1. IGCSE textbook Chapter 5, p. 42
Turning Forces
IGCSE textbook Chapter 5, p. 42

2. 1.25 know and use the relationship between the moment of a force and its distance from the pivot: moment = force × perpendicular distance from the pivot 1.26 recall that the weight of a body acts through its centre of gravity 1.27 know and use the principle of moments for a simple system of parallel forces acting in one plane 1.28 understand that the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam

3. Turning Forces
Who wins, the woman trying to open the door or the man trying to keep it closed?
Why?

4. Turning Effect
When a force is applied to an object with a fixed pivot point (or fulcrum) it can have a turning effect.
This turning effect is called a moment.
A lever is a simple machine which uses moments.
So how does it work?

5. Levers
The length of the lever seems to increase or reduce the ability of the applied force to turn things.
The same turning effect can be achieved by a bigger force nearer the pivot or a smaller force further from the pivot
Distance and force seem to compensate for each other

6. Archimedes: "Give me a place to stand, and I shall move the Earth with it"

7. Moment moment = force  perpendicular distance
from pivot to line of action
newton metres
(Nm)
force (N)
metres (m)

8. Spanners Tough nut Force on spanner causes turning effect
Moment = 10 x 0.1 = 1 Nm
Bigger spanner has a bigger distance from pivot .
Moment = 10 N x 0.2m = 2 N m
Much easier to turn nut for person.
To get the maximum moment you need to push at right angles to the spanner.
0.1m
10 Newtons
pivot
0.2m
10 Newtons

9. Force times which distance?
Size of force
times
perpendicular distance from pivot to line of action of the force

10. Force on the hammer, F = 50 N
Distance from pivot, d = 0.30 m
Moment = 50 N  0.30 m
= 15 Nm

11. Changing the force Force on the hammer = 70 N
Distance from pivot = 0.30 m
Moment = 70 N  0.30 m
= 21 Nm

12. Changing the distance Force on the hammer = 50 N
Distance from pivot = 0.20 m
Moment = 50 N  0.20 m
= 10 Nm

13. Principle of Moments
If a body is acted on by more than one turning force and remains in equilibrium (doesn’t turn), then:
Sum of clockwise moments
Sum of anticlockwise moments =
3 m ?
500 N
750 N

17. a) Who is lighter, Dawn or Jasmine?

18. b) Jasmine weighs 425 N WD  2.5 m = 425 N  2.0 m
Dawn
Jasmine
425 N
WD = ? N
2.5 m
2.0 m
b) Jasmine weighs 425 N
Now divide
both sides
by 2.5 m
WD  2.5 m = 425 N  2.0 m
WD = 425 N  2.0 m/2.5 m
= 340 N

19. c) John weighs 450 N 450 N  d1 = 425 N  2.0 m
Jasmine
425 N
450 N d1
2.0 m
c) John weighs 450 N
Now divide
both sides
by 450 N
450 N  d1 = 425 N  2.0 m
d1 = 425 N  2.0 m/450 N
= 1.9 m (to two s.f.)

21. Centre of Gravity
The centre of gravity of an object is the point at which all its weight seems to act.
To balance an object, it must be supported in line with its centre of gravity
why?
For a symmetrical object it will be where the lines of symmetry cross.
... G

22. Finding Centre of Gravity
Step 1
Suspend the object:
Centre of gravity lies somewhere along this line

23. Finding Centre of Gravity
Step 2
Suspend the object from a different point:
Centre of gravity lies where the lines cross

24. To Find the Centre of Gravity
Hang the shape from a pin and let it swing freely.
Use a ‘plumb line’ to draw a vertical line on the shape from the pin downwards.
Now repeat this procedure with the shape suspended from a different point to give another line.
The centre of gravity of the shape is where the two lines meet.

25. Pivot not at Centre of Gravity
If the pivot point is at the centre of gravity, the weight of the object has no moment.
If the pivot is offset from the C of G, the weight produces a turning effect

26. Click to find the centre of gravity.

28. Click to find the centre of gravity.

30. Click to find the centre of gravity.

32. Click to find the centre of gravity.

34. Click to find the centre of gravity.

36. Stability & Toppling
The position of the centre of gravity affects an object’s stability.
If an object is tilted and the line of force from the centre of gravity remains within the base, it will not topple over.
If it is tilted so far that the line of force from the centre of gravity moves outside the base, it will topple over.
Can you explain the design of this C18th ship’s decanter?

42. When will it topple over, when will fall back upright?
“Stable” means it will fall back to its upright position: the moment acting restores it as it was.
“Unstable” means the moment acting will move it further away from its original position.

43. What is done to make these more stable? Buses Tractors F1 cars
As wide a base as possible
As low a centre of gravity as possible.
Buses
Tractors
F1 cars
High chairs for baby
Skittles

44. Forces on a beam
If the boy and beam are not moving, the forces must be in equilibrium
The total downward force (his weight) is equal to the total upward force (from supports at each end)
How much support force is provided by each end?
What happens if he moves towards one end?

45. Forces on a beam L R
The support force provided by each end depends on the object’s position
Object position
Support Force L R
The two support forces always add up to the object’s weight
Left support
Right support

46. 1.25 know and use the relationship between the moment of a force and its distance from the pivot: moment = force × perpendicular distance from the pivot 1.26 recall that the weight of a body acts through its centre of gravity 1.27 know and use the principle of moments for a simple system of parallel forces acting in one plane 1.28 understand that the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam