Centre of Gravity and Stability

Centre of Gravity and Stability of an object GCSE Physics

Learning Intentions By the end of the lesson we will be able to…   

Understand the term Centre of Mass/Gravity (CoG) Mass/Gravity  (CoG) Recognise the factors that affect an object’s CoG Investigate how to find the CoG of an irregular object.

Centre of Gravity 

This is the point at which the whole weight of the body can be taken as acting through

or 

the point from which the object will balance

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

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

For non-uniform shapes 

When more than one material makes up an object the centre of gravity will be affected.

External CoG 

For some objects the CoG is not located on the actual object Stool

Ring

2 uniform perpendicular planks

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

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) Recognise the factors that affect an object’s CoG Investigate how to find the CoG of an irregular object.

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

CoG of an irregular shape 

Find the CoG of an irregular shaped lamina Pin held by clamp a c b

b

c a

c a

hole plumb line

b

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

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?

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

Position of Centre of Gravity

Stability An object will be in STABLE EQUILIBRIUM when it returns to its original position after given a small displacement 

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 Anti-Clockwise

Pivot

W Stable

Pivot

W Stable

W Unstable

• 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

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

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 and label the one which shows all passengers on - the lower level - the upper level - both levels 

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

Question Time  

Page 56 Questions 37-39