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Structures and Forces
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Structures and Forces
What is a Structure?
Every object that provides support is a structure.
Exploring
What amazing STRUCTURES can you think of in our world?
Brainstorm at your table!!
Think of the word “Structure”
Write down 10 words/phrases that come to mind.
Think of the word “Force”
Write down 10 words/phrases that come to mind.
1.1 - Classifying Structural Forms
- Every object that provides support is a structure.
Structural Strength
They vary in structural strength - their capacity to hold itself, as well as any weight added.
ex) person holding weights
ex) desk holding books
Structural Stability
Structural stability is the structure's ability to maintain its position even when it is being acted on by a force.
Ex) Tower and Wind
Ex) Water rushing over a Dam
Types of Structures:
1) Solid Structure: A structure that is one piece. There is no space in the center.
Make a fist! This is a solid structure.
Apple, Hockey puck, Rock, Cement block
Why is it strong?
It has little or no inside space.
It relies on its own mass to resist the forces that act on it.
Usually strongest
Heavy
Hard to move
Types of Structures:
2) Frame Structure: A structure that is made from many pieces fixed together.
Make a tent with your hands! This is a Frame Structure.
Human skeleton, House frame, Bike, Wheelchair
Why is it strong?
From the way the components are joined.
Not one component is as strong as all of it combined.
Lighter than solid - less materials.
Types of Structures:
3) Shell Structure: A structure that has a solid outer component and a hollow middle.
Cup your hand. This is a Shell Structure.
Human skull, Soccer ball, Egg, building
Why is it strong?
There is a lot of surface area, causing the weight of what it is holding to be well distributed.
Lighter than solid structure
Usually stronger than frame structures.
1.2 The Function of Structures
Once you’re done them all, read pages 269 – 275.
Now re-read the statements and state whether truly are true or false.
Include what page number you found that answer on.
Read the statement and write whether or not you think it is true or false.
Correct any false statements by making them true
1.2 The Function of Structures
Function - the use or purpose of a structure.
ex: Function:
-Thermos?
-Chair?
-Car?
1.2 The Function of Structures
Many structures are designed to serve more than one function. An
airplane, for example, provides both movement and shelter. So does
a train. Sometimes these two types of structures are built to move
and shelter cargo. Other times they are built to move and shelter people.
When a designer knows what all the functions of a planned
structure are to be, he or she will be better able to design a structure
that will be used.
https://www.facebook.com/watch/?v=660996804308351
https://www.facebook.com/watch/?v=2194453820875344
https://www.smashinglists.com/most-beautiful-buildings-world/
Other Factors when Building:
- Design
--> can vary
- Aesthetics - pleasing appearance or effect that an object has on a design.
--> decoration
--> important?
Common Functions/Different Designs
1.3 Human Built Structures Around the World
Design of houses differ around the world due to:
-climate
-precipitation
-culture
Roof Types for Different Climates
The steepness of your roof – known as the roof pitch or slope – is an important decision to make for your home. Knowing the difference between a steep pitch, a low pitch or somewhere in between is something you want to be comfortably informed about when considering roofing changes.
2.0 External and Internal Forces Act on Structures
Section 2.1 Measuring Forces
What is a force??
A push or pull that tends to cause an object to change its movement or shape.
What Can Forces Do?
1) Move a stationary object.
2) Stop a moving object.
3) Change the direction of an object in motion.
4) Change the speed of an object.
5) Change the shape of an object.
Magnitude, Direction and Location
The actual effect of a force on a structure depends on three things…
1) Magnitude (size of the force)
2) Direction of the force
3) Location of the force.
Magnitude
With the person beside you:
Partner 1 hold your hand straight out with palm facing up.
Partner 2, push gently down on partner 1’s hand.
Partner 1, try and resist the pressure (try and hold your arm where it is.
Partner 2, now try pushing down a little harder
Switch roles…
Magnitude
Does magnitude (size of force) have an effect on your ability to resist it?
Was the softer pressure or harder pressure easier to resist?
