1. Last lesson Know what the turning effect of a force is
Know that the moment of a force depends on force and the distance from the pivot
Know the principal of moments

2. What happens when you try to open a door with one finger?
Where do you push?
Hi, my name is Mr Stick
hinges

3. Do you push near the hinges?
I can’t do it!

4. Do you push far from the hinges?
That’s easier!

5. Moments – Turning forces

6. The turning effect of a force depends on two things;
The size of the force
Obviously!

7. The turning effect of a force depends on two things;
The distance from the pivot (axis of rotation)
Not quite so obvious!
Axis of rotation

8. Turning effect of a force
The turning effect of a force is called the moment of the force
The moment is calculated by multiplying the force by the distance from the pivot

9. Turning effect of a force – moment of a force
Moment (Nm) = Force (N) x distance from pivot (m)
Note the unit is Nm, not N/m!

10. A simple example!
50 N
0.15 m
nut
spanner (wrench)

11. A simple example! Moment = Force x distance from pivot
Moment = 50 N x 0.15 m
Moment = 7.5 Nm
50 N
0.15 m
nut
spanner (wrench)

12. What do you do if the nut won’t move and you can’t push harder?!
spanner (wrench)

13. Get a longer spanner! Moment = Force x distance from pivot
Moment = 50 N x 0.25 m
Moment = 12.5 Nm
50 N
0.25 m
nut
spanner (wrench)

14. Do you think we’ll be safe in this power point?
More than one force
Take an uneven see-saw for an example
It’s not looking good!
Do you think we’ll be safe in this power point?
pivot

15. If the see-saw is balanced, what must be the weight of the dog on the left?
pivot
? N
110 N

16. The force on the left is trying to turn the see-saw anticlockwise about the pivot
1.2 m
2.2 m
pivot
? N
110 N

17. The force on the right is trying to turn the see-saw clockwise about the pivot
1.2 m
2.2 m
pivot
? N
110 N

18. If the see-saw balances, the turning effect anticlockwise must equal the turning effect clockwise
pivot
? N
110 N
Anticlockwise moment =
clockwise moment

19. Anticlockwise moment = clockwise moment. X 1. 2 = 110 x 2. 2. X 1
Anticlockwise moment = clockwise moment ? X 1.2 = 110 x 2.2 ? X 1.2 = 242 ? = 242/1.2 ? = N
1.2 m
2.2 m
pivot
? N
110 N
Anticlockwise moment =
clockwise moment

20. Questions
We then answered 2 sheets of questions on moments. Those who were absent last lesson (PE trip) will have to complete these for homework due FRIDAY.

21. The ruler trick!

22. With a uniform ruler the C of G is at the mid-point.
Centre of gravity
The centre of gravity of an object is the point where the objects weight seems to act.
With a uniform ruler the C of G is at the mid-point.

23. How do you find the centre of gravity of more complex shapes?
Complex shape man

24. Draw the sentence 1. 2. 3. 4. 5. 6. 7. 8.

25. Draw the following sentences
i. Place a compass or needle through any part of the shape.
ii. Make sure that the shape “hangs loose”.
iii. Hang a pumb line on the needle.
iv. After it has stopped moving, carefully draw a line where the plumb line is.
v. Place the needle in any other part of the card.
vi. Repeat steps ii to iv.
vii. Where the two drawn lines cross is where the centre of mass is.
viii. Physics is the most interesting science subject.

26. Stability

27. ¿Pueden ustedes copiar este por favor?
Stability
Stable objects have a wide base and a low centre of gravity.
¿Pueden ustedes copiar este por favor?

28. Stable!

29. Let’s try some questions

30. Forces and springs

31. Robert Hooke

32. Investigating forces and springs
You are going to investigate how much a spring stretches when a force is applied to it.
The amount a spring stretches is called its extension. This is the difference in length between the stretched spring and the length of the spring when it was unstretched.
(Remember we are looking at the force on the spring. A mass of 100g will have a weight (force of gravity pulling it down) of 1 Newton.
Add masses to your spring and carefully measure its extension. You can do this until the spring breaks! (but you must wear goggles and be careful during this investigation).
Your experimental report will include the following
A table of results
A graph of your results
A conclusion describing what you have discovered (think about this!)

33. Table of results Force (N) Length of spring (cm) Extension (cm) 3.4 1
3.4 1
5.4 2
7.4 4 3
9.4 6

34. Table of results
Force (N)
Length of spring (cm)
Extension (cm) 1 2 3

35. Graph
Extension (cm)
Force (N)

36. Go!