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# Centre of Gravity and Stability

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GCSE Physics double award notes

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• can a fallen tree can stand of its own after the CG is put in its base due to cutting of its upper bark and made thinner than its base???

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• Metre rule each, there are three different edges that is possible to balance the rule on. Try to find out what they are and balance the rule on each of the edges. If an imaginary line was to be marked vertically along each of the axis that the ruler was balanced on, where would the exact point be that all three line crossed? This is the point where the ruler’s centre of Gravity is located.
• Mark on where the centre of the shape is for the three shapes shown.
• Attempt to find the centre of the triangles shown
• Try finding the centre of gravity of the tube Why has the CoG changed? Check out the bird balancing on its beak!
• Again draw the object and mark on their centre Explain how and why the CoG would change if the bottom plank was twice as dense as the other plank… roughly mark the new CoG
• Lake Tasman where the terminal merrain of the Tasman glacier is located in NZ, South Island close to Mount Cook in McKenzie country. The end of the glacier has large chunks of ice that randomly break away and float in the lake. This is ice is thousands of years old as it has slowly inched its way down the Tasman valley to finally end its journey in the lake.
• The water lapping against the iceberg melts the edges and also the Sun’s energy changes the shape of the mass. The iceberg will have to tilt or turn over slightly to re-balance itself and the underside of the iceberg becomes visible when this happens. The surface and colour of the underside is very different from the stained, sun-beaten exposed side.
• The smooth deep blue colour is partially transparent and it is possible to see the pieces of rock suspended in the iceberg which the glacier would have ripped from the sides of the valley as it rubbed up against the edges.
• Ans 5 kg
• Back of the book – order of stability, give a reason why!
• Buskers’ festival – they love to show off how great their stability is…
• Put the cones in order of stability (come back to why they are in that order in a few slides time!) Why does the drinking bird alternate between being stable and toppling over? Can you explain this in term of moments, the turning effect of a force? Where is the pivot, what is the force causing the turning? As soon as the ‘weight’ is concentrated more in the bird’s head he then becomes unstable and topples over. A turning effect of a force/moment is needed to cause the object to rotate. Why does the can roll up hill? The CoG acts vertically down, but if it acts outside the object’s base then it will cause the object to rotate in that direction, i.e. the can moves up hill due to the extra 100 g mass which changes where its CoG’s location is. The same is true for the bird. The bird’s CoG has in fact moved up and over. Once it acts outside the base of the bird, he tips over.
• Check out the weebles- also masters of stability, impossible to topple over!
• Back to the starting activity
• Board sports require stability and a low CoG. The CoG is continually shifting to direct the board and the rider. The back needs to be kept straight while the hips, knees and ankles provide all the turning. The head too needs to be kept still and in line with the core so that it doesn’t cause any unbalance. As soon as the CoG acts outside the area of the board, the rider is in danger of losing control.
• Rock climbing too relies on the body’s position and the potential turning effect of the body’s weight. Whenever a hand-hold is used as a pivot, the body’s CoG will need to be shifted closer to the hold before the climber’s fatigue cause them to let go. The body needs to be flexible to quickly position itself and reduce the size of the turning effect.
• Some of the World’s greatest footballers, fantastic dribblers with close control and nearly impossible to topple over due to their low CoG. Pocket rockets who are small yet unstoppable because they have such precise balance.
• ### 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>