Forces and Structuresmmeschroeder.weebly.com/.../6/0/24605898/forces_and... · Forces External forces act from the outside. Ex: Wind blowing on a house wall. Ex: Gravity Ex: Load

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Forces and Structures

Vocabulary

Frame: A rigid structure that surrounds or encloses something such as a door or window. Skeleton

Shell: Mostly empty structures surrounded by a thin layer. Bee hive

Solid: Matter that has a definite shape and volume packed tightly together. Brick Castle

Vocabulary

Centre of Gravity: The point at which a structure’s mass is concentrated; the structure is equally balanced in all directions at this point.

Stability: Ability to maintain or resume its position when an external force is applied.

Compression: A type of internal force that squeezes or presses a structure together. Ex: Crumpling a can when done your drink.

Vocabulary

Tension: The pulling or stretching force.

Ex: When a beam bends under a load, the bottom surface becomes longer – it is stretched.

Tug-of-war

Shear: When parallel forces act in opposite directions on an object.

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Vocabulary

Torsion: Type of internal force that twists a structure.

Internal Forces: Any push or pull acting on a structure that can change the structure’s shape, speed, or direction.

External Forces: Any push or pull acting on a structure that can change the structure’s shape, speed, or direction.

Vocabulary

Structural Stress: The effect of all the internal and external forces acting on a structure over a long period of time.

Structural Fatigue: Occurs when a combination of external and internal forces weaken components of a structure.

Structural Failure: The breakdown of a structure due to internal and external forces acting on it (collapse).

Vocabulary

Load: Loads need to be supported by structures. They are caused by forces acting on the structure.

Magnitude: The size of the force being placed on a structure.

Prototype: Model used to test and evaluate a design.

Vocabulary

Point of Application: Exact location where force meets a structure.

Plane of Application: This is the side of the structure affected by the force.

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Vocabulary

Efficiency: Structural efficiency is a single number that compares the mass of a structure with the load it supports.

Structure: Any small or large object that supports and has at least one function.

Function: To perform a specified action or activity.

Vocabulary

Newton: The standard unit for measuring force (N).

Truss: Interlocking triangles making up a framework within a structure.

Gusset: Thick piece of steel used to reinforce a joint within a structure.

Vocabulary

Strut: A support that resists compression.

Tie: A support that resists tension.

Mobile joint: Movable part in which 2 or more parts connect.

Between the humorous and the ulna

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So

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Solid structures are solid through and through even though it may have some gaps and small holes in it. The majority of its structure is solid. A dam, for example, may have service tunnels and electrical lines running through it, but is otherwise solid.

Sh

ell Stru

ctures

Shell structures are mostly hollow, but have a solid outer skin. They use little material for their size and make good containers.

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Fra

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Frame structures are made of many individual parts that are connected to each other in complex ways. The key to strong and useful frame structures is the specific way that parts are attached and the manner in which they are attached.

Fram

e Stru

ctures

Frame structures are more flexible than other comparable structures made from the same material. They usually require greater knowledge and skill to assemble, causing greater construction cost, but use significantly less material.

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Stru

ctures

Many structures are combinations of two or all three types of structures. A house, for example, has a solid foundation, frame walls and roof trusses and is covered by flat sheets of wood and drywall, making it a large shell structure.

Forces

A force is a push or a pull. Forces act on all structures, big or small, and the structure must be able to withstand the forces. Structures experience both internal as well as external forces.

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Centre of Gravity

1. Where along the length of the ruler did you place your finger to balance the ruler?

2. What force was pulling on the ends of the ruler?

3. Draw and cut out a variety of shapes and find the point at which they balance.

Centre of Gravity

Belt and washer experiment

1. What was the position of the washer in relation to the student’s feet and hips when he or she lost balance?

2. The centre of gravity of a person is located in the mid-abdomen region, over the hips. Where must the centre of gravity be in relation to your feet for you to remain balanced while standing?

Centre of Gravity

Belt and washer experiment

3. Of the four demonstrated stances/positions, which one provided the most stability? Why?

4. How is this knowledge of stability, balance, and centre of gravity used in sports?

Balance, Force and Centre of Gravity Investigation

Working in pairs, take turns sitting on a chair and the standing up without using your hands.

One partner sits in the chair while the other partner stands in front of the chair, placing their outstretched hand on the seated partner’s forehead.

The seated partner should try to stand. The standing does not move.

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Balance, Force and Centre of Gravity Investigation

Forces

Forces are measured in newtons (N) –named after Sir Isaac Newton in recognition of his work in classical mechanics.

Forces

External forces act from the outside.

Ex: Wind blowing on a house wall.

Ex: Gravity

Ex: Load (dynamic load, static load)

Forces

Internal forces act from the inside of the structure, like the tension in a tendon or ligament that holds a joint together.

One part of the structure exerts onto another part of the same structure.

Ex: Press your palms against one another and push.

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Describing Forces

Many quantities in nature/science have just one “dimension”. They are basically a count in a certain quantity and that’s it. Ex: I could try to find out what the length

of a piece of wood is. The answer is a simple number, like 24cm.

