centre of gravity and stability
TRANSCRIPT
Centre of Gravity and Stability of an object
GCSE Physics
Learning IntentionsBy 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.
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 -
RingStool
2 uniform perpendicular planks
Why does the object balance? Linked to Principle of Moments
When the object is suspended from a point it will come to rest when the clockwise moment is equal to the anti clockwise moment
The Humble metre rule
Anti- Clockwise Moment = Clockwise Moment
Icebergs!
As these guys melt their CoG changes and they roll over!
Learning IntentionsBy 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
a
bc a bc
b
a
c
plumb linehole
Pin held by clamp
Balancing Act! Now, try to balance the object at that
point…
Question- explain in terms of moments why the object can be balanced at this point?
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.
Example 1 m long spade balances in a string loop when
the loop is 30 cm from the end of the blade.
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?
2 kg
Learning IntentionsBy 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 it-
W W WStable
Pivot Pivot
Rotates Clockwise Rotates Anti-Clockwise
Stable Unstable
• An object becomes unstable when its Centre of Gravity falls outside of the object’s base.
Why?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