Science 8 Topic 2 – Reflection



Unit 4 : Structures and Forces

Topic 3 – Mass and Forces

Mass

a. When was the metric system created? _________________. It again changed to SI in? ______

b. The Definition of mass- the measure of the _______________________________________.

So if an elephant has a large mass it has _____________ of particles whereas an egg

sized piece lump of lead has less mass so it has ______________ particles.

c. A cylinder was used to by scientists to measure mass in 1795. This is called? ______________

It weighed ________________ kg.

d. 1kg = _________ g 1 g = _________ mg

Force

You have just learned that the elephant's mass of 5000 kg would not change whether it was on Earth __________________________________________.

This is because the amount of matter making up the elephant would be the same wherever it is in the universe.

So why does its weight change?

To understand how mass and weight are different, you must first learn _____________________.

Forces are stresses such as _____________________________.

The standard SI unit of force is called a ______________________.

One newton (1 N) is only a small force, just enough to stretch a thin rubber band a bit.

To understand and predict how forces affect structures, you need to find the ___________________________.

Who is this Isaac Newton and why is he important?

Sir Isaac Newton (1643-1727)

Isaac Newton was the greatest English mathematician of his generation. He laid the foundation for differential and integral calculus. His work on optics and gravitation make him one of the greatest scientists the world has known.

Laws of Motion

1. An object at rest tends to stay at rest or an object in motion ________________________________.

2. The acceleration of an object depends directly upon the net force acting upon the object, and inversely _______________________________________________________,

3. For every action there is an equal and _________________________.

Weight

Weight is a force and, like other forces. It is properly measured in _____________________. Did you notice that we said the elephant's mass is 5000 kg, not the elephant's weight?

Weight was carefully investigated by Isaac Newton in the seventeenth century. According to a famous story, Newton once sat under an apple tree and began to wonder why the apples always fell down, toward Earth. They never fell up into the sky or just floated in mid-air. Newton realized that there is a force between any two objects, anywhere in the universe, that tries to pull them together.

REMEMBER: Mass is the amount of matter an object is made of and weight is the force with which gravity pulls on an object.

A standard bathroom scale is a type of spring scale.

Springs within the scale are stretched when a person stands on the scale.

The springs are connected to parts that move a numbered scale below a fixed needle.

Topic 3 Review

1. Suppose you want to measure the weight of a pencil.

a) What measuring instrument should you use?

b) What units of measure should you use for your answer?

2. In a lab report, two students reported that they had applied a force of 6.5 N to a brick. Their answer received only half marks. What extra information should they have reported?

3. a) Mass is the number of particles in an object measured in ______________

b) Weight is the force of gravity measured in _____________

4. a) Express the mass of a 125 g tube of toothpaste in kilograms.

b) Calculate the weight of the tube of toothpaste.

Unit 4 : Structures and Forces

Topic 4 – Forces, Loads, and Stresses

EXTERNAL FORCES on structures are stresses that act on a structure from the _____________________.

These forces produce INTERNAL FORCES, or stresses, within the materials from which a structure is made.

Such internal stresses can change the shape or size of the structure. This is called _______________.

External Forces

Engineers divide the forces that affect buildings into 3 groups.

1. A dead load is a permanent force acting _________________________. This includes

the weight of_____________________________.

2. A live load is a changing or non-permanent force ____________________________.

This includes the force of the wind and the weight of things ______________________.

3. Impact forces, caused by objects colliding with the structure, are another

_________________________________________.

Internal Forces

1. Tension forces stretch a material __________________________________________.

2. Tensile strength measures the largest tension force the material ______________ ________________________________________________________.

3. Compression forces crush a material ________________________________________.

4. Compressive strength measures the largest compression force the material can withstand before _____________________________________________________________________.

5. Shear forces bend or tear a material by pressing different parts in ____________________ ____________________________________.

6. Shear strength measures the largest shear force the material _______________________.

7. Torsion forces twist a material by turning the ends ________________________________.

8. Torsion strength measures the largest torsion force the material can withstand and still spring back _______________________________________.

9. Bending is a combination of ________________________________________________.

Resisting Stress - The Inside View

What determines the strength of a material?

Scientists trace strength, and many other properties, to forces between the between the tiniest particles of the materials.

[pic]

• Steel has high __________________________________. It has strong forces pulling its __________________.

A very strong tension force is needed to separate the particles and ____________________________________.

• Graphite (a form of carbon) has low _________________________________. Its particles are arranged in

layers, but the forces between the layers are ________________________________. Because the layers slide

over one another easily, graphite is slippery and makes a ___________________________. The layers of

graphite in a pencil "lead" rub off and leave a mark on the paper __________________________________.