Such quantities are called scalar quantities. They are fully described by a magnitude alone.

Describing Forces

Forces, on the other hand, have both a magnitude and a directionality. A force pushing left will have a dramatically different effect to one pushing right, even if they have the very same strength (magnitude).

Describing Forces

The direction in which the force is acting is an essential part in fully describing it. Such quantities are called vectors and are represented with an arrow. In terms of what a force will do to a structure, there is even more to a force.

Examples of Scalars and Vectors

Scalar Mass

Length

Distance

Speed

Power

Energy

Work

Vector Displacement

Velocity

Acceleration

Force

Weight

Friction

Momentum

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Describing Forces: Magnitude

The magnitude of a force describes its strength and is represented by the lengthof an arrow.

***A bad habit by some textbooks and old diagrams is to base the magnitude by the thickness of the arrow – DO NOT DO THIS!!

Describing Forces: Direction

The direction of a force describes the direction in which it is acting and is represented by the direction of the arrow.

Ex: Gravity is a force that pulls things downward – so the arrow would be pointing down.

External Forces Application

Create force diagrams identifying the forces in the following scenarios about a boat.

**The force acting to keep the boat up is called buoyancy.

**Gravity acts to keep a boat from flying in the air. It acts downward.

External Forces Application

1. The boat is floating in one place on a calm lake.

2. The boat gets caught in a current of water that begins to move it backwards.

3. The driver starts the motor and goes against the current moving forward.

4. The driver has picked up 10 more people at the dock and the boat is sitting a little lower in the water.

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Describing Forces: Point/Plane of Application

The point of application of a force is where it is applied on a structure. This is a third aspect of a force that applies when we consider the effect of forces on structures.

Describing Forces: Point/Plane of Application

Consider a lever, for example. Where we push on a lever determines the mechanical advantage the lever will produce for us.

We will instinctively slide back on a lever if we find it too hard to use it. That changes the point of application while keeping everything else the same.

The point of application of a force is expressed by where the arrow attaches to the structure (tip or origin).

Loads

Structures need to be able to support loads. Loads are caused by forces acting on the structure.

The total load on a structure is the sum of its static and dynamic load.

Loads: Static

The static load is the load due to gravity on the structure itself.

Many strong building materials are also very heavy, which adds to the static load on a structure before we have even begun to load it up with anything useful.

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Loads: Dynamic

The dynamic load is the load due to forces that change while they are acting on the structure.

Consider a truck driving across a bridge. It constantly changes its point of application on the bridge. The force due to wind change magnitude, direction and point of application with changes in the wind pattern.

Internal Forces/Stress

Loads produce internal force or stress within a structure and we must design them with that in mind.

Internal Forces/Stress: Compression

Compression occurs when two forces push towards one another (squeezing). The object may respond by squeezing together or buckling. Solid structures tend to be used the most if compression is expected.

Ex: A house foundation is made of poured concrete.

Internal Forces/Stress: Tension

Tension occurs when two forces pull away from one another (stretching). The object may respond by stretching or ripping. Depending on the circumstances we may use elastic materials or materials with high tensile strength like steel cables.

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Internal Forces/Stress: Tension

Some materials that are great at resisting compression are terrible at resisting tension. Concrete, for example, is great compression but must be reinforced with steel bars if any degree of tension is expected.

Internal Forces/Stress: Torsion

Torsion is a stress that results from a twisting motion. In nature, torsion can be extremely destructive. Structures that need to resist torsion are often designed with some level of built in flexibility.

Internal Forces/Stress: Torsion

Frame structures have such a built in flexibility, stemming from a small amount of mobility in each of its many joints.

Japan experiences many earthquakes so has to consider mobility in their building structures.

Internal Forces/Stress: Shear

Shear is a stress that occurs when two forces just slide past one another. The structure rips, or is cut, right along the line where the two forces slide past one another. We make use of this in just about every cutting tool, especially scissors. Resisting shear tends to be a simple question of structural strength. You either have it, or you don’t.

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Licorice Experiment

1. Bend a licorice strip in half and observe where the bend occurs. In your journals, draw a diagram of the licorice and label the top and the bottom part of the bend with arrows to indicate the direction of the forces acting within the licorice. Write a brief description of what is happening.

Licorice Experiment

2. Straighten the licorice and, holding one end steady, turn the other end of the licorice. In your journal, draw a diagram of the licorice and label the directional forces. Write a brief description of what is happening.

Licorice Experiment

Which of the examples best illustrates the following:

Compression is the result of squeezing together

Tension is the pulling apart of a structure.

Shear is the result of forces acting in opposite directions of each other.

Torsion is a twisting force.

Preventing Failure

Every day, we depend on structures doing their job. Cars and buses bring us safely to where we want to go, bridges and houses stay strong, carrying their burden. Sometimes, though, we witness the failure of such structures, often with tragic consequences.

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Collapse of SampoongDepartment Store June 1995

Ultimately this collapse was caused by the installation of large air conditioning units and restaurants on the the

roof and top floors. The collapse killed 501 people.

