1. Grade 7/8 Science Ms. Budzinski
Forces & Structures
Grade 7/8 Science
Ms. Budzinski

2. Structures What is a structure?
something that has a definite size and shape, in order to serve a definite purpose or function
to perform its function, every part of the structure must resist forces that could damage its shape or size

3. Classifying Structures
Structures can be natural OR manufactured
Natural: not made by people; occur naturally in the environment
Manufactured: built by people; often modelled after natural structures

4. Types of Structures Solid Structures
Made of solid pieces of a strong material
Can be formed by piling up or forming similar materials into a particular shape
Natural examples: mountains, coral reefs
Manufactured examples: sand castles, dams, brick walls

5. Types of Structures Advantages of Solid Structures:
Held in place by their own weight
Losing a small part usually has little (if any) effect on the overall strength of the structure

6. Types of Structures Frame Structures
Made of a skeleton of strong materials, which is then filled or covered with other materials, supporting the overall structure
Most of the inside part of the structure is empty space
Parts are connected in a set arrangement

7. Types of Structures Natural examples: human skeleton, spider webs
Manufactured examples: wooden houses, staircases, bicycles
Frame structures are the most common building choice because they are easy to design and materials are inexpensive

8. Types of Structures Disadvantages of Frame structures:
Weakest points are at joints
Must design braces, anchors and joints so that the structure does not collapse

9. Types of Structures Shell Structures
keep their shape and support loads, without a frame or solid material inside
use an outer layer of material to provide their strength and rigidity
shape spreads forces throughout the whole structure, so every part of the structure supports only a small part of the load

10. Types of Structures Natural examples: turtle shell, beaver dams
Manufactured examples: igloos, egg cartons, pop cans
Balloons and parachutes are also shell structures, but they are FLEXIBLE

11. Types of Structures Advantages of Shell structures:
Completely empty (useful for storage)
Thin outside layer uses less material

12. Types of Structures Disadvantages of Shell structures:
A tiny weakness or imperfection can cause the whole structure to fail
When the shell is formed from hot or moist materials, uneven cooling can cause some parts to weaken other parts by pushing or pulling on nearby sections
Flat materials are difficult to form into the rounded shell shape

13. Types of Structures Combination Structures Football helmets –
Warehouses –
Airplanes –

14. Words to Know The Three different types of structures are…
Frame structure
Shell structure
Solid structure

15. What types of structures are in this room?
In in your table groups, look or walk around the room and try to identify at least 1 of each type of structure.
The more you can find the better.
Fill in the chart on your page as you identify them.

16. The object in the room
The type of structure
skin
Garbage can
Kassandra’s chair
Bohr model
Floor
Filing cabinet
Human skeleton
Brick wall
Heater
Mushu’s hut
Umbrella
Fan
Desk
Smartboard
Windows
Door
Cardboard box
Shell
Frame/shell
Frame
Solid
Shell/solid
Solid/frame
shell

17. Activity Complete “Structure ID Search”
There are a number of structure pictures placed around the MPR. Your job is to identify each one as one of the following…
Frame
Shell
Solid

18. Force & Center of Gravity
What is force?
The push or pull on an object
What is the “center of gravity?”
The specific point where the mass of the structure is evenly distributed, so that the force of gravity acts evenly on all the pieces
Engineers need to know the center of gravity in order to ensure that their structures are stable

19. Drawing Diagrams of Force
Force is represented by arrows called “vectors”
Longer arrow represents more force
Point indicates the direction of the force
Diagrams do not include all the forces that are acting - usually just show the most important or strong force

21. Activity Tower Contest!
In groups of 4, build the tallest tower possible with the materials that have been provided.

22. Activity & Exit Slip
Why is it difficult to stand up from a seated position with someone’s hand on your forehead?
You must use the words “balance,” “force” and “center of gravity” to explain your answer

