Unit D: Structures and

ForcesTopics 4-7

Topic 4: Forces, Loads, and Stresses

Types of Forces Internal

Tension, compression, shear, torsion, bending

External Dead Load

Permanent force acting on a structure Includes the weight of the structure itself

Live loadChanging or non-permanent force acting

on a structure (wind, weight of things in a structure)

Forces & Loads

External Forces• Dead Load:

• Permanent Force Acting on a Structure

• Includes the Weight of the Structure Itself

• Can cause Structures to break

• Live Load:• Changing or Non-Permanent

Force Acting on a Structure

Internal Forces Tension Forces:

Stretch a material by pulling its ends apart

Tensile Strength: Measures the largest

tension force the material can stand

before breaking

Compression Forces: Crush a material by squeezing it together

Compressive Strength: Measures the largest compression force the

material can stand before losing its shape or

breaking

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 stand before ripping

apart

Torsion Forces: Twist a material by

turning the ends in opposite directions

Torsion Strength: Measures the largest torsion force the material can stand and still spring back to its original shape

See pg 308, forces on a bike

Use big marshmallows to show forces

See pg 309, samples of forces

Pg 309 Internal ForcesExample Dead load? Live Load?

River (water) Live load (shape the waterway through erosion)

Beaver/bear Live load (he moves around, not a permanent force acting on the ground)

Water wheel Live load

Bridge Dead load (people or cars on bridge would be live loads)

People sitting on a stool Live load

Precipitation from clouds Live load

Wind sock Live load (wind inside)

Pg 309 External ForcesExample Compression, tension,

torsion, shear, bendingTent ropes tension

Fishing line tension

Saw shear

People sitting on a stool Compression (tension on the bottom surface of the stool seat)

Dog chewing on stick Compression (shear if it breaks)

Bike (see pg 308) handle bars torsion

Handle on the well torsion

Internal ForcesExample Dead load? Live Load?

Wind blowing against a tree Live load

Tree rooted in the earth Dead load

Compression of a spring (pen) Live load

External ForcesExample Compression, tension,

torsion, shear, bendingWires in braces

Wringing out a wash cloth

Spider web

Elastic band

Chain connecting a ship to the anchorSitting on an exercise ball

Bolt you are tightening with a wrenchLegs of the chair you are sitting on

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Unit D: Structures and Forces

Topic 5: How Structures Fail

Types of Failure Shear

Crack, break Due to compression, twisting (torsion),

bending Bend and buckle

Compression

Types of Failure Torsion

Twisting brittle items (spaghetti, plastic cutlery) causes it to shear

Flexible structures such as cords, hoses, rubber bands, shear less easily

Metal Fatigue Bending a material back and forth,

weakening the metal, causing it to break (staples, paper clips)

Making Use of Stress

How can forces such as buckling, twisting, bending and shearing be

used in a positive way?

Buckle Car bumpers and sheet

metal buckle in a collision. They absorb

the crash impact Grass on a soccer field will buckle, so it absorbs

the impact when someone falls

Shear Used in boat motors to

help keep them from getting tangled in weeds Used in the clutch and automatic transmissions

of cars

Twist Yarn is twisted together

to make it stronger Wires are twisted into

cables Hair is twisted into

braids

Unit D: Structures and Forces

Topic 7: Stable Structures

How can we make structures more stable?

Account for the center of gravity (need to be balanced)

Have a solid foundation Use pilings Make a solid layer (underneath roads

there is gravel, rough ashphalt) Cement “foundation” for house footings

Make them symmetrical, or use a counter-weight (cranes)

• Stable - Less Likely to Tip• Centre of Gravity - The

point where all of the gravitational

• Pilings – Large metal, concrete or wood cylinders

• Footing – Concrete foundation under basement walls

Pilings

Spin Stabilization Gyroscopes will balance

if they are spinning Throwing footballs in a

spiral helps them travel in a straight line

Space satellites spin to keep the antennas pointed at earth

Frisbee/yo-you, top

Assignments Review Q’s T 4+5

Pg 320 # 1-5

Topic 6: Designing with Forces

Read Topic 6 pg 321-324,326