Designing a Roof for a Rain Barrel
A school wants to collect rainwater in a barrel to water its garden. The barrel has an open top with a diameter of 60 cm and can hold 200 liters. Students must design a cover that keeps leaves and debris out while letting rain in, and that will not collapse under heavy rain or wind. They are given these materials: plastic mesh, clear plastic sheeting, wooden dowels, string, tape, and rubber bands. The cover must be removable so the barrel can be cleaned. Constraints: it must cost less than $10, be built in one class period, and survive a simulated wind test (a fan blowing for 2 minutes) without blowing off.
1. Describe a design that meets all the criteria and constraints, and identify at least one trade-off the students are making.
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One reasonable design: Build a shallow dome frame from wooden dowels tied with string to span the 60 cm opening. Stretch plastic mesh over the frame and secure it with rubber bands or tape around the rim, leaving a small gap. The mesh lets rain through but blocks leaves. The dome shape sheds debris and resists wind. The cover is held on by rubber bands so it can be lifted off for cleaning.
Trade-off: The plastic mesh blocks large debris but lets small particles (tiny leaves, grit) through, whereas solid plastic sheeting would block everything but also block the rain and could sag and collect water. Choosing mesh trades some debris-blocking for rain passage and low cost. The dome frame also adds build time, trading simplicity for wind resistance.
Checking constraints: Materials are inexpensive (under $10), the build fits one class period, and the rubber-band attachment plus dome shape should resist the fan test. The design is removable, meeting the cleaning requirement.
2. During the wind test, the cover lifts off the barrel. The students want to fix it. Describe how they should use the engineering design process to improve the cover, and give one specific modification with reasoning.
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The students should treat this as an iteration in the engineering design process:
- Define the problem: the cover lifts off under wind.
- Identify the cause: the wind catches the edge of the cover and the rubber bands are not strong enough to hold it down.
- Brainstorm and choose a fix, then test again and redesign if needed.
One specific modification: Add a lip or skirt around the rim that hooks under the barrel's edge, or increase the number of rubber bands and anchor them to hooks on the barrel. Reasoning: A lip or more anchors resist the upward and sideways force of the wind better than a flat cover resting on top, because the wind can no longer get under the edge. After making the change, they should repeat the same 2-minute fan test to see whether the modification actually solved the problem - comparing results before and after is how engineers judge whether a fix worked.