Forces Structures Grade 78 Science Ms Budzinski Structures

Forces & Structures Grade 7/8 Science Ms. Budzinski

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

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

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

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

Types of Structures n Frame Structures n 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 n Parts are connected in a set arrangement

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

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

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

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

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

Types of Structures n Disadvantages of Shell structures: n A tiny weakness or imperfection 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 n Flat materials are difficult to form into the rounded shell shape

Types of Structures n Combination Structures Football helmets – n Warehouses – n Airplanes – n

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

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. n The more you can find the better. n Fill in the chart on your page as you identify them. n

The object in the room 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 The type of structure Shell Frame/shell Frame Solid Shell/solid Shell Frame/shell Frame Solid/frame Solid shell

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

Force & Center of Gravity n What is force? n n 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 n Engineers need to know the center of gravity in order to ensure that their structures are stable n

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

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

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

Words to Know Center of gravity n Stability n

Internal Forces n Compression n The n Ex: result of forces squeezing together

n Tension n The n Ex: pulling apart of a structure

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

n Torsion n Twisting n Ex: force

Activity n Activity Licorice and Forces

External Forces n n n 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… n n n Gravity Buoyancy Thrust Current Wind

What are the external forces acting on the following object?

Words to Know Compression n Tension n Shear n Torsion n Internal forces n External forces n

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

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

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

Structural Fatigue on a Bridge in Minnesota n http: //www. youtube. com/watch? v=z. Tz. Pn 4 xo. Zg&safe=active

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

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

Words to Know Structural fatigue n Structural failure n

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

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

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

Words to Know Load n Magnitude n Point of application n

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

Strong paper bridge n https: //www. youtube. com/watch? v=gt. UZc. I Drwoo

Shapes and Strength n There are many different shapes and they all have their strengths and weaknesses. n 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.

Archs n n 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.

Types of Arch's

Struts & Ties

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

Pillars n 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.

n Pillars resist compression force.

Gussets provide extra support to structures. n 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. n a bracket strengthening an angle of a structure. n

Words to Know Arches n Struts n Ties n Pillars n Gussets n

Activity n Straw House

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

Efficiency of Structures n n 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.

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

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

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

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