56
 Questions 37-39
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Centreofgravityandstabilitystuver 100518122326-phpapp02

  1. 1. CHAPTER 9 : Stability
  2. 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. 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. 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. 5. Centre of Gravity
  6. 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. 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. 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. 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. 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. 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. 12. External CoG Stool Ring 2 uniform perpendicular planks
  13. 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. 14. Position of Centre of Gravity
  15. 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. 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. 17. Improving the stability of objects
  18. 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. 19. Applying the principle of stability
  20. 20. Applying the principle of stability
  21. 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. 22. Icebergs!
  23. 23. As these guys melt their CoG changes and they roll over!
  24. 24. 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.
  25. 25. 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
  26. 26. 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.
  27. 27. 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?
  28. 28. 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
  29. 29. Stability  An object will be in STABLE EQUILIBRIUM when it returns to its original position after given a small displacement
  30. 30. Page 53  Neutral Equilibrium  Unstable Equilibrium
  31. 31. 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
  32. 32. • 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
  33. 33. 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
  34. 34. Balance and Sport
  35. 35. Balance and Sport
  36. 36. Balance and Sport
  37. 37. 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
  38. 38. 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
  39. 39. Question Time  Page 56  Questions 37-39

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