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

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

3. Forces & Loads

4. External Forces Dead Load: Live 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

5. Stretch a material by pulling its ends apart
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

6. Crush a material by squeezing it together
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

7. 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

8. Twist a material by turning the ends in opposite directions
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

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

10. 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
Precipitation from clouds
Wind sock
Live load (wind inside)

11. Pg 309 External Forces Example
Compression, tension, torsion, shear, bending
Tent ropes
tension
Fishing line
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

12. 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)

13. External Forces Example Compression, tension, torsion, shear, bending
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

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

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

17. Types of Failure Torsion Metal Fatigue
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)

18. Making Use of Stress

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

20. 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

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

22. Twist Yarn is twisted together to make it stronger
Wires are twisted into cables
Hair is twisted into braids

23. Unit D: Structures and Forces
Topic 7: Stable Structures

24. 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)

25. 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

26. Pilings

27. 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

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