1. UNIT 5 STRUCTURES

2. 1. FORCES AND STRUCTURES

3. 1.1. FORCES
Gravity

4. Wind

5. Mechanical forces

6. Pressure of gases

7. Our muscles

8. Effects of heat

9. A force is anything that can deform a body or change its state of movement or rest.

10. 1.2. STRUCTURES

11. A structure prevents a body from breaking or becoming too deformed
A structure prevents a body from breaking or becoming too deformed. They may be natural structures or artificial (human-made) structures.
Natural structure
Artificial structure

12. The forces that act on a structure are called LOADS
Fix or permanent loads don´t vary over time
Variable loads are occasional and changeable

13. 2. STRESSES
A stress is an internal tension which happens to all bodies subjected to the applying of one or more forces on them

14. 2.1. TRACTION Forces try to STRETCH the body.
The body tends to become longer

15. 2.2.BENDING
Forces try to bend a body

16. 2.3. COMPRESSION
Forces try to compress a body

17. 2.4. TORSION
Forces try to twist a body

18. 2.5. SHEAR OR CUTTING OR
The forces act like two scissors blades

19. Page 93
Exercises 6, 7, 8, 11

20. 3.STRUCTURAL CONDITIONS

21. STABILITY: capacity of a structure to remain upright.
The centre of gravity of the structure must be centred over its base

22. RESISTANCE: capacity of a structure to bear tensions without breaking
It depends on the shape and the material

23. RIGIDITY: all objects become slightly deformed when a force is applied to them

24. Page 95
Exercises 12, 13, 14, 15

25. 4. TYPES OF ARTIFICIAL STRUCTURES

26. MASSIVE VAULTED LATTICE TRIANGULATED SUSPENDED
MASSIVE VAULTED LATTICE TRIANGULATED SUSPENDED PNEUMATIC ROLLED GEODESIC

27. 4.1.MASSIVE STRUCTURES
Carved on rock or built by stacking rock or stone

28. 4.2.VAULTED STRUCTURES
A vault is made from a number of arches placed side by side

29. 4.3. LATTICE STRUCTURES
These are used in modern blocks or flats

30. 4.4.TRIANGULATED STRUCTURES
They´re used to create large spans, and in verttical structures

31. 4.5. SUSPENDED STRUCTURES
This type of structure is hung from cables called rods

32. 4.6. PNEUMATIC STRUCTURES
This structures are light and can be dismantled

33. 4.7. GEODESIC STRUCTURES
They are three-dimensional triangulated structures

51. Page 99
Exercise 22
Page 100
Exercises 1, 2

52. UNIT 7 MECHANISMS
5. MECHANISMS

53. Mechanisms are devices that transmit and convert forces and motions from a driving force (input) to an output element. They enable us to carry out certain tasks with greater comfort and less effort.

54. We can clasify mechanisms in this way:

55. 5.1. LINEAR MOTION MECHANISMS
A. LEVERS
A lever is a rigid bar that it is used with a point of support or a fulcrum

56. LAW OF THE LEVER: F X f= R x r F:effort, R:load
TYPES OF LEVERS:

57. EXAMPLES TYPE 1 LEVERS
SCISSORS
SEESAW
PLIERS

58. EXAMPLES TYPE 2 LEVERS
LEVERS
HANDCART
BOTTLE OPENER
NUTCRACKER

59. EXAMPLES TYPE 3 LEVERS
EYEBROW TWEEZERS
FISHING POLE
STAPLER REMOVER

60. A fixed pulley is balanced when the effort, F, is equal to the load, R
B. FIXED PULLEYS
A fixed pulley is a wheel that rotates around an axle that is fixed to an immobile surface.
A fixed pulley is balanced when the effort, F, is equal to the load, R
F=R

61. C. Moveable pulleys
A moveable pulley has two pulleys, one is fixed while the other can move in a linear direction.
F = R / 2

62. Page 103
Exercise 2, 3, 5

63. 5.2. ROTARY MOTION MECHANISMS A. Friction drive
Made up of two or more wheels that are in contact. The motion of the first wheel makes the other wheel turn, transmitting the motion. The relation between the rotation velocities of the wheels depends on the sizes of the wheels

64. N1: velocity of the primary drive wheel
N2: velocity of the output wheel
D1: diametre of the primary drive wheel
D2: diametre of the output wheel

65. B.Pulleys with belts
These are two pulleys or wheels that are a certain distance apart.
They rotate simultaneously because of the belt.

66. C. GEAR MECHANISMS AND COGWHEELS
Cogwheels are sets of wheels that have teeth called cogs. These teeth mesh together so that one wheel moves the other.

67. N1 x Z1 = N2 x Z2 Z1 /Z2 = N2 /N1 N1 : velocity of the wheel 1
Z1 : number of teeth of the first wheel
Z2 : number of teeth of the second wheel

68. D. GEAR MECHANISMS WITH A CHAIN
N1 x Z1 = N2 x Z2 Z1 /Z2 = N2 /N1

69. Page 107
Exercises 6, 9, 10, 11, 12, 13

70. 5.3. MECHANISMS THAT TRANSFORM MOTION
A. FROM ROTARY INTO LINEAR MOTION
RACK AND PINION L= P X Z X N L: velocity of the movement of the rack
P: distance between two of the teeth, in milimetres.
Z: number of teeth in the pinion.
N: number of rotations per minute of the pinion

71. b). WINCH (AND CRANK HANDLE)
It consists of a crank handle attached to the axle of a cylindrical drum (or winch)

72. B. FROM ROTARY INTO RECIPROCATING MOTION
a) CRANK-LINK-SLIDER
As the wheel rotates, the crank transmits the rotary motion to the link, which moves the slide with a reciprocating motion

73. Page 111
Exercise 14, 18
Page 112
Exercise 1, 4

74. Two boys are sitting on a seesaw
Two boys are sitting on a seesaw. The first boy weights 42 kg and is 0,70 m from the fulcrum. The second boy is 1,10 m from the same point. How much does this second boy weight?

75. What is the velocity of rotation of the driven wheel in a cogwheels mechanism if the drive wheel is moving at 20 rpm? The driven wheel has 40 teeth and the drive wheel has 25.