Preventing Failure

Designers try to anticipate the intensity and duration of use, the exposure to factors like heating and cooling that might fatigue materials, and the human factor, which often means that people will abuse and misuse objects.

Preventing Failure

While designers can’t, and shouldn’t, be held accountable for the poor judgment of other people, it is still a good idea to build in a safety cushion to account to lapses in common sense. Such extra capacity in ability to support loads, for example, is called a “margin of safety”. Think of expiration dates on food –

usually it is still fine even a couple days past the expiry, but you are pretty much guranteed safety before that day.

Preventing Failure

Consider that if an elevator fails catastrophically, the human cost may be very high. The inside of elevators list the maximum load that should be in the cabin, but who’s going to calculate the total load, much less ask for the individual weight of passengers, before getting on. It’s much better to make the actual ability of the elevator higher than advertised. Tacoma Bridge Failure

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Preventing Failure

Another way to increase the safety of structures is the add sensors to it that monitor crucial values.

Ex: An elevator might have an overload sensor that disables the cabin from moving if a critical load value is surpassed

Ex: Houses are now required to have functioning carbon monoxide and smoke detectors, etc…

The World’s Most Famous Tower

Read the handout.

Create, in storyboard/comic strip fashion, a series of pictures that depict the internal and external forces that have caused the structural stress to occur and the attempts to correct it.

Include brief captions or speech bubbles that provide textual information to support each concept.

Strength

As far as structural strength is concerned, there is, of course, the material’s innate strength. Some materials are simply stronger than others simply based on the nature of the material.

Steel is simply stronger than a piece of pine, but a beam made of wood is lighter than a steel beam.

Strength

In order to maximize strength while keeping weight to a minimum we can employ a number of tricks:

Triangles, Curves, Domes

I-beams

Corrugation

Lamination

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Strength: Triangles, Curves and Domes

Using triangle shapes, curves and domes we can distribute the load over more than one location and lessen the impact.

Strength: Triangles, Curves and Domes

Trusses are extremely common in building and bridge construction because they provide significant reinforcement with little material.

Notice how they are basically a collection of triangles.

Strength: I-beams

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Strength: Corrugation Strength: Lamination

Strength: Lamination

Notice how the direction of the wood is switched in each layer. Wood breaks most easily along the direction of its grain. We could consider wood to have a directional weakness. By gluing each layer with the grain facing a different way by 90 degrees, we eliminate this directional weakness. A piece of plywood is much stronger than a piece of solid wood of the same kind and size.

Stability

Stability measures how much it takes to make a structure fall over. A structure will fall over as soon as we bring its centre of gravity outside of its support base (assuming it’s not anchored or tied down…).

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Stability

Wide support bases and less height both bring the centre of gravity lower to the ground, which makes it harder to bring outside the support base by tipping.

Lifespan of a Product

Every product has a lifespan. When a manufacturer designs a product, they have to answer the question of how long it should last before it wears out. If it wears out too quickly, consumers will feel like they’re not getting their money’s worth.

Lifespan of a Product

If it lasted a very long time, either the manufacturer wouldn’t make a profit or the item would cost too much because longer life span usually comes with using superior, and thus more expensive, raw materials and manufacturing methods.

Lifespan of a Product

When a manufacturer deliberately designs a product with a limited life span, it is called planned obsolescence. Finding the right length of time before a product fails is a delicate balance between the desires of consumers (long life and low purchase cost) and those of the manufacture (short life span and high purchase cost).

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Lifespan of a Product

When a product has reached the end of its life cycle, it needs to be disposed of. Often enough the entire item just ends up in our landfill and that pollutes our environment and is wasteful. Designing a consumer item should also keep in mind how much of an item can be recycled and reclaimed and how easy it is to do that .

Spaghetti Beam Bridge

Determine whether the magnitude of weight needed to cause structural failure of a “spaghetti bridge” differs at various points along the plane of a bridge.

Identify the independent variable and dependent variable

Discuss some of the controlled variables that need to be addressed

Spaghetti Beam Bridge

Develop a hypothesis, list of materials needed, and method

Chart or graph the magnitude versus the point of application

Develop a conclusion based on your data. The conclusion should show the relationship between magnitude of weight and point of application, and also identify the strongest and weakest points along the plane of the bridge.

Penny-Paper Bridge

Place a piece of paper between two stacks of books to create a beam bridge.

Place pennies on the bridge until structural failure occurs.

Plan and build a paper beam bridge that will hold more pennies than the previous one.

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Penny-Paper Bridge

The type of paper must be the same (no limit on the quantity)

The paper can be folded

Small amounts of tape and glue may be used to create box beams or I-beams as bridge spans.

Penny-Paper Bridge

Determine the efficiency of your structure using this formula:

Structural efficiency = Maximum mass

Mass of structure

Tower Building Contest

In groups of 3, students will have 60 seconds to build the tallest tower from the material provided.

Follow the following guidelines:

Prior to building, create a plan for your tower.

During construction, you may not modify the structure once the blocks are in place. However, you may modify your plans for subsequent layers.

Draw the final structure – include measurements.