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  Activity: Suspension Bridge
Activities Index | Handout | Educator Ideas  

Time
30&-40 minutes

Materials
(per group of two)
• 7 drinking straws
• masking tape
• 100 cm (about 4 ft.) dental floss or thread
• scissors
• 4 large paper clips
• paper cup
• pennies, metal washers, or other small weights
• ruler

Video Connection
Before the activity, show "Brooklyn Bridge" from Bridges to explain the forces supporting a suspension bridge. (See the Program Description to locate the show segment.)

See Bridge Overview.

For more information, see Additional Resources.

Icebreaker
Tie a piece of string to a shoe or other weight. Ask: Who can lift this shoe in the air only by pulling down on the string? (Guide kids to the solution of passing the string over the back of a chair or other support and using it as a pulley to lift the shoe.) Discuss how this design converts the pulling-down force into a force that pulls up on the weight.

Lead the Activity
• This basic suspension bridge design can be applied using other materials to build larger, stronger bridges. For example, kids can use paper-towel tubes and string to build the bridge deck and cables, and use the backs of two chairs as the towers.  
• To help kids understand how forces act in a suspension bridge, have them experiment with attaching the cables from the bridge deck only to the tops of the towers, instead of extending them back down to the surface at the ends of the bridge. Ask: How strong is the bridge this way? Why? (This model is less strong than the model in which the cables extend back down to the ground on the other sides of the towers. A load on this bridge deck pulls the tips of the towers inward. There is no balancing tension pulling the towers back out toward the ground.)

The Big Idea
Kids should find that adding the cables to their straw bridge and anchoring the cables on both sides significantly increases the load that the bridge can support.  
Illustration of suspension bridge, showing direction of compression and tension forces.
A suspension bridge's cables and towers transmit the dead load of the bridge deck and the live load of traffic to the massive anchor blocks at each end of the bridge. The tension in the cables leading up from the bridge deck is balanced by the tension in the cables leading to the anchor blocks, as well as the compression in the towers. The anchor blocks must be massive enough to resist the tension in the cables caused by the weight of the bridge deck.

Build on It
Possible outcome: Kids can make their bridge decks longer by creasing the end of one straw and inserting it into the end of another straw. Kids can try to build as long a bridge as possible that can support a given amount of weight.

Make Connections
Social Studies After showing "Brooklyn Bridge" from Bridges (see Program Descriptions) have kids each choose a member of the Roebling family and write or record an entry from that person's diary describing his or her role on and feelings about the bridge. 

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