23. Words to Know
Center of gravity
Stability

24. Internal Forces Compression The result of forces squeezing together
Ex:

25. Tension
The pulling apart of a structure
Ex:

26. Internal Forces
Shear
Result of forces acting in opposite directions of each other
Ex:

27. Torsion
Twisting force
Ex:

28. Activity
Activity Licorice and Forces

29. External Forces
There are many external forces that can act on an object.
These forces differ depending on the situation or area in which the object is in.
Some common external forces include…
Gravity
Buoyancy
Thrust
Current
Wind

30. What are the external forces acting on the following object?

31. What are the external forces acting on the following object?

32. What are the external forces acting on the following object?

33. Words to Know Compression Tension Shear Torsion Internal forces
External forces

34. Assignment External Forces & Boats DUE DATE: Thursday, December 17th.
You will be given 10 min to complete this now.

35. Structures & Loads
The effect of forces acting on a structure is called the load
Static
Caused by the force of gravity
Dynamic
Caused by forces other than gravity

36. Effects of Force on Structures
Structural Stress
Occurs when a combination of external and internal forces act on a structure at one time
Structural Fatigue
Occurs when a combination of external and internal forces weaken components of a structure
Structural Failure
Occurs when structure collapses

37. Structural Fatigue on a Bridge in Minnesota

38. The Leaning Tower of Pisa
The Leaning tower of Pisa is located in Italy and it is a major tourist attraction.

40. Assignment Read “Anti-Gravity in Pisa”
You are going to create a comic strip using the story Leaning tower of Pisa Story.
Leaning Tower of Pisa Facts

42. Words to Know
Structural fatigue
Structural failure

43. Application of Force
When force is applied to structures, there are 3 factors that affect the structure
1. Magnitude
measurement of the strength of a force
Shown by length of vector arrow

44. Application of Force 2. Direction
The “push” or “pull” of the force affects what will happen to the structure
Example:

45. Application of Force 3. Point of Application
Where the force is applied determines how the structure moves or changes

46. Words to Know
Load
Magnitude
Point of application

47. Activity: Spaghetti Bridge Test
Using the spaghetti given to you, follow the instructions on the activity page and complete the activity.
Make sure to record your data as you go.

48. Strong paper bridge

49. Shapes and Strength
There are many different shapes and they all have their strengths and weaknesses.
Architects need to keep the function of the object in mind when designing it so that they can decide what shapes are needed to be able to function properly.

50. Archs
An arch is a curved structure that spans a space and may or may not support weight above it.
There are many different types of arches.
After more than 2,000 years of architectural use, the arch continues to feature in many structures.
Its semicircular structure distributes compression through its entire form and diverts weight onto its two abutments, the components of the bridge that directly take on pressure.

51. Types of Arch's

52. Struts & Ties

53. Ties – Ties resist Tension.
Strut – Struts resist
Compression

54. Pillars
Pillars have been used for centuries to provide support for various structures. They are very common in ancient architecture as well as our current homes.

55. Pillars resist compression force.

56. Gussets Gussets provide extra support to structures.
a piece of material sewn into a garment to strengthen or enlarge a part of it, such as the collar of a shirt or the crotch of an undergarment.
a bracket strengthening an angle of a structure.

58. Words to Know
Arches
Struts
Ties
Pillars
Gussets

59. Activity
Straw House

60. Quiz!
You will have a quiz next class! Look over the information presented in the unit so far.

61. Efficiency of Structures
A structure needs the correct ratio between its load and its own mass in order to be stable
A structure that has a load greater than its own mass will not be very efficient in carrying the load.

62. A structure that is much heavier than its load is also not an efficient use of a structure

63. Efficiency of Structures
How can we compare how efficient structures are at carrying loads and resisting forces?
Efficiency = maximum mass mass of structure
The higher the number, the higher the efficiency.

64. Which of the following bridges would be the most efficient?
Bridge A: holds 100 kg loads and has a mass of 10 kg
Bridge B: holds 100 kg loads and has a mass of 50 kg
Bridge C: holds 100 kg loads and has a mass of 5 kg

65. Final Design Project Time.
The Unit test with be in 2 weeks.