Science 7 – Structures and Forces
Review:
How Structures Fail:
1) Levers  strong winds cause bending which puts a larger force on one side
of base, base weakens and structure can fall over!
2) Shear  Small cracks or weaknesses enlarge or break when structure is
compressed. Ex. Weight of building can compress soil causing it to shear or slip
sideways, the ground sinks and building tilts or collapses.
3) Bend or Buckle  Thin panels bend on inside of curve (like a pop can). Boat
bodies are reinforced to prevent buckling.
C
4) Torsion  Brittle structures break when twisted (spaghetti).
Flexible materials (rubber bands, hoses, cords) fold up, twist or
tangle.
A
B
Metal Fatigue  Weakening of metal when it is bent and twisted over and
over again. (Think of paper clip activity)
Designing With Forces 
Using Frictional Forces 
Fasteners like nails or screws provide friction between wooden parts to keep the joint rigid.
Some nails provide more friction than others and screws are better yet – but take longer to
put in.
Pilings 
Building Stable Structures 
- Need to consider center of gravity – class demo
- Do “Tip It!” on page 330
- Procedure 1
a)
b)
c)
- Procedure 2
a)
b)
c)
- Procedure 3
a)
b)
Summary 
Objects are more stable if they rest on a ____________ area and have most of their mass
close to the _____________.
Centre of Gravity – central point of a structure that falls in a straight line (straight down).
All other points rotate around the centre of gravity.
See page 332 and page 333.
Stable structures need a firm foundation!
Question – What factors prevent foundations from being
firm?
How do builders construct a stable structure on shifty
ground?
1) Dig Deep  hit bedrock OR use pilings (concrete
cylinders) that do down to the bedrock. Going down 1.5
meters is often deep enough for lighter buildings – past
the frost line.
2) Make a solid layer  pack the loose surface soil 
what they do when making highways, will also use layers
of gravel and pave on top of solid layer. Packed gravel
foundations are also used for dams and other mass
structures.
3) Spread the Load  Footings are used where the
concrete foundations at base of walls is wider than walls
and using many footings or pilings rather than a few
means less force on either of them.
Read Ask an Expert on page 342 – 343.