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# Centre of Gravity and Stability

GCSE Physics double award notes

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### Centre of Gravity and Stability

1. 1. Centre of Gravity and Stability of an object GCSE Physics
2. 2. Learning Intentions <ul><li>By the end of the lesson we will be able to… </li></ul><ul><li>Understand the term Centre of Mass/Gravity (CoG) </li></ul><ul><li>Recognise the factors that affect an object’s CoG </li></ul><ul><li>Investigate how to find the CoG of an irregular object. </li></ul>
3. 3. Centre of Gravity <ul><li>This is the point at which the whole weight of the body can be taken as acting through </li></ul><ul><li>or </li></ul><ul><li>the point from which the object will balance </li></ul>
4. 4. Regular and uniform objects <ul><li>The geometrical centre of some common shapes- </li></ul><ul><li>This is also where the centre of gravity and the object’s weight, W , can be considered to act </li></ul>W W W
5. 5. For a triangle <ul><li>The centre of gravity (and geometrical centre) is found by drawing a line from the corner to the opposite line’s midpoint </li></ul>W W W
6. 6. For non-uniform shapes <ul><li>When more than one material makes up an object the centre of gravity will be affected. </li></ul>
7. 7. External CoG <ul><li>For some objects the CoG is not located on the actual object - </li></ul>Ring Stool 2 uniform perpendicular planks W W W
8. 8. Why does the object balance? <ul><li>Linked to Principle of Moments </li></ul><ul><li>When the object is suspended from a point it will come to rest when the clockwise moment is equal to the anti clockwise moment </li></ul>The Humble metre rule Anti- Clockwise Moment = Clockwise Moment
9. 9. Icebergs!
10. 10. As these guys melt their CoG changes and they roll over!
11. 12. Learning Intentions <ul><li>By the end of the lesson we will be able to… </li></ul><ul><li>Understand the term Centre of Mass/Gravity (CoG) </li></ul><ul><li>Recognise the factors that affect an object’s CoG </li></ul><ul><li>Investigate how to find the CoG of an irregular object. </li></ul>
12. 13. When an object is suspended <ul><li>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 </li></ul>See Page 52 and 53 for extra notes in the book
13. 14. CoG of an irregular shape <ul><li>Find the CoG of an irregular shaped lamina </li></ul>a b c a b c b a c plumb line hole Pin held by clamp
14. 15. Balancing Act! <ul><li>Now, try to balance the object at that point… </li></ul><ul><li>Question- explain in terms of moments why the object can be balanced at this point? </li></ul>a b c 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.
15. 16. Example <ul><li>1 m long spade balances in a string loop when the loop is 30 cm from the end of the blade. </li></ul><ul><li>A 2 kg mass is placed at the handle and the balance point is at the centre of the spade. </li></ul><ul><li>What is the mass of the spade? </li></ul>2 kg
16. 17. Learning Intentions <ul><li>By the end of the lesson we will be able to… </li></ul><ul><li>Recall how the stability of an object depends on the… </li></ul><ul><ul><li>position of the centre of gravity </li></ul></ul><ul><ul><li>size of the object’s base </li></ul></ul>
17. 18. Position of Centre of Gravity Put these in order of STABILITY 1 2 3 4
18. 19. Stability <ul><li>An object will be in STABLE EQUILIBRIUM when it returns to its original position after given a small displacement </li></ul>
19. 20. Page 53 <ul><ul><li>Neutral Equilibrium </li></ul></ul><ul><ul><li>Unstable Equilibrium </li></ul></ul>
20. 21. Stability <ul><li>For an object to start rotating it needs to have an unbalanced moment acting on it- </li></ul>W W W Stable Pivot Pivot Rotates Clockwise Rotates Anti-Clockwise Stable Unstable
21. 22. <ul><li>An object becomes unstable when its Centre of Gravity falls outside of the object’s base. </li></ul><ul><li>Why? </li></ul><ul><li>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 </li></ul>
22. 23. The Traffic Cone <ul><li>A great example of a VERY stable object. </li></ul><ul><li>What features of the cone make it so stable? </li></ul><ul><li>Low Centre of Gravity </li></ul><ul><li>Wide base </li></ul>
23. 24. Balance and Sport
24. 25. Balance and Sport
25. 26. Balance and Sport
26. 27. The Double Decker Bus <ul><li>Each of the diagrams on the right represent a Double Decker bus with passengers inside. </li></ul><ul><li>Draw out and label the one which shows all passengers on </li></ul><ul><li>the lower level </li></ul><ul><li>the upper level </li></ul><ul><li>both levels </li></ul>
27. 28. Delete a word to complete the sentences <ul><li>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 </li></ul><ul><li>The bus will fall over when the CoG acts inside/outside of the bus’ base </li></ul><ul><li>A stable object has a low/high CoG and a narrow/wide base </li></ul>
28. 29. Question Time <ul><li>Page 56 </li></ul><ul><li>Questions 37-39 </li></ul>