This document discusses equilibrium and centre of mass. It defines moment or torque as the turning effect of a force. The moment of a force depends on both the size of the force and the distance from the pivot. An object is in equilibrium when the sum of clockwise moments equals the sum of anticlockwise moments. For an object to be in equilibrium, the forces must balance and the principle of moments must apply. The centre of mass is the point where the entire weight of an object can be considered to act. Several diagrams show how to determine the centre of mass for different objects.

1. EQUILIBRIUM AND CENTRE OF MASS

2.  The turning effect of a force is called its
moment or torque
Turning Effect of Forces 2

3.  The moment of a force is bigger if the force is
bigger.
 The moment of a force is bigger if it acts
further from the pivot.
 The moment of force is greatest if it acts at
90ᵒ to the object it acts on
Turning Effect of Forces 3

4. Turning Effect of Forces 4
Moment of a Force
=
Force × Perpendicular distance from the line
of action of the force to the pivot
= F × d

5.  Moment can be clockwise or anticlockwise.
 When an object is in equilibrium, the sum of
clockwise moments about any point is equal to the
sum of anticlockwise moments about the same point.
Turning Effect of Forces 5

6.  If an object is in equilibrium, the forces on it
must balance as well as their turning effect.
 So:
◦ The sum of the forces in one direction must equal
to the sum of the forces in the opposite direction.
◦ The principle of moments must apply.
Turning Effect of Forces 6

7. 10. The diagram shows a boy of weight 500 N
sitting on a see-saw. He sits 2.0 m from the
pivot.
1. What is the force F needed to balance the
see-saw?
Turning Effect of Forces 7
A 250 N B 750 N C 1000 N D 3000 N
A

10. GCSE Physics

11. 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.

12.  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

13.  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

14.  The centre of gravity (and geometrical centre)
is found by drawing a line from the corner to
the opposite line’s midpoint

15.  For some objects the CoG is not located on
the actual object -
Ring
Stool
2 uniform
perpendicular
planks

16.  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

17.  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

18.  Find the CoG of an irregular shaped lamina
a
b
c
a b
c
b
a
c
plumb line
hole
Pin held by
clamp

19. 1. A piece of card has its centre of mass at M.
2. Which diagram shows how it hangs when
suspended by a thread?
Turning Effect of Forces 19
A

20. 3. A piece of uniform card is suspended freely
from a horizontal pin.
4. At which of the points shown is its centre of
gravity?
Turning Effect of Forces 20
C

21. 4. A tractor is being used on rough ground.
5. What is the safest position for its centre of
mass?
Turning Effect of Forces 21
D

22. 5. An empty glass is placed on a join between
two tables as shown. The glass remains
stable.
6. Which point is the centre of mass of the
glass?
Turning Effect of Forces 22
C

23. 6. A light aircraft stands at rest on the ground.
It stands on three wheels, one at the front
and two further back.
7. Which point could be its centre of mass?
Turning Effect of Forces 23
B

24. 8. A student uses a stand and clamp to hold a
flask of liquid.
9. Which diagram shows the most stable
arrangement?
Turning Effect of Forces 24
B

25. 9. Some containers are made from thin glass.
10. Which empty container is the most stable?
Turning Effect of Forces 25
A

26. 10. The diagrams show the cross-sections of
different glasses.
11. Which one is the least stable when filled with
a liquid?
Turning Effect of Forces 26
B

27. 11. The diagram shows four models of buses placed
on different ramps.
How many of these models will fall over?
A. 1
B. 2
C. 3
D. 4
Turning Effect of Forces 27

28. 12. The diagram shows four objects standing on
a flat surface.
13. The centre of mass of each object is marked
M.
14. Which object will fall over?
Turning Effect of Forces 28
C

29. 14. The stability of a bus is tested by tilting it on
a ramp. The diagram shows a bus that is just
about to topple over.
15. Where is the centre of mass of the bus?
Turning Effect of Forces 29
C

30. 15. Passengers are not allowed to stand on the
upper deck of double-decker buses.
Turning Effect of Forces 30

31. 1. Why is this?
A. They would cause the bus to become unstable.
B. They would cause the bus to slow down.
C. They would increase the kinetic energy of the bus.
D. They would lower the centre of mass of the bus.
Turning Effect of Forces 31

32. ENERGY,
WORK
AND
POWER