pg5

                                                              Unit Review - Pages (186 - 189)

Please note that only some questions are selected for the unit test. Do not study the rest of the questions.

Understanding Concepts:

1)
a) Pyramid: exact geometric centre
   Tripod: where the three legs meet
    Table: centre of tabletop
    Beach umbrella: at the top of the umbrella where all the spokes meet

b) The most stable is the pyramid (large base, most of the mass on the lower part of the structure, centre of gravity lower than the highest part of the structure)
c) the least stable is the beach umbrella (mass is concentrated on the top half of the structure, high COG, very small base)
d) Increasing the size or mass of the base, increasing the concentration in mass to the lower part of the structure, building a structure attached to the existing structure to act as a support system, lowering the COG by burying the structure partway.

2) When the canoeist is considered along with the canoe, the "system" has a high COG as well as a relatively small base. The mass of the canoeist and canoe is concentrated on the upper portion of the canoe.

3)
a) False: the lower the COG, the more stable the structure
b) True: Tension is a pulling force, which means stretching the material
c) False: the nail clipper exerts a pinching or compression force that causes structural failure in the fingernail.
d) False: This is the less expensive way, as well as the safest way for some situations. Other ways include building and stress-testing models
e) False: Dynamic loads are the changing loads, like live loads and the forces in weather. Gravity stays the same.
f) True: When the materials are glued together, the structure is less likely to twist.

4)
a) The CD player falling off is a dynamic load. The impact and the squeezing in the compactor are compression forces.
b) The dog jumping is a dynamic load. The landing of the dog on the owner's lap may exert a compression force. The breaking of the frame chair may be a result of torsion, compression, tension, or shear; depending on the way the dog lands on the owner's lap and/or the type of chair and/or the size of the dog.
c) The force here is shear; which results in a structural failure, also called shear, of the house (across the fault line).

6) The elevator can safely support 300 kg. However, it is built to support a maximum of 18 x 300 kg = 5400 kg.

9) 200 = 8. The strength-to-mass ratio of the ladder is 1:8
      25

Applying Skills:

10) 1-D; 2-K; 3-L; 4-A; 5-C; 6-F; 7-H; 8-I

12)
a) Flaws:
for the bridge: no structure below roadway to support the span; the load (unknown) may require further support; the material (nnknown) may need to be changed; the fact that the slats are close together may affect the bridge's wind resistance.
For the bookshelf: no bracing, ties, or struts to prevent twisting.
For the CD carrying case: materials are not rigid enough to be carried or to withstand a blow; CDs not separated, perhaps causing abrasion damage; fastener may not be of sufficient size or strength.

Making Connections:

22)
a) True, because structures are built with a high factor of safety that overcompensates (in favour of safety) for any load the structure will have to withstand
b) False, since many problems with structural strength are not apparent until a prototype is tested.
c) True, information that is organized often reveals a repeating pattern, which can be helpful to te researcher.
d) False, although you can probably predict some of the results of a product test, you can rarely predict all of them.

31) The bridge expands and contracts with heat and cold, as well as sways in wind and storm conditions. This apparent weakness in the structure is required so that the bridge does not suffer structural failure under normal conditions.