• Rubber has high _____________________________. Each particle is attracted in all directions to the other

________________________________________. The particles hold together even when a piece of rubber is

_______________________________________________.

Topic 4 Review

1. List examples of deformation that a building could suffer.

2. Name three types of stress that a force can cause inside an object.

3. a. What is the difference between a live load and a dead load?

b. Study the following loads that are acting on a tree.

Classify them as "live" or "dead."

• wind blowing against a tree.

• the weight of the tree

• the weight of a bird in the tree

4. Identify the type of strength that is shown by

a. the chain that connects a ship to its anchor

b. a piece of very tough dried meat you are chewing

c. a bolt you are tightening with a wrench

d. the legs of the chair you are sitting on

Unit 4 : Structures and Forces

Topic 5 – How Structures Fail

Levers Create Large Forces

What is a lever? ____________________________________________________________________

__________________________________________________________________________________

_________________________________________________________________________________.

Some types of levers consist of a long arm that rests on _____________________________________.

When effort is applied as an external force to the lever, a large enough force is created

_______________________________.

A lever is divided into 3 different parts;

• The pivot

• The applied force

• The resulting force

Material Failure

External forces can cause internal forces ______________________________________________.

Shear

• Solid materials are never perfectly uniform. They nearly always have microscopic

______________________. When a solid material is compressed, the crack can enlarge or break

apart. One section may shear (slide over another section along the weakness). The weight of a building

can compress the soil causing the soil to _________________________________________________.

The ground beneath the building sinks, and the __________________________________________.

Bend or Buckle

If you put pressure on a metal can, the thin metal folds and the can buckles (gives

way) under the ___________________________________________________________________.

The same thing happens to a piece of paper when you push the ends together. All thin panels tend to

bend and buckle when __________________________________________.

Compression forces cause material to bend on the inside of the curve, and pull and snap on the

_____________________.

Shell structures that use thin panels to support their entire load, such as boats and aircraft, are

reinforced to __________________________________________.

Torsion

Twisting forces can cause material failure, too. Brittle structures, such as dry spaghetti and

plastic cutlery, often shear when ________________________________________.

Sections of the structure slide past each other, and the structure cracks or

_________________________________.

Very flexible structures, such as rubber bands, hoses, and electrical cords, _____________________.

Instead, torsion forces make them fold up and twist into _____________________________________.

Although the structure is unbroken, it has lost its shape, which is a ____________________________.

Metal Fatigue

In a bent or twisted part of a metal structure, the arrangement of particles ______________________.

Where particles move apart, the forces holding them together ________________________________.

If enough particles are affected, small __________________________________________________.

Eventually the material may fail under only a small stress, one that it could easily resist when

_______________________________________. Engineers call this weakening metal fatigue, and it is

still a problem, especially in lightweight, flexible _____________________________________.

Topic 5 Review

1. (a) Identify three dead loads and three live loads that are acting on your classroom.

(b) Explain your reasoning for deciding which loads are "dead" or "live."

2. Name three ways that materials fail, and identify the type of internal force that causes each kind of failure.

3. (a) Which type of structure (mass, frame, or shell) is most likely to be damaged when its parts act as levers and create very strong forces?

(b) Why are the other two types of structures not also weakened by lever action?

4. Which type of material failure occurs when you

(a) leave a trail of footprints in a carpet?

(b) sprain your ankle in a soccer game?

(c) accidentally hit a baseball through a window?

(d) crinkle a new $5 bill as you stuff it into your pocket?

(e) twist the lid of a partly opened tin can back and forth until it breaks off?

Topic 3-5 Homework

1. Classify each statement as referring to force (F) or mass (M). (4 points)

a) measured in newtons

______________

b) stays the same no matter where the object is located

______________

c) measured with a balance

______________

d) your weight

______________

2. Using the diagram: (3 points)

(a) Name the type of force stressing the top of the bookshelf.

(b) Name the type of force stressing the bottom of the bookshelf.

(c) Describe what might happen if more books were piled on the bookshelf.

3. Describe what is metal fatigue. (2 points)

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

4. What is the name of the legendary scientist who is rumoured to have based his theory of gravity on an apple falling from a tree? (1 point)

_________________________

5. Describe how structure failure can be beneficial in certain circumstances.

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

__________________________________________________________________________________

Topic 3 Review Notes: Mass and Forces

Mass

The mass of an object is the measure of the amount of matter in it.

The mass is the number of particles the substance has

A small cylinder of metal was used, as a standard, by which, to compare different substances. This standard (in the metric system) is called the primary standard of mass, and the amount of material in it is called one kilogram (kg). Smaller masses are measured in grams (g).

'Kilo' means one thousand (1000) and is equal to 1000 grams. Very small masses are measured in milligrams (mg). 1000 mg. equal 1 g.

