Centre of Gravity, Moments &
Levers
Levers
A lever is a simple machine that involves moving a load
around a fulcrum (pivot) using a force.
Examples
1.The handle of a door
2.A wheelbarrow
3.A revolving door
How a lever works
There are 3 terms
associated with
levers
• Effort
• Load
• Fulcrum
Moment
A Moment is a force that would
tend to make a lever turn.
A moment equals force times
perpendicular distance from the
fulcrum.
M=Fxd
M=Fxd
Moment = (40)(0.2)
= 8 Nm
A wrench 30 cm long is used by a mechanic to
turn a nut. If the force he exerts on the end is 5
Newtons, calculate the moment of the force.
The Law of the Lever
If a lever is balanced then the sum of
clockwise moments equals the sum
of anti-clockwise moments.
Exp:To investigate the turning effect of forces on a
lever
Method:
1. Suspend a metre stick from a
stand at its centre of gravity.
2. Arrange the weights at each
end until the stick balances.
3. Calculate the turning effect
(weight x distance from fulcrum)
on each side.
Result: When the metre stick is balanced the turning effect on the right hand
side is equal to the turning effect on the left hand side.
Conclusion: When the stick is balanced the total of the turning effects on one
side is equal to the total of the turning effects on the other
side. This is the law of the lever.
Clockwise Moments = Anti-clockwise Moments
M=Fxd
or
F1 x d1 = F2 x d2
?
39
Find the missing force?
Clockwise Moments = Anti-clockwise Moments
F(39) + 8(12) = 13(7) + 6(18) + 2(46)
F(39) + 96 = 91+ 108+ 92
F(39) = 195
F=5
•Edward weighs 500 N and is sitting at one end
of a see-saw which is 4 m long and balanced in
the middle.
•John is 2000 N. Where should he sit in order to
balance the see-saw?
The centre of gravity of an object is the
point through which the weight of the
object appears to act.
Exp:To find the centre of gravity of a thin lamina.
Method:
1.Hang the card as shown.
2.Draw a line under the plumbline.
3.Reposition and repeat.
Result: Where all the lines meet is the centre of gravity.
Conclusion: The centre of
gravity of the card is the
centre and it should balance
on the base of a pencil.
Stability
A body is more stable if:
(a) its centre of gravity is
close to the ground
(b) it has a wide support
base.
In physics the term equilibrium is used when
talking about stability.
Equilibrium
If something is in equilibrium it is balanced
A body can have 3 different states of
equilibrium
A.Stable
C.Neutral
B.Unstable
• Stable Equilibrium
– A body is in stable equilibrium if it returns to
its equilibrium position after it has been
displaced slightly.
• Unstable Equilibrium
– A body is in unstable equilibrium if it does not
return to its equilibrium position after it has
been displaced slightly.
• A body is in neutral equilibrium if it stays in the
displaced position after if has been displaced
slightly.
Equilibrium?