Keeping in mind that this is an intended physics project for a child in elementary school, here are some additional ideas that may meet your criteria:
1. Build a Rube Goldberg machine that demonstrates a chain reaction using simple machines like levers, pulleys, and inclined planes.
2. Construct a homemade electromagnet and explore its properties, including its magnetic field strength and the types of materials it can attract.
3. Design a rollercoaster using cardboard tubes, paper clips, and marbles, and study the laws of motion as the marbles travel along the track.
4. Use simple materials like balloons and straws to build a hovercraft or rocket and explore the principles of lift, thrust, and drag.
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