II. UNIT 1:  Static Forces and Bridges


    Time:  Entire quarter


  1. Unit Introduction:

Bridges can be designed in different ways to withstand certain loads and the forces that act on them.

  1. Standards


1.     C23 Describe the qualitative relationships among force and mass.

2.     C30 Explain how beam, truss, and suspension bridges are designed to withstand the forces that act on them.

3.     CINQ1 Identify questions that can be answered through scientific investigation.

4.     CINQ2 Read, interpret and examine the credibility of scientific claims in different sources of information.

5.     CINQ3 Design and conduct appropriate types of scientific investigations to answer different questions.

6.     CINQ4 Identify independent and dependent variables, and those variables that are kept constant, when designing an experiment.

7.     CINQ5 Use appropriate tools and techniques to make observations and gather data.

CT State Grade Level Expectations (Draft)

GRADE-LEVEL CONCEPT u Bridges can be designed in different ways to withstand certain loads and potentially destructive forces.

GRADE-LEVEL EXPECTATIONS (Forces GLEs 1-4 are also in 8.1)


1.     Force is a push or a pull and is described by its strength and direction and can be caused by a moving or a stationary object.  Forces are measured in newtons or pounds using scales.

2.     Forces can act simultaneously on an object from all directions with different strengths (magnitudes).  When the magnitude and direction of all the forces acting on an object are combined, or added together, the total force (net force) determines the object’s motion.  Forces in opposite directions are subtracted; forces in the same direction are added.

3.     If the strength of all the forces acting on an object from one direction is equivalent to the strength of the forces from the opposite direction, then the forces cancel each other out, and are said to be balanced.

4.     Bridges are elevated structures designed to support the movement of objects over a span.  Two important forces at work in bridges are tension and compression.

5.     Bridges must support their own weight (dead load) and the weight of those objects that will cross over them or act on them from time to time, such as wind, snow and ice (live load).  Bridges are kept stable by balancing the load forces with the supporting forces of the structure. These forces can cause parts of the bridge structure to push together (compression) or pull apart (tension).

6.     Different bridge designs distribute tension and compression forces in different ways, depending on the shapes of the parts of the structure. The biggest difference among bridge designs is the distances they can cross in a single span. Shapes commonly used in bridge design include arches, triangles and rectangles.

7.     Bridges are constructed of different materials whose properties and costs vary.  Some materials are strong against compression forces but weak against tension forces; some materials resist fire, corrosion or weathering.  Materials commonly used in bridge design include wood, rope, aluminum, concrete and steel.

8.     A beam bridge balances the load by concentrating it entirely onto the two piers that support the bridge at either end.  When a force pushes down on the beam, the beam bends. Its top edge is pushed together (compression), and its bottom edge is pulled apart (tension).  The amount of bend depends on the length of the beam.

9.     A truss bridge uses rigid, interlocking beams to form a system of triangles that distribute the load among all parts of the structure, increasing the structural strength of the bridge.

10.  A suspension bridge uses cables suspended from tall towers to hold up the deck and distribute the load.  The tension and compression forces acting on the beam are distributed among the cables (which experience tension) and the towers (which experience compression).

Engineers and scientists build models of bridges, conduct controlled experiments to learn how they will withstand various stresses, and consider the benefits and trade-offs of various design alternatives. 

11.  Bridge design is influenced by the length of the span, the properties of the materials and the environmental conditions, as well as by practical considerations, such as the bridge’s appearance, cost of materials or construction site challenges.

12.  Bridges can fail because they have faulty parts, are used in ways that exceed what was intended by the design, or were poorly designed to begin with.


SCIENTIFIC LITERACY TERMINOLOGY:  balanced/unbalanced forces, net force, load, tension force, compression force, beam bridge, truss bridge, suspension bridge

  1. Essential  Questions


    1. What is the relationship between mass and weight?
    2. What affect do balanced and unbalanced forces have on an object’s motion?
    3. What keeps a bridge from falling down?
    4. How does a bridge support its own weight and the weight of a load?
    5. How do different bridge designs balance the forces that act on them?


  1. Essential Content


    1. Force is a push or a pull and is described by its strength and direction.
    2. Mass is the measure of the amount of matter in an object; weight is the force of gravity that depends on mass. 
    3. Net force is the combination of all forces acting on an object.  They can add or cancel each other depending on direction and strength (magnitude).
    4. Unbalanced forces acting on an object cause a change in the object’s motion.
    5. The most fundamental rule of bridge design is that the net force acting on a bridge must be zero.
    6. Bridges are kept from falling down by balancing action/reaction forces.
    7. Compression is a “pressing together” force.  Tension is a “stretching or  pulling apart” force.
    8. A beam bridge balances the load of the bridge with the piers that support the bridge.
    9. A truss is a structure composed of thin horizontal and vertical members which is used to reinforce the structural strength of bridges.
    10. The load on a suspension bridge creates tension forces on the cables.  The towers and abutments create reaction forces.


  1. Essential Skills:


    1. To identify dependent and independent variables in an experiment.
    2. To measure force using spring scales.
    3. To design an experiment that tests how changing different properties on bridges affects bridge strength.
    4. To read and interpret different scientific sources of information.


  1. Vocabulary:


    1. Force
    2. Mass
    3. Weight
    4. Gravity
    5. Friction
    6. Net Force
    7. Balanced Forces
    8. Unbalanced Forces
    9. Action/Reaction Forces
    10. Tension
    11. Compression
    12. Pier
    13. Abutment
    14. Member
    15. Load/Decking
    16. Beam Bridge
    17. Truss Bridge
    18. Suspension Bridge
    19. Magnitude

      g.   Science Misconceptions:



h.   Suggested Labs and Activities:



III. Significant Task:  Which bridge is the strongest?