Science 7 – Unit D: Structures and Forces

Science 7 – Unit D:
Structures and Forces
Topic 5: How Structures Fail
Levers Create Large Forces
Lever: device that can
change the amount of
force needed to move
an object. When
enough applied force
is put on one of ends
of the lever, it can be
used to lift very heavy
A lever is an arm
rotating on a pivot,
called the fulcrum.
Lever Action
When an external or
internal force acts on
a one end or side of a
frame structure,
deformation can
The joints between
the components are
weakened and, if
nothing is fixed, the
structure will fail.
Resisting Structural Failure
Basically any force, from bending to
twisting to shearing can cause
 The right materials and designs must
be chosen to deal with the forces that the
structure will have to face. For example,
the cables in a hanging sign should be
metal, because metals have a high tensile
strength. Any component dealing with
compression (eg. parking lot columns)
should be made of concrete.
Why do Materials Fail?
Materials fail because the forces acting on them
are strong enough to rearrange or break apart
the connections between the particles of the
If you bend a paper clip back and forth enough
times, you will notice that it becomes easier to do
this: the connections between the particles are
weaker. It’s only a matter of time before a crack
or break will form.
When this weakening occurs, engineers call it
metal fatigue. It is an issue in everything from
buildings to cars to airplanes.

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