Magnitude
The bigger a force’s magnitude, the stronger it is and the more effect it will have on a structure.
More force = more effect on a structure
Direction
With your partner:
Partner 1 hold arm straight out, palm up
Partner 2, apply gentle but firm pressure in different directions (down, up (from the other side of the hand) left, right)
Partner 1, try to resist the force.
Switch roles…
Direction
Does the direction of the force have an effect on your ability to resist it?
Which directions did you find most difficult?
Direction
The direction in which a force acts on a structure also determines what effect that force will have.
What is easier…
Pushing a wheel barrow uphill, or down hill?
Why?
Location
With your partner:
Partner 1, hold arm straight out, palm up
Partner 2, apply pressure downwards on the finger tips.
Partner 1, resist it.
Partner 2, now apply pressure on the forearm
Partner 2, then apply pressure near the elbow.
Switch roles…
Location
Did the location have an effect on your ability to resist the force?
Which location was the most difficult to resist?
Why do you think this is?
Location
If I am trying to move a filing cabinet, and I push it at the top of it, what will happen?
Where might be a better location to push (apply a force) on the filing cabinet?
Location
The location on a structure where a force is applied affects the outcome.
Applying a force at a point high up on an object that you
are trying to slide along the floor may cause it to topple over.
http://www.angrybirdsgames.com/games/angry-birds
The Newton
- Named after Sir Isaac Newton
- In 1687, he was the first person to describe the “law of gravity”.
What is the law of gravity??
- Every object in the universe is attracted to each other, and it depends on the mass (size) how strong the attraction is.
Ex: The earth is so big- everything is pulled (attracted) to the center of it.
Meaning everything is pulled down on by the earth.
The Newton
Newton (N) is the standard unit for measuring force.
1 Newton holds up 100g of mass
100g = 1 Newton (N)
1kg book = about 10 N.
Example: if a box has a mass of 3 kg, how much force (N) is required to hold it?
1 kg = 10 N so 3 kg = 30 N
2.2 External Forces Acting on Structures
External Force: forces applied to the structure from the outside of the structure AND the weight of the structure itself.
Examples:
- Gravity - pushes down on us. To stand, all structures on Earth must be able to resist the force of gravity.
- Wind – the force you feel acting on your body is an external force.
How are we able to withstand these external forces??
We must consider:
- Center of gravity
- Symmetry
- Load
Center of Gravity
It is the imaginary point in an object where the force of gravity acts on the structure.
When a structure is supported on its center of gravity, it will remain balanced despite external forces.
Example:
Grab your pen or pencil. Where do you have to place your finger so the pen/pencil will not fall? That is its center of gravity.
Imagine someone wanted to push you over... How would you stand to WITHSTAND their force?
Center of Gravity
How can we increase or improve our balance or stability in sports?
In sports, is it important to know where your center of gravity is?
Hint: When a structure is supported on its center of gravity it will remain balanced.
Center of Gravity
Best way to increase balance and stability is to increase the width of the base (relative to its height).
Predict which table is more stable.
Symmetry
is the balanced arrangement of mass around the center of gravity.
Important factor to maintain balance and C.O.G.
Mass is the amount of matter in an object.
The more mass an object has, the more force of gravity acts on it.
Symmetry
The force of gravity is equal on either side of the center of gravity.
When you balanced a ruler on your thumb, on either side of your thumb the mass will be equal.
What would happen to the ruler if you moved your finger one cm to the right?
Why?
Load
Load: is an external force on a structure.
Types of Load we must consider for a structure...
Two types of loads:
1)Static (Dead) Load
2) Dynamic (Live) Load
Static Load: weight of the structure and non-moving load it supports.
Static means NOT moving
Parts of the structure are also considered a static load because they are an external force (gravity is acting on them)
Ex. Wood, nails and screws in a bookcase
2) Dynamic Load: external force that moves or changes with time. Can change very quickly.
Ex. Wind, earthquakes, snow, people, cars, etc.
