1. AQA GCSE Physics 3-1a Turning Forces Moments & Stability
GCSE Physics pages 214 to 233
April 10th 2010

2. AQA GCSE Specification
MOMENTS & STABILITY
13.1 How do forces have a turning effect?
Using skills, knowledge and understanding of how science works:
• to describe how to find the centre of mass of a thin sheet of a material
• to calculate the size of a force, or its distance from an axis of rotation, acting on a body that is balanced
• to analyse the stability of bodies by considering their tendency to topple.
Skills, knowledge and understanding of how science works set in the context of:
• The turning effect of a force is called the moment.
• The size of the moment is given by the equation:
moment = force × perpendicular distance from the
line of action of the force to the axis of rotation
• The centre of mass of a body is that point at which the mass of the body may be thought to be concentrated.
• If suspended, a body will come to rest with its centre of mass directly below the point of suspension.
• The centre of mass of a symmetrical body is along the axis of symmetry.
• If a body is not turning, the total clockwise moment must be exactly balanced by the total anticlockwise moment about any axis.
• Recognise the factors that affect the stability of a body.
• If the line of action of the weight of a body lies outside the base of the body there will be a resultant moment and the body will tend to topple.

3. The turning effect of a force
Also known as the moment of a force.
The moment of a force about any point is defined as:
force x perpendicular distance from the turning point to the line of action of the force
moment = F x d
Unit: newton-metre (Nm)
Moments can be either CLOCKWISE or ANTICLOCKWISE
Force F exerting an ANTICLOCKWISE moment through the spanner on the nut

4. Question
Calculate the moment exerted with the claw hammer if the person exerts a force of 80N and distance d equals 25cm.
moment = F x d
= 80N x 25cm
= 80N x 0.25m
= 20 Nm CLOCKWISE

5. Complete: Answers Force (N) Distance Moment (Nm) 40 3 m 120 200 5 m
1000 50
4 m
3000
20 cm
600
120
5 m 50
600

6. Question Choose appropriate words to fill in the gaps below:
The ‘moment of a force’ is another name for the ‘________ effect of force’.
The moment of a force is equal to the ________ multiplied by the perpendicular _________ between the line of ________ of the force and the turning point.
Turning effect is measured in _________ metres.
__________ can be either clockwise or anticlockwise.
turning
force
distance
action
newton
moments
WORD SELECTION:
moments
force
distance
newton
turning
action

7. Moments Notes questions from pages 214 & 215
What is meant by the ‘moment’ of a force?
Copy Figure 2 on page 214 and explain how a lever can used to lift a heavy load.
Copy and answer questions (a) and (b) on page 214.
Copy the equation at the top of page 215 along with the units used.
Copy and answer questions (c) and (d) on page 215.
Copy the ‘Key points’ table on page 215.
Answer the summary questions on page 215.

8. Moments ANSWERS In text questions:
Either the spanner bends or it deforms the nut.
A long one.
The moment increases.
21 Nm
Summary questions:
(a) Anticlockwise
(b) (i) Increased
(ii) Decreased
(a) (i) The moment of the force you apply is greater so more force is applied to the nail to remove it.
(ii) The moment needed to turn a rusty hinge is greater so more force is needed compared with an oiled hinge.
(b) 72 N

9. Centre of mass
The centre of mass of a body is that point at which the mass of the body may be thought to be concentrated.
The centre of mass of a symmetrical body is along the axis of symmetry.
centres of mass of regular shapes

10. If suspended, a body will come to rest with its centre of mass directly below the point of suspension.

11. Finding the centre of mass of a card
Pierce the card in at least two places.
Suspend the card from one of these holes.
Hang a plumbline from the point of suspension.
Using the plumbline as a reference draw a vertical line on the card.
Repeat for the other hole(s).
The centre of mass is where the lines cross on the card.

12. Centre of mass Notes questions from pages 216 & 217
What is meant by the ‘centre of mass’ of an object?
Copy and answer question (a) on page 216.
Draw both diagrams of Figure 3 on page 217.
Describe an experiment to find the centre of mass of an irregular shaped object. You should include a diagram like that of Figure 4 on page 217 as part of your answer.
Copy the ‘Key points’ table on page 217.
Answer the summary questions on page 217.

13. Centre of mass ANSWERS In text questions:
If the ruler is uniform, the centre of mass is at the centre.
Summary questions:
(a) and (c) At the point where the diagonals meet.
(b) At its centre.
2. The weight of the child has a turning effect about the top of the swing when the child’s centre of mass is not directly below the top. The turning effect makes the child return to the centre.

