Centre of Gravity The centre of gravity (centre of mass) of a body is an imaginary point where the whole …………….. of the body seems to act in any orientation. –The CG of a regular object is at its …………... –The CG of an ……………….. object is determined using a plumb line. If a body is hanging freely at rest, its centre of gravity is always vertically below the …………., thus the plumb line method works. It can only be used for ………….., irregular objects. pivot flat irregular weight centre

Stability Objective To understand what stability is and describe how the centre of gravity of an object affects it. 24 May 2015

Outcomes All must be able to carry out an experiment that shows how an objects shape affects its stability. Most should be able to explain how the centre of gravity of an object effects its stability. Some could be able to define the stability of an object and apply this definition to different situations.

v=U45lggf2OkM Why do you think that weebles wobble but they don’t fall down?

Stability Stability is a measure of the body’s ability to maintain its original position. What 2 things can change an object’s stability?

Stability Stability is a measure of the body’s ability to maintain its original position. a)The CG is lowered b)The area of the base is really wide

Stability Stability is a measure of the body’s ability to maintain its original position. Explain why the stability of an object is improved if: a)The CG is lowered b)The area of the base is really wide

Stability Stability is a measure of the body’s ability to maintain its original position. Explain why the stability of an object is improved if: a)The CG is lowered b)The area of the base is really wide –Lowering the CG (A lower CG will allow the line of action to act within the base area of an object) –Area of its base should be as wide as possible (allow line of action to act within base area)

Definition If the line of action of the weight of a body lies outside the base of the body there will be a resultant moment and the body will tend to topple.

Draw the diagrams and explain what will happen in each diagram (a), (b) and (c)

Weebles Wobble but they don’t fall down Design a high chair that will not fall down very easily.

Part Route 23 Y 226 TSC CANONMILLS Line of Action Centre of Mass

Line of Action Pivot

Line of Action Pivot

What happens to the centre of mass when all he passengers are on the top deck? How would this affect the stability? Part Route 23 Y 226 TSC CANONMILLS Line of Action Centre of Mass

v=U45lggf2OkM Why do weebles wobble but they don’t fall down? Draw a weeble and label it to explain why it doesn’t fall down.

High Chair design Design a high chair that will not fall down very easily.

High Chair design WWW Is the design practical Would it fall over easily What else is good EBI How could the design be improved?

Practical Time!! Stability Prac Set up the equipment as above. You have a selection of objects to try and topple. You will place the objects at the bottom of the board and measure the angle at which it just starts to topple.

Stability Practical Results Object (sketch if necessary) Angle (degrees) 123average Analysis: Explain which shape toppled first and which toppled last.

What have you learnt today? How could what you have learnt today be useful?