1. Structures and design unit: forces
Warden Ave PS Integrated 7/8 Science Unit

2. Learning goals By the end of this presentation we will be able to:
Be able to define the term “Force”
Give multiple examples of the various kinds of force
Identify various forces from examples found in the real world

3. Questions about force A few starter questions: What is force?
Where do forces come from?
What kind of forces effect structures?
Are forces the result of natural occurrences or are they created by people?

4. The many faces of force
Forces are present in the world around us all the time. Some forces such as wind, rain, ice and snow are part of our natural environment while other forces are created by people. Forces can be both internal (inside the structure) and external (from outside).

5. Internal Forces
Internal forces are from within a structure and may be affected by external forces acting upon the structure. Shear, torsion, tension and compression are internal forces. These internal forces, for example within a bridge, are affected by external forces such as high wind or ice which may cause the bridge to twist or buckle.

6. Some definitions
Shear: Force acting on a substance in a direction perpendicular to the extension of the substance. For example the pressure of air along the front of an airplane wing. Shear forces often result in shear strain.

7. Some Defiinitions (cont’d)
Torsion: The twisting of a body by two equal and opposite torques (opposite forces).

8. Some definitions (cont’d)
Tension: The act of stretching or straining (pulling apart). Compression: The state of being compressed (pushed together).

9. External forces
External forces act upon structures from outside. Strong wind, heavy snow and ice are examples of naturally occurring external forces.

10. Guess the force
For each example, identify and explain (briefly) which force is at work: A) B)

11. Guess the Force
C) D)

12. Guess the force
E) F)

13. Question for journal
Of the Internal Forces we looked at today, which do you think is A) the most common and B) the most dangerous to human created structures? Don’t forget to write your reasons why.