Supporting the Load
Different structures are designed to withstand different loads and forces
For Bridges engineers think about:
1) what the bridge is crossing (water or land)
2) what kinds of loads the bridge will be supporting
Measuring a Structure’s Load Performance
Ex. Performance requirements of waterslide:
Must be able to support weight of 200 people at one time plus the weight of water.
Structure must be able to withstand high winds and heavy snow in winter without becoming unstable.
How well a structure withstands the load it was designed to carry is important for safety, cost and efficiency reasons.
Standards or Conditions that structures must meet are called Performance Requirements.
2.3 - Internal Forces Within a Structure
- You crumple a piece of cellophane into a ball and put it on the table. Slowly, the cellophane opens up again. Why?
- You’ve used an elastic band to hold a collection of cards together. Suddenly, the elastic snaps. Why?
- To remove excess water from a sponge, you squeeze the sponge. To remove excess water from a towel, you could also squeeze the towel, but wringing it works better. Can you explain why?
Internal Force
An internal force is a force that one part of a structure exerts on other parts of the same structure.
Press the palms of your hands together firmly. Can you feel the internal force your muscles are exerting?
Compression, Tension, Shear
Internal forces can be classified by the direction in which they act within an object. Three internal forces are compression, tension, and shear.
Compression
- A force that acts to squeeze an object or push parts within an object together.
- Structures with parts that must resist compression include the human body, chairs, shelves, and architectural columns.
Tension
- A force that acts to stretch and pull apart something. It can cause the lengthening or even snapping of a component.
- Structures with parts that must resist tension are ski lifts and running shoes.
Shear
- A force that acts to push parts that are in contact with each other in opposite directions.
- Structures with parts that must resist shear include doors, airplanes, and scissors.
COMPLEMENTARY FORCES
- When different kinds of internal forces act on a structure at the same time, they are called complementary forces.
- Bending is an example of complementary forces at work.
http://www.pbs.org/wgbh/buildingbig/lab/forces.html
#1-3, page 299
2.4 -
Designing Structures to Resist
Forces and Maintain Stability
STRUCTURAL COMPONENTS
Arches
- An arch is a common shape in structures such as bridges.
- The arch can support a large load because the force of the load is carried down through the arch to the foundation.
- This spreads out the load.
STRUCTURAL COMPONENTS
beams
- Beams are common components in a wide range of structures.
- A simple beam is a flat structure that is supported at each end.
- If too much weight is put on a beam in the middle, it will bend in a U-shape and may even break.
- Changing the shape of a beam, however, can increase its strength.
STRUCTURAL COMPONENTS
I-beam
- The shape of an I-beam gives it strength.
- I-beams have less mass than solid beams.
Girder
- Girders, or box beams, are long beams in the shape of hollow rectangular prisms.
STRUCTURAL COMPONENTS
Truss
- A truss is a framework of beams joined together.
- Trusses are usually in the form of interlocking triangles.
cantilever
- A cantilever is a beam that is supported only at one end.
STRUCTURAL COMPONENTS
Column
- A column is a solid structure that can stand by itself.
- Columns can be used to support beams.
STRUCTURAL STRESS, FATIGUE, OR FAILURE
- Structural Stress: The weakening of a structure due to external and internal forces.
- Structural Fatigue: a permanent change in a structure caused by internal forces such as compression, tension, and shear. Cracks, for example, might start appearing in the material.
- Structural Failure: occurs when a structure can no longer stand up to the forces acting on it.
3.0 Structural Strength and Stability Depend on the Properties of Different Materials and How They Are Joined Together
3.1 MATERIALS AND THEIR PROPERTIES
Properties
Brittleness: how easily the material breaks.
Ductility: how easily the material can be made into a thin wire, and bent.
Strength: how much force can be applied to the structure/ object before it changes it structural shape.
3.1 MATERIALS AND THEIR PROPERTIES
Properties
Deformation: the change in shape of an object due the forces acting on it.
Compressibility: how much the material can be squished.
