Unit D Structures and Forces Topics 4 7

Unit D: Structures and Forces Topics 4 -7 Topic 4: Forces, Loads, and Stresses

Types of Forces w Internal w Tension, compression, shear, torsion, bending w External w Dead Load w Permanent force acting on a structure w Includes the weight of the structure itself w Live load w Changing or non-permanent force acting on a structure (wind, weight of things in a structure)

• 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 w Tension Forces: w Stretch a material by pulling its ends apart w Tensile Strength: w Measures the largest tension force the material can stand before breaking

w Compression Forces: w Crush a material by squeezing it together w Compressive Strength: w Measures the largest compression force the material can stand before losing its shape or breaking

w Shear Forces: w Bend or Tear a Material by pressing different parts in opposite directions at the same time w Shear Strength: w Measures the largest shear force the material can stand before ripping apart

w Torsion Forces: w Twist a material by turning the ends in opposite directions w Torsion Strength: w Measures the largest torsion force the material can stand still spring back to its

w See pg 308, forces on a bike w Use big marshmallows to show forces w See pg 309, samples of forces

Pg 309 Internal Forces Example 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 Forces Example Compression, tension, torsion, shear, bending Tent 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 Forces Example 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 Forces Example Wires in braces Wringing out a wash cloth Spider web Elastic band Chain connecting a ship to the anchor Sitting on an exercise ball Bolt you are tightening with a wrench Legs of the chair you are sitting on Compression, tension, torsion, shear, bending

Unit D: Structures and Forces Topic 5: How Structures Fail

w Shear Types of Failure w Crack, break w Due to compression, twisting (torsion), bending w Bend and buckle w Compression

w Torsion Types of Failure w Twisting brittle items (spaghetti, plastic cutlery) causes it to shear w Flexible structures such as cords, hoses, rubber bands, shear less easily w Metal Fatigue w Bending a material back and forth, weakening the metal, causing it to break (staples, paper clips)

How can forces such as buckling, twisting, bending and shearing be used in a positive way?

w Car bumpers and sheet metal buckle in a collision. They absorb the crash impact w Grass on a soccer field will buckle, so it absorbs the impact when someone falls

Shear w Used in boat motors to help keep them from getting tangled in weeds w Used in the clutch and automatic transmissions of cars

w Yarn is twisted together to make it stronger w Wires are twisted into cables w Hair is twisted into braids

Unit D: Structures and Forces Topic 7: Stable Structures

How can we make structures more stable? w Account for the center of gravity (need to be balanced) w Have a solid foundation w Use pilings w Make a solid layer (underneath roads there is gravel, rough ashphalt) w Cement “foundation” for house w footings w 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 w Gyroscopes will balance if they are spinning w Throwing footballs in a spiral helps them travel in a straight line w Space satellites spin to keep the antennas pointed at earth w Frisbee/yo-you, top

Assignments w Review Q’s T 4+5 w Pg 320 # 1 -5 w Topic 6: Designing with Forces w Read Topic 6 pg 321324, 326