1. Unit D Structures and Forces
Section 2.0
External and Internal Forces Act on Structures

2. Section 2.1 Measuring Forces
Definition
Force: push or pull that tends to cause an object to change its movement or shape
Let’s Do An Activity
Page. 281
Figure 2.1, 2.2, and 2.3

3. Section 2.1 Measuring Forces
Magnitude, Direction, and Location
The actual effect of a force relies on these three things:
Magnitude (size of the force)
Direction (where the force is going)
Location (where the force is applied)
Copy and Draw out pg.282 (the three mini blurbs on magnitude, direction, and location)

4. Section 2.1 Measuring Forces
Newton
Unit of Force is referred to as a NEWTON (N)
1 Newton (N) = amount of force needed to hold up a mass of 100 grams (g)
Check and Reflect
Pg.284 Questions 1-3

5. Direction and Location of a Force
Lab Time
Refer to page 283
Direction and Location of a Force
Step 1: Write out all the information you are already given in terms of the lab (refer to lab report format)
Step 2: Construct and Organize a Table for your results to be recorded
Step 3: Proceed with the Lab
Step 4: Finish your lab according to the “Lab Report Format”, answer any of the Analyzing and Interpreting Questions
Step 5: Write a Conclusion based on your results

6. Section 2.2 External Forces Acting on Structures
Definitions
External Force: force which is applied on a structure by something else
Example: walking through the water and waves hit your body
Gravity is a pull of earth, therefore Gravity is an External Force
Mass: amount of matter in an object
Greater the Mass = Greater the Gravitational Force

7. Section 2.2 External Forces Acting on Structures
Centre of Gravity
Activity Time!!!!!!!
Everyone Stand up and find a place in the room
Hold your arms out straight for 2 minutes, What is happening?
Stand on one foot and try to keep balance, What are you experiencing?
Answer
GRAVITY!!!!!
Is having an effect on your stability

8. Section 2.2 External Forces Acting on Structures
Centre of Gravity
Definition
Centre of Gravity: The point in any solid where a single applied force could support it; the point where the mass of the object is equally balanced
Location of the Centre of Gravity will determine the stability of a structure

9. Section 2.2 External Forces Acting on Structures
Symmetry
Definition
Symmetry: balanced ratio of mass which occurs on both sides of a line/plane or around the centre of the axis
** the force of gravity on either side of the center point is equal
For a symmetrical structure to be stable, the mass of that structure must be distributed equally around the centre of the structure’s base, meaning force of gravity around the centre is also equal

10. Section 2.2 External Forces Acting on Structures
Load
When structures are built the idea of having weight put on them is something every engineer or designer must think about.
Static and Dynamic Loads
Load: external force on a structure
Two Types of Loads
Static : weight of a structure and the non-moving load it supports Example: books on a bookshelf
Dynamic : external force that moves or changes with time
Example: moving students on a staircase at school
Note: When you build a dynamic structure you must consider you may have both static and dynamic loads, therefore the structure must maintain both

11. Section 2.2 External Forces Acting on Structures
Supporting the Load
Engineers need two conditions to decide what type of bridge is suitable:
What the bridge is crossing (water/land)
What kinds of loads the bridge will be supporting Pg
Draw and Define each of the four different types of bridges.
(Figures 2.15, 2.16, 2.17, and 2.18)

12. Section 2.2 External Forces Acting on Structures
Performance Requirements of a Structure
** How well will a structure hold up under the load it was designed to carry out.
Important for safety, cost and efficiency
Performance Requirements should always be expressed in Maximum Weight

13. Section 2.3 Internal Forces within Structures
Definition
Internal Force: one part of a structure exerts on other parts of the same structure
Forces that act within a structure
Three Types of Internal Forces
Compression
Tension
Shear
Compression: a force that acts on an object or push parts together within an object
Tension: a force that acts to stretch and pull apart something
Shear: a force which push parts which are in contact with each other in opposite directions

14. Section 2.3 Internal Forces within Structures
Complementary Forces
Definition
Complementary Forces: when different kinds of internal forces act on a structure at the same time
Example: Bending
Compression is put forth on the top and tension is put forth on the bottom
Failure to resist either results in the beam breaking

15. Structural Components
Section Designing Structures to Resist Forces and Maintain Stability
Structural Components
Arches
Common structure used in bridges due to supporting large loads because the force of the load is carried through the arch to the foundation
Draw an Arch
Beams
Flat structure which is supported at each end
There are many different types of beams, how many and what are they?
Name, Draw and Describe each of them

16. Structural Components Structural Stress, Fatigue, or Failure
Section Designing Structures to Resist Forces and Maintain Stability
Structural Components
Columns
Solid structure that can stand by itself
Structural Stress, Fatigue, or Failure
When internal and external force is applied to a structure it can weaken it therefore resulting in STRUCTURAL STRESS
When a structure can no longer stand up to forces acting on it results in STRUCTURAL FAILURE
When a structure is repeatedly used out of it’s original function it results in STRUCTURAL FATIGUE