14. The principle of moments
When an object is not turning (e.g. balanced):
The total clockwise moment equals the total clockwise moment
If the ruler above is balanced:
clockwise moment = anticlockwise moment
W2 x d2 = W1 x d1
Lever - Fendt

15. Question 1
On a see-saw Mary, weight 600N balances John, weight 200N when she sits 1.5m away from the pivot. How far from the pivot is John?
Applying the principle of moments:
Mary’s weight x distance = John’s weight x distance
600N x 1.5m = 200N x distance
900 = 200 x distance
900 ÷ = distance
John is 4.5m from the pivot
Lever - Fendt

16. Question 2 Applying the principle of moments:
Calculate the weight of the beam, W0 if it is balanced when:
W1 = 6N;
d1 = 12 cm;
d0 = 36 cm.
Applying the principle of moments:
W1 x d1 = W0 x d0
6N x 12 cm = W0 x 36 cm
W0 = 72 / 36
W0 the weight of the beam = 2N
Lever - Fendt

17. Complete for a balanced ruler: W1 d1 W2 d2
20 cm
10 N
10 cm
4 N
15 cm
6 N
12 cm
2 N
36 cm
8 N
25 cm
100 cm
10 cm
6 N
12 cm
8 N
Lever - Fendt

18. Moments in balance Notes questions from pages 218 & 219
Copy Figures 1 and 2 on page 218 and then copy and answer question (a) on page 218.
What is the Principle of Moments?
Copy and answer questions (b), (c) and (d) on pages 218 and 219.
Explain how a wheelbarrow allows the transportation of heavy objects. Copy the top diagram of Figure 4 on page 219 as part of your answer.
Copy the ‘Key points’ table on page 219.
Answer the summary questions on page 219.
Lever - Fendt

19. Moments in balance ANSWERS
In text questions:
Her moment about the pivot must equal the moment of the boy about the pivot. The moment is her weight x the distance from the pivot. Because she is heavier, she needs to be nearer the pivot than the boy to give an equal moment to the boy’s moment.
2.5 N
1.2 N
The effort acts further from the pivot than the load does. So a smaller effort gives an equal and opposite moment to a larger load.
Summary questions:
(a) Jasmin.
(b) 340 N
(c) 1.89 m
2. (a) 1.5 N
(b) 1.5 N

20. Stability
stable
balanced
unstable - toppling
A body is stable as long as the line of action of its weight lies inside the base of the body.
If this is not the case there will be a resultant moment and the body will tend to topple.

21. Question What factors make a modern racing car as stable as possible?
1. A wide wheel base.
2. A low centre of mass.

22. Question Choose appropriate words to fill in the gaps below:
The turning effect of a ________ is also called the ________ of a force. Moment is measured in _________ metres.
If a body is balanced the total clockwise moment is _______ to the total ____________ moment.
A body will be _______ if the line of action of its weight lies inside the _______ of the body. A tractor has a large wheel base to reduce the possibility of it __________.
force
moment
newton
equal
anticlockwise
stable
base
toppling
WORD SELECTION:
toppling
anticlockwise
newton
moment
equal
stable
force
base

23. Stability Notes questions from pages 220 & 221
Copy Figure 1 on page 220 and explain the process by which the brick becomes unstable as it is tilted.
Copy and answer questions (a) and (b) on page 220.
Copy Figure 2 on page 220 and explain why the wheelbase of a tractor should be as wide as possible.
Copy and answer questions (c), (d) and (e) on pages 220 and 221.
Copy the ‘Key points’ table on page 221.
Answer the summary questions on page 221.

24. Stability ANSWERS In text questions:
Its weight acts through the point of contact so it has no turning effect.
People open an upper drawer that produces a turning force that topples the filing cabinet.
So its centre of mass is as low as possible, which makes it more stable.
Less stable.
To provide a wider base if the child loses balance.
Summary questions:
1. (a) A table, a low-loader vehicle, a traffic cone, etc.
(b) A supermarket trolley, a tall electric kettle, etc.
(a) A chair would topple over if the baby in the chair leans too far sideways.
(b) The lower the centre of mass, the harder it is to topple it over.

25. Moments & Stability Simulations
See-Saw - Explore Science
See-saw forces - uses g - NTNU
Lever - Fendt
Torque - includes affect of angle - netfirms
Leaning Ladder - NTNU
BBC KS3 Bitesize Revision:
Moments - includes formula triangle applet
Centre of mass - Explore Science
Stability of a block - NTNU
Blocks and centre of gravity - NTNU
Why it is easier to hold a rod at its centre of gravity - NTNU

26. Turning issues Notes questions from pages 230 & 231
No questions.

27. How Science Works ANSWERS
Force, mass or moment.
The distance the tip of the board moves down.
Range based on diver masses and/or the position from where the diver will take off.
At least five.
Probably millimetres
Depends on chosen independent variable.
For example with force – controls would be position on board, width of board etc.
(g) Table should be clear and contain headings for the independent and dependent variables along with their units. Space should also be provided for repeated measurements and average calculations.