The Mag Mile and...Torque?

Part of content:

The Mag Mile andTorque?











A Bridge in Balance
p. 1

C
HICAGO
R
IVER
S
CHOOLS
N
ETWORK

The Mag Mile andTorque?
Chicago River Classroom Activity



Grade Level:

9
th
12
th



Duration:

45 min


Objectives:
1. Students will understand the
way the gear train in the Michigan Avenue Bridge works.
2. Students will understand
how the amplification of torque enables a small motor to lift a huge bridge.

Materials:
Sturdy large paper plates made of heavy cardboard (4 per group of 3 students)

Sturdy small paper plates (3 per group of 3 students)

Poster board (1 per group of 3 students)


Paper fasteners (4 per group of 3 students)

Standards:
12.D.4a, 12.D.5a
Summary

Students learn about gears and torque. They then build a model of the Michigan Avenue Bridge gears and

calculate
the increased torque provided by the gear chain.


Background

Built in 1920, the Michigan Avenue Bridge is a double leaf, double decker, fixed counterweight bascule trunnion bridge. The bridges two leaves are the two spans that come from either bridge tower and meet in the middle. Each leaf is also double decker, allowing for traffic to move on two levels. The leaves are actually part of Michigan Avenue itself. When the bridge opens to let boats through, each leaf moves up into an almost vertical position. Each leaf weighs 3, 750 tons. The word bascule is French for see saw. The bridge works in just that way. Each leaf balances on a trunnion (the same things used to fix a cannon to its stand). A trunnion is like a big pin sticking through the bridge leaf so it can pivot. Because the pivot point is not in the middle of the leaf, but rather close to one end, a counter weight is needed on the shorter end. The counter weight balances the torque (torque = force X distance, and refers to rotational motion) of the two ends. Torque takes into account both the differences in length and the effe