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# Centreofgravityandstabilitystuver 100518122326-phpapp02

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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