Tensile strength: how much the material can be stretched.
3.1 MATERIALS AND THEIR PROPERTIES
Properties
Plasticity: how easily the material can be shaped.
Resistance to heat/ water
Flexibility: how bendable the material is. How much force can be applied to it, and still allow it to return to its original shape?
3.1 MATERIALS AND THEIR PROPERTIES
Properties
Texture: the feel of the material. Is it bumpy? silky? soft? Rough?
Colour
Hardness – how tough is it, how easily is it scratched?
3.1 MATERIALS AND THEIR PROPERTIES
Other Considerations
Aesthetics
Consumer Demand
Availability
Cost
Environment
Waste (Recycled or Reused?)
3.2 JOINING STRUCTURAL COMPONENTS
Joints – the points at which parts are joined
Friction – force that results when the one surface moves against another.
Joiners
Ex’s: Nails, Screws, Rivets, Tacks, Staples – hold components together
Interlocking Pieces = increase the amount of surface area so there is more friction.
Mass and Friction
Bonding Joints
Joined together by a common material (a bonder).
Ex: glue, cement, tape
Mass – when you stack blocks the friction between the base of one and the other surface keep it in place.
Ex: stacking bricks
Types of Joints
Fixed Joints – rigid to prevent movement
Ex: welding, cementing, gluing, nails.
Movable Joints – are flexible and mobile to allow parts to move when required.
Ex. scissors, human bones, doors.
3.3 Properties of Materials in Plants and Animal Structures
Our body is a structure!
- bones, ligaments, cartilage form a structural frame that work together
- muscles (656) allow your frame to move
- joints allow for movement
-skin – human shell
Materials in a Tree Structure
Trees are structures with layers of different materials too...
- Bark
- Phloem
- Heartwood
- Sapwood
- Cambium
These structures maintain the strength of the tree.
4.1 BUILDING SAFE STRUCTURES IN ALL
ENVIRONMENTS
Margin of Safety
Margin of Safety – limits within a structure's performance is thought to be acceptable.
Ex’s – speed limits, tire pressure, crash requirements.
We perform tests in order to determine structural safety. Monitor structures to see how they fair.
ENVIRONMENT FACTORS
Climactic – snow, rain, wind, heat, cold, humidity, dryness.
Terrain – the ground (certain soils, hills, slopes, mountainsides) as well as avalanche/rockfall areas.
Earthquake Risk – built to resist the external and internal forces
https://www.businessinsider.com/round-homes-survive-major-hurricanes-dorian-2019-9?utm_content=bufferab9ab&utm_medium=social&utm_source=facebook.com&utm_campaign=buffer-insider-main&fbclid=IwAR3NYssEsbmmBR_BjN5k1pIepWfepUW9F_1udATYuHJG-7vtvJFHUBLF7W4
4.2 ALTERING MATERIALS FOR STRENGTH
- we solve structural problems by combining materials and arranging them in new ways.
- we take advantage of the best qualities of each material.
CORRUGATION
The process of forming a material into wave like ridges or folds.
Corrugated cardboard and corrugated metal are common examples.
Ex: corrugated cardboard and corrugated metal
LAMINATION:
Gluing layers of a material together to create a strong bond.
Laminated materials are stronger than a single piece of the same material of the same thickness.
Ex: automobile glass, kitchen counter tops, plywood.
NEW MATERIALS:
Science and technology are creating new materials all the time that provide solutions to creating stronger, lighter and more stable structures!
For example, composites of carbon fibers have properties that are superior to steel and other metals.
4.3 Evaluating Designs from an Overall Perspective
A question we ask when designing is, "Will it do the job I want it to?"
However, that is not the only question we can ask... Some factors to consider are:
- Cost
- Benefits
- Safety
- Impact on the Environment
CASE STUDY!
Read "How Rocky Mountain Bicycles Make Bikes" on page 335.
Using the information provided, answer questions 1-5 on page 334.
THIS IS INDIVIDUAL WORK.