A balance is used to measure the amount of mass in a particular substance. Standard scientific balances include the triple beam balance and the equal arm balance.

Mass stays the same no matter where you are in the universe.

Forces

Force is a push or pull on an object. The standard unit of force is called a newton (N). (1 newton of force will stretch a thin rubber band, or will be what it takes to lift up a D-cell battery). A force meter (spring scale) is used to measure the amount of force - the pull of gravity - on a mass. To describe a force accurately, you need to determine its direction and size.

Weight

Weight is a force and should properly be measured in newtons. Sir Isaac Newton (Did You Know - p. 300) described the force that pulls objects together as the force of gravity. The gravitational forces between two objects depends on the masses of the objects and the distance between them. This gravitational force is called weight. Because gravitational force depends on the distance between two objects, an object's weight changes depending on where it is. (the farther away from the earth, the less the weight.

REMEMBER:

Mass is the amount of matter an object is made of and

Weight is the force with which gravity pulls on an object.

Picturing Forces

A force diagram is a simple picture that uses arrows to show the strength and direction of one or more forces (a longer arrow represents a larger force and a wider arrow represents a stronger force).

Topic 4 Review Notes: Forces, Loads and Stresses

External forces on structures are stresses that act on a structure from outside the structure. These forces produce internal forces, or stresses, within the materials from which the structure is made. These internal stresses can change the shape or size of a structure and is called deformation. This deformation can lead to repair of the damage to the structure, or failure of the structure.

External Forces

A dead load is a permanent force, acting on a structure.

This includes the weight of the structure itself.

A live load is a changing, or non-permanent force acting on a structure.

This includes the force of the wind and the weight of things that are in or on a structure.

Impact forces (things that collide with the structure) are another type of live load.

Internal Forces

Tension forces stretch a material by pulling its ends apart

Tensile strength measures the largest tension force the material can withstand before failing.

Compression forces crush a material by squeezing it together.

Compressive strength measures the largest compression force the material can withstand before it loses its shape or fails.

Shear forces bend or tear a material by pressing different parts in opposite directions at the same time.

Shear strength measures the largest shear force the material can withstand before it rips apart.

Torsion forces twist a material by turning the ends in opposite directions.

Torsion strength measures the largest torsion force the material can withstand and still spring back into its original shape.

A bending force is a combination of tension and compression

Shear and torsion forces are also a combination of tension and compression

Resisting Stress - The Inside View

Strength of materials can be traced to the forces between the tiniest particles of the materials.

Topic 5 Review Notes: How Structures Fail

Forces acting on structures can cause them to fail to perform their function.

Failure can occur if the force is too strong for the structure's design or if the force is acting on a vulnerable part of the structure (that part of the structure that will likely fail the most often).

Levers

A lever is a device that can change the amount of force needed to move an object. When a force is applied to the effort arm, a large force, which can move the structure, is created. This can be intentional - like when a crowbar is used to move a heavy rock, or it can be unintentional - like when a gust of wind knocks down a flagpole.

(Crowbar) (Flagpole)

Material Failure

Shear - minor weaknesses in a material can cause failure because the particles move farther apart and are less attracted to each other. This can be caused by compression.

Bend or Buckle - compression can also cause a material to bend and buckle - like a pop can that is stepped on. To prevent these reinforcements - stringers and ribs - are used to strengthen the structure.

Torsion - Twisting can cause material failure. When sections of the structure slide past each other the structure can crack or break in two. When the twisting action makes the structure unusable (even though it is not broken) it has failed because it has lost its shape.

(Slinky)

Making Use of Stress

Knowing that materials fail when external forces are applied can be useful.

(Crash Test Dummies)

Buckle - Car bumpers are designed to buckle in a collision - as the metal fails, it absorbs some of the energy of the impact, which protects the occupants of the vehicle. Blades of grass on a sports field buckle as players land, which absorbs some of the impact forces on the players body.

Shear - Shear pins are used in outboard motors to prevent failure of the motor (when the propeller gets tangled in weeds, a shear pin breaks and the propeller becomes disengaged with the motor and gears. The clutch and automatic transmission in a vehicle take into account shear forces, which enable parts to slip past each other and produce a smooth ride.

Twist - Spinning wheels twist cotton or wool fibres so they lock together - making them strong enough to make cloth. Controlled twisting can also be useful in hair braids, ropes and telecommunication cables.

Metal Fatigue

(Definition - The phenomenon leading to fracture under repeated or fluctuating stress. Fatigue fractures are progressive beginning as small cracks and grow under the action of fluctuating stress.)

Metal breaks down over time and extended use. (They get bent and twisted over and over). The particles in the metal move further apart and have less attraction to each other. When a crack develops it weakens the metal - metal fatigue - and can eventually fail even if a